Abstract
This article is designed to serve as a roadmap for understanding the fundamentals, the key advantages and the potential applications of microwave-assisted hydrothermal/solvothermal (MAH/S) processing. MAH/S synthesis is a versatile chemical method for preparing a diversity of materials such as metals, semiconductors, electroceramics, graphene and their composites as bulk powders, thin films, or single crystals. The key to improve performance of these materials is achieving controlled morphologies (0 to 3D dimensionality) that favor desirable physical-chemical phenomena at the surface, and in the bulk of these advanced materials. The main features related to the improvement of the thermal and non-thermal effects associated with the use of microwave power concurrently with hydrothermal or solvothermal methods are discussed. Furthermore, the main crystal growth mechanisms (Ostwald ripening and oriented attachment) of these solids in solution under MAH/S treatment are described. Products synthesized by the MAH/S, particularly of interest in the development of gas sensors, batteries, fuel cells, solar cells and photocatalysts are emphasized. We conclude by envisaging new future directions for the use of this rapid and versatile processing approach.
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1 Introduction
Microwave-assisted hydrothermal/solvothermal (MAH/S) synthesis is a chemical method for preparing a diversity of materials such as metals [1,2,3], semiconductors [4,5,6], electroceramics [7,8,9,10], graphene composites [11,12,13,14,15], as well as many other types of materials. The process involves the use of water (hydrothermal), different solvents (solvothermal) or mixtures of both, as synthetic medium under pressure, typically above the solvent vapor pressure and temperature near or above its boiling point in a sealed pressurized vessel during microwave (MW) irradiation (typically 2.45 GHz frequency). The method is quite versatile and can be used in preparation of bulk powders [7, 9, 16], thin films [17, 18], single crystals [19,20,21], polycrystalline compounds [22,23,24,25], and composites [11, 13, 26, 27]. In addition, it is possible to obtain different crystal morphologies — three-dimensional (3D) [28,29,30,31,32], two-dimensional (2D) [33,34,35], one-dimensional (1D) [36,37,38], or zero-dimensional (0D) [2, 39,40,41,42,43], by controlling the solvent supersaturation, precursor concentration, use of templates, pH adjustment, kinetic control, and other parameters. Furthermore, this procedure represents an interesting alternative to others for the preparation of new thermodynamically stable or even metastable materials. Over the last decade, a large number of reports regarding MAH/S synthesis have focused on controlled materials morphology (Fig. 1). This review highlights the advances in fundamental aspects of MAH/S synthesis, and provides examples and applications of materials prepared by this method.
The number of published items in each year on microwave-assisted hydrothermal/solvothermal processing. Data collected from the “Web of Science” in February 2, 2018. The words “microwave and (hydrothermal or solvothermal)” were input into the “topic” search box.
We present an overview of the experimental aspects and the basic principles concerning morphology and size control, thermodynamic and kinetic factors, classical and non-classical crystal growth mechanisms during MW heating. This is followed by describing the main features that induce the choice of this technique and correlate it with the versatility of obtaining large numbers of advanced materials of different compositions ranging from metallic nanoparticles, n- and p-type semiconducting metal oxides, electroceramics, graphene and diverse composite materials. In addition, it will be shown that these MW-assisted synthesized materials have potential application in certain technological areas of high interest such as gas sensors, photocatalysts, fuel cells, batteries and solar cells. Figure 2 shows a schematic representation of the possibilities arising from the methods discussed here. Finally, through an examination of the trends described in the literature, new future research directions for materials synthesis by the MAH/S method will be described.
An overview of the possible applications of the MAH/S method for obtaining materials with different compositions and morphologies of relevance to advanced technological applications
2 Effects of microwave power on hydrothermal/solvothermal synthesis
In general, the conventional hydrothermal/solvothermal methods refer to the processes taking place in closed systems where reagents and solvents are treated under both medium to high pressure and medium to high temperature. The most recognized delineation of the “hydrothermal” method relates to a heterogeneous reaction medium containing water as a main solvent (usually at a temperature above 100 °C and at a pressure above 1 atm) [44,45,46,47]. Modified conditions are typically achieved either by using an autoclave, a vessel allowing the exposure of selected materials to relatively elevated pressure and temperature by subjecting them to high-pressure saturated steam, or by employing a specific reactor to generate autogenous pressure through the heating of the closed reactor [48].
On the other hand, the addition of microwave power under hydrothermal/solvothermal conditions can contribute to remarkable improvements in the organic materials synthesis process/organic materials synthesis process. This is related to increased reaction rates induced by high temperatures; improved control of reaction parameters; improved reproducibility; superior yields; improved synthesis product selectivity resulting from reduced side reactions; indirect interaction between the microwaving source (magnetron valve) and the solvents and/or reactants; selective heating (if there are products with different microwave susceptibility in the reaction mixture); automatization and high throughput synthesis [49]. Varma and co-authors [50,51,52,53,54,55,56,57] described several microwave approaches leading to greener methods for producing nano-catalysts; use of recyclable mineral oxides or supported reagents; synthesis of pharmaceutically active heterocycles; organics and nanomaterials. Such advantages arise from the physical mechanisms inherent to the dielectric fluid heating phenomenon via microwave susceptibility described below.
Conventional heating of a solution occurs by heat conduction along the walls of the reaction vessel followed by diffusion to the central volume of the solution. In this less efficient process, there is loss of heat energy as well as non-uniform heating of the reaction medium. This leads to unequal distribution with some regions more heated than others, which results the need of extra time to obtain the final product. In the case of solid growth, this favors crystallization with a broad size distribution.
To circumvent these problems, using microwave as the heat source is a more efficient alternative. Microwaves induce dielectric medium heating and do not rely on heat conduction. The two most widely accepted mechanisms for explaining the instantaneous heating caused by microwaves are dipole rotation and ionic conduction [58,59,60]. In the first mechanism, polar molecules of certain solvents interact with the electric field component of the microwave through dipole rotation, with these movements leading to heating of the entire solution. In the second case, the ionic species present in solution respond quickly to the microwave driven force, causing rapid heating due to Joule heating. As the temperature increases, the energy transfer is more efficient and the whole process is facilitated.
Microwaves contribute to high yield given the rapid consumption of the initial reagents promoted by increased reaction kinetics and initial rapid heating [61]. In contrast to conventional heating where heat is dissipated at the walls of the reactor vessel, heating by microwaves takes place uniformly within the entire volume susceptible to the radiation (Fig. 3). Thus, microwave-based system may reach the boiling temperature of the solvent by 10 to 1000 times faster than the conventional system [62].
(Left). The microwave effect on the temperature profile after 60 s exposure. (Right). Comparison with a temperature profile of an oil bath. Heating by microwaves is more homogeneous in the central volume of the solution. The temperature range is in Kelvin and the position of the meniscus is indicated at the zero vertical axis. Reproduced with permission from Ref. [62], Copyright (2003) Springer
It is noteworthy that the microwaves do not have sufficient energy to break chemical bonds, but it is speculated that non-thermal microwave effects can increase the effective collisions between the molecules of the reagents and solvents [63,64,65]. Moreover, the conformation of the microwave apparatus can also influence the final results and the reproducibility of the experiments [66]. The Arrhenius equation [k = A exp.(−Ea/RT)], where k is the rate, A is the molecular collisions pre-exponential constant, Ea is the activation energy and R is the gas constant; can be used to indicate the relationship between k ∝ A at constant temperature (T) under MW heating. Typical chemical reactions result from specific molecular collisions, which classically increase with increasing temperature but also result from MW non-thermal effects. Nevertheless, for an specific reaction it is difficult to differentiate the specific molecular collisions effect from the impact of the temperature average molecular motion [67].
Accordingly, the choice of a suitable solvent (with appropriate permanent dipole moment) is vital to achieve the desired selective heating for the growth of homogeneously crystallized particles while achieving improved size distribution in reduced time. Importantly, microwave heating can accelerate slow reactions with associated large activation energies.
To illustrate the synergistic use of microwave power with the hydrothermal/solvothermal methods, Table 1 lists materials for which the addition of microwave power was observed to accelerate synthesis compared to the conventional hydrothermal/solvothermal process. Reaction times can be reduced from hours for conventional methods to minutes for microwave synthesis of the same crystalline structure materials.
3 Morphology control and growth mechanisms during MW irradiation
To improve the performance of functional materials, it is very important to have fine control over the shape and size of particles. In many cases the form of a crystalline solid derives from its unit cell structure. Wulff’s thermodynamic study [81] indicates that solids may have different morphologies because of the different surface energies of the crystal face during growth in a given environment. The lowest free energy will be associated with the more stable faces during the growth process, culminating in the formation of crystals with morphology having lowest surface energy [82].
Considering that the Wulff criteria also relates to growth within microwave-assisted hydrothermal/solvothermal reactions, Andrés et al. [83] developed a combined theoretical (first-principles calculations) and experimental study to improve the understanding of the electronic energy and structural properties that induce the control of the morphology of different metals and metal oxides, such as TiO2, BaZrO3, α-Ag2WO4 and Ag. Figure 4 shows the good agreement between the scanning electron microscopy (SEM) image of BaZrO3 crystals obtained by microwave-assisted hydrothermal (MAH) and the theoretical surfaces predicted by the Wulff simulation.
(Left). The Wulff crystal representation of BaZrO3 material with surface energy in Joules per square meter. (Right). Scanning electron microscopy image of BaZrO3 crystals obtained by microwave-assisted hydrothermal method. Reproduced with permission from Ref. [83], Copyright (2015) IOP Publishing
The classic process of solid crystallization in supersaturated solution follows a bottom-up design, starting from atoms, molecules or clusters using thermodynamics as the driving force. In order to prepare supersaturated solutions, changes in temperature or pressure, additions of soluble salts and/or changes of pH in solution are necessary [84, 85]. Under the MW-assisted hydrothermal/solvothermal method, different types of nucleation and growth processes are observed which are earlier than the conventional heating technique. In this section, the two main nucleation and growth mechanisms (Ostwald ripening and oriented attachment) that govern crystallization of inorganic solids will be described and illustrated. The growth of materials in the hydrothermal/solvothermal medium is impacted by both thermodynamic and kinetic parameters. Figure 5 illustrates the steps for mineral crystallization which arises from changes in the activation energy barriers related to nucleation, crystal growth and possibly phase changes [86].
The influence of the kinetics and thermodynamics in the crystallization process. Mineral crystallization can occur in a single stage (A) or through a sequence of steps (B) depending on the free energy of activation in accordance with the processes of nucleation, growth and phase transformation with amorphous phases representing possible kinetic reaction intermediates. Reproduced with permission from Ref. [86], Copyright (2003) Wiley-VCH
The Ostwald ripening (OR) mechanism [87] occurs due to the change in solubility of the nanoparticles in solution. Smaller particles have larger surface energy and a higher solubility facilitating their redissolution and subsequent aggregation onto existing larger particles. For this type of growth to take place, it is essential that the chemical potential of the nanoparticles increases in the process of dissolution and new precipitation, culminating into the growth of particles at the expense of the decrease in the total number of particles present in solution until the equilibrium state is reached. According to the Thomson-Gibbs equation, the fluidity of the smaller particles to the larger surface occurs due to the shift of the local chemical potential of the solution. This process is usually controlled by diffusion; and the final result is the production of near-spherical particles which are more thermodynamically stable [88]. Thus, the events described above take place during the growth of the particles in the MAH/S method. However, the nucleation process is faster and more homogeneous with the aid of microwave heating, promoting more effective maturation of the desired crystals. There are many published studies indicating that OR mechanism is responsible for the production of different types of materials [76, 89,90,91,92].
The oriented attachment (OA) mechanism was first described by Penn et al. [93] to explain how titania nanoparticles aggregate during hydrothermal synthesis leading to their growth into the final crystalline form. In the OA process, there is a spontaneous local organization (in planar interface) of close particles having similar crystallographic orientation [94], which requires the continued alignment of common crystal planes during crystal growth [95]. This type of non-classical mechanism helps to explain the formation of nanocrystals with controlled morphology by the assembly of primitive oriented crystallites available in solution. However, it is necessary to use modern techniques of characterization to identify and to explain in detail the mechanism in solution [96]. The use of microwave radiation provides a rapid and homogeneous heating of solution, in addition to possibly assisting in the alignment of the atomic planes by dipole rotation. Recent literature shows that materials grown by MAH/S occur via OA [40, 69, 97, 98]. Some reaction conditions, morphology and nucleation/growth mechanisms for metal oxides/hydroxides are summarized in Table 2.
4 Examples of microwave-assisted hydrothermal/solvothermal materials synthesis
The first paper which describes the MAH/S synthesis was reported in 1992 by the Komarneni group, in which crystalline oxides, such as TiO2, ZrO2, Fe2O3, KNbO3 and BaTiO3, were prepared using a combination of 2.45 GHz microwaves and hydrothermal techniques [102]. In recent years, many papers describing the use of microwave-assisted methods have been published, and these include Bilecka and Niederberger, who demonstrated the liquid phase routes for synthesizing inorganic nanomaterials [49], and the Kappe group, that elucidated the role of microwave irradiation in the synthesis of colloidal inorganic nanocrystals [103], drug discovery [104], and organic synthesis [105, 106]. In the following sections, we present examples of metallic nanoparticles, metal oxide semiconductors, electroceramics, and composites synthesized by MAH/S method. In addition, we described the morphologies, reaction time, temperature, solvents, precursors, and the surfactant agents.
4.1 Metallic nanoparticles
4.1.1 Ag, Au, Pt and Pd
A rapid microwave-polyol method for preparation of silver (Ag) and platinum (Pt) nanoparticles (NPs) was described by Komarneni and co-authors [107]. They produced NPs in 15 min, much faster than the conventional polyol method. They also showed that the size of Pt nanoparticles depends on the molecular weight of the polyvinylpyrrolidone (PVP) polymer. Nadagouda and Varma [53] also reported the synthesis of Ag nanoparticles without the use of surfactants or reducing agents, showing that the NPs were only obtained in higher polyethylene glycol concentration. In Gao’s work [1], Ag nanoparticles were prepared under microwave-assisted solvothermal conditions, in which different ratios of ethylene glycol and dodecylthiol were used to modify the particle morphology, a result of changes in reducing rate caused by the interaction between dodecylthiol and silver ions. Ag, Au, Pd and Pt nanoparticles were successfully prepared under microwave irradiation using glutathione as reducing and capping agent in aqueous medium. For Ag nanoparticles, the size ranged from 5 to 10 nm, while for the other metals larger particles were obtained (100–120 nm) [57]. Palladium NPs with about 3.8 nm of diameter were successfully prepared by MA solvothermal method, using ethanol solution of Pd(OAc)2 and PVP. In this case, 5 min of microwave exposition was sufficient to reduce Pd(II) to Pd(0) [3].
4.1.2 Metallic alloys
Au/Pd particles were synthesized starting from palladium powder and muriatic solution of gold by microwave and conventional hydrothermal methods. The microwave treatment was more efficient in achieving homogeneity of particles than conventional heating [108]. Nguyen et al. [109] reported the synthesis of FePt using platinum acetylacetonate and disodium tetracarbonylferrate as precursors with oleyl amine and octyl ether at 280 °C. They verified that the microwave heating at high pressure resulted in a mixture of face-centered cubic and face-centered tetragonal structured FePt particles. The alloy Au/AuxPd1-x, in which Au is the core and AuxPd1-x is the shell, was processed via layer-by-layer growth, resulting in a uniform morphology for all nanoparticles. The synthesis by the MAH method was performed in only 3 min using gold nanoparticles, gold and palladium salts, and sodium citrate as reducing agent [110]. A layer-by-layer formation of Au/Ag nanoparticles, with Au core and Ag shell, was also reported [111]. Figure 6 shows the images from transmission electron microscopy (TEM) of two nanoparticles prepared at 120 °C, where the number of shell layers is equal to 10. It shows the formation of the core-shell structure along with the dimensions of the shell thickness and core diameter.
(a, b) High resolution transmission electron microscopy (TEM) images of Au/Ag nanoparticles. dc = core diameter, ts = shell thickness. Adapted with permission from Ref. [111], Copyright (2011) American Chemical Society
4.2 N-type metal oxide semiconductors
Metal oxide semiconductors (MOS) have attracted a great deal of interest given their wide range of applications, as discussed in the final section of this review. Among the different n-type MOS, the most common are SnO2, WO3, In2O3, Fe3O4, ZnO and TiO2, which will serve as the focus of our discussion. An overview of the morphologies obtained for these n-type materials can be seen in Fig. 7. It includes SnO2 nanosheets [112], WO3 spheres [113], ZnO flower-like [114] and In2O3 bundle-like structures [115].
SEM images of: (a) SnO2 nanosheets (scale bar denotes 100 nm), inset: high resolution TEM (scale bar denotes 1 nm), showing the lattice spacing of SnO2. Adapted with permission from Ref. [112], Copyright (2014) American Chemical Society; (b) WO3 hierarchical spheres prepared in 10 min, inset: a single hierarchical sphere. Adapted with permission from Ref. [113], Copyright (2014) The Royal Society of Chemistry; (c) ZnO flower structure obtained using 450 W for 6 min. Adapted with permission from Ref. [114], Copyright (2014) American Chemical Society; (d) In2O3 bundle-like structure assembled out of 1D nanorods. Adapted with permission from Ref. [115], Copyright (2015) Elsevier. All materials were prepared by the MAH/S method
4.2.1 SnO2
Birkel et al. [71] closely monitored the crystal growth of SnO2 nanorods prepared by a microwave-assisted reaction and recorded the process by a series of photographic images. In the early minutes of the reaction (15 min.), ill-defined rod-like structures were obtained mixed with other morphologies (spheres and cubes). These structures served to initiate the growth process resulting in nanorods final morphology after 45 min of reaction. Keeping the synthesis running for longer times (60–90 min.), rods size and crystallinity further increased.
Wang et al. [112] compared the performance of photoanodes of dye-sensitized solar cells that consists of SnO2 nanosheets prepared by MAH and conventional hydrothermal synthesis. Both methods led to the same general nanosheet morphology, but the sheet size produced by MAH was smaller due to faster heating resulted in a high nucleation density within a short time. This led to enhanced photoelectrical energy conversion efficiency. Figure 7(a) presents nanosheets prepared by the MAH method.
The influence of ethanol/water ratio on the growth of SnO2 nanoparticles was studied by Mansek and co-workers [116]. It was demonstrated that the higher the ethanol/water ratio, the bigger the nanoparticle size, but when only ethanol was applied as solvent no particles were formed, thus, revealing a mechanism of SnO2 synthesis based on the hydrolysis process.
4.2.2 WO3
Phuruangrat et al. [117] prepared the hexagonal phase of WO3 by both MAH and conventional hydrothermal methods which led to nanowire and nanorod morphologies, respectively. The WO3 nanowires produced by MAH within 3 h exhibited both higher crystallinity and aspect ratio (over 625) than the WO3 nanorods prepared by CH method in 12 h, both at 150 °C. It was found that the WO3 nanowires had superior electrocatalytic activity for the hydrogen evolution reaction from water, compared to WO3 nanorods due to their high crystallinity and aspect ratio.
WO3 hierarchical nanospheres were synthesized by MAH at 180 °C for 10, 30, 60 and 240 min, in order to evaluate the effect of synthesis time. The morphology of the nanospheres produced within 10 min is illustrated in the SEM image of Fig. 7(b). The diameter and dispersion of the spheres increased with time until 240 min, after which the homogeneity of the spheres disappeared [113]. Sungpanich et al. [118] also studied the influence of time on the synthesis of WO3 nanoplates. For reaction times equal to or below 120 min, two different phases (monoclinic WO3 and orthorhombic WO3.H2O) were obtained, while at reaction time of 180 min only monoclinic WO3 nanoplates were obtained.
WO3 hydrates have also been prepared by the MAH method. Perfecto et al. [119] demonstrated the synthesis of nanostructured WO3∙0.33H2O by a synthetic approach combining ultrasonic spray nozzle and MAH methods. The crystal growth was discussed in two steps, as depicted in Fig. 8. First, the ultrasonic spray nozzle step enhances the nucleation rate of H2WO4 clusters. Then, during the MAH step, the particles grow in nanocolumn shape by Ostwald ripening process and are self-assembled into a hierarchical morphology. As a result of the developed synthetic method, the hierarchical WO3∙0.33H2O nanocolumns showed a high specific surface (125 m2 g−1) area and exhibited excellent isopropanol sensing performance at room temperature.
Crystal growth illustration of nanostructured WO3∙0.33H2O using a combination of ultrasonic spray nozzle and MAH methods. The growth mechanism can be divided in two steps: (a) high nucleation rate of H2WO4 clusters and (b) growth of nanocolumns by Ostwald ripening followed by the self-assembly of the structures until the final hierarchical morphology is achieved. Reproduced with permission from Ref. [119]. Copyright (2017) The Royal Society of Chemistry
4.2.3 ZnO
Different morphologies of ZnO were effectively obtained using a MAH method by changing the solvents and precursors, as well as applying different reaction times, in the absence of templates or surfactants (Fig. 9). The growth processes for each reaction condition were discussed, showing the role of the solvents (water, ethanol and ethylene glycol) in achieving marigold flower-like, jasmine, sea urchin-rod, calendula, and rice grain shape-like morphologies [78].
Several morphologies of ZnO (flower-like marigold, multipod jasmine flower-like, sea urchin-rod flower like, calendula flower-like, and rice grain shape-like) obtained by MAH/S method by changing the zinc source and solvents. Reproduced with permission from Ref. [78], Copyright (2015) The Royal Society of Chemistry
The nanorod morphology is commonly observed for ZnO synthesized by MAH. Although the reaction time was shortened, surfactant cetyltrimethylammonium bromide was required as a structure directing agent in the synthesis [120]. Pimental et al. [114] studied the effect of time and microwave power on ZnO synthesis. They observed that increasing MW power, resulted into thinner nanorods, until flower-like structures appeared. For example, Fig. 7(c) shows a flower-like structure of ZnO that was obtained using 450 W for 6 min. In this study, the same morphology was achieved by conventional hydrothermal method, taking at least 4 h compared to 6 min for the MAH treatment. Flower-like ZnO morphology obtained by MAH method also exhibited excellent radiation sensing response at room temperature [121].
4.2.4 In2O3
Rapid synthesis of In2O3 using vitamin C as a structure-directing agent was reported by Li et al. [115]. They discussed the formation of cubic In2O3 with nanorod bundle-like morphology (Fig. 7(d)), achieved at 140 °C. The morphologies appeared to evolve following the increase in temperature; e.g., samples prepared at 115 and 120 °C were elliptical nanoparticles with smooth surfaces or with small protuberances on the surfaces, respectively; while the ones synthesized at 130 °C were composed of 1D nanorods. Finally, nanorod bundles were obtained at 140 °C. Motta and co-workers [122] described the synthesis of In(OH)3 by the MAH method, followed by annealing at 500 °C for 5 min in a microwave oven. Cubic and rectangular shapes of In(OH)3 and In2O3 were reported, with collapse of the structures due to water elimination following annealing.
4.2.5 Fe3O4
Magnetite (Fe3O4) has attracted attention for in Li-ion battery applications, for electronic devices and as support for catalyst immobilization, due to its magnetic properties. Rizzut et al. [43] found that the morphology of Fe3O4 was influenced by the different additives used in the synthesis. The use of PVP polymer promoted the formation of hexagonal nanoplates, similar to that found in synthesis without additives, whereas the use of trisodium citrate and oleic acid resulted in spheroidal and quasi-spherical nanoparticles, respectively.
Chen et al. [123] reported the synthesis of Fe3O4 by MAH using only FeCl2 and NaOH. By increasing the reaction temperature to 50 °C trace amounts of Fe metal were observed, at the same time there was an improved development of the Fe3O4 faces. Fe3O4 nanoparticles were synthesized with low reaction time (30 min.) and changes in the precipitating and nucleating agents [90]. Pure Fe3O4 cubic phase could only be obtained using aqueous ammonia, while the use of (NH4)2CO3, NH4HCO3 or NH4Ac led to the formation of other phases (FeO(OH) and α-Fe2O3). Furthermore, polyhedral nanoparticles with uniform-shapes were obtained with aqueous ammonium and ammonium bicarbonate, whereas clusters appeared when ammonium carbonate or acetate was used.
4.2.6 TiO2
Titanium dioxide (TiO2) is central to a number of applications such as solar cells [80, 124, 125], batteries [126], and photocatalysis [127]. For the dye-sensitized solar cell application, a low-cost, clean and rapid synthesis of TiO2 using MAH method has been proposed. Nanorods and V-shaped twin morphologies of TiO2 were prepared by using mixtures of titanium tetraisopropoxide, tetramethylammonium hydroxide (TMAOH), and citric acid in aqueous medium, heated at 180 °C for 60 min. This method resulted in particles of TiO2 with a highly crystallized nature, exhibiting high incident photon-to-current conversion efficiency (85.6%) [80]. Shen et al. [125] reported a solvothermal route for the synthesis of TiO2 nanocrystals, carried out by reacting titanium isopropoxide, isopropanol and water for 30 min at 220 °C.
Yoon and co-workers [126] demonstrated that different polymorphs of TiO2 could be obtained by the use of diverse alcohols as solvents and changing the titanium source (TiCl3 or TiCl4) in the synthesis. For instance, when using ethanol and TiCl4 or TiCl3, the resulting phase was tetragonal anatase. On the other hand, the use of butanol with TiCl4 led to the formation of tetragonal anatase, but with TiCl3 orthorhombic brookite, with a small amount of tetragonal rutile, was generated. Meanwhile, employing octanol as solvent with TiCl4, resulted into the tetragonal rutile phase while tetragonal anatase with a small quantity of tetragonal rutile were obtained with TiCl3. A change in the morphology followed the change in crystalline phase of TiO2. The influence of reaction time was also investigated by Moura et al. [128]. They found that 1 min was sufficient to produce coalescence and misshaped spheres of TiO2. The sphere shape was preserved until 30 min of reaction; however, after 60 min, rods also appeared to be mixed with the spheres.
4.3 P-type metal oxide semiconductors
Many p-type MOS have been synthesized by MAH/S method, including Cu2O [129], Cr2O3 [130], CoO [130, 131], Mn2O3 [130, 132], Mn3O4 [131], and MnOx (with Mn3O4 as principal component) [133]. However, we focused only on four metal oxides: CuO, NiO, Co3O4, and Bi2O3, discussing their synthesis in more detail. Figure 10 illustrates the morphology of such materials, for example NiO flakes [134], Co3O4 nanocubes [135], and Bi2O3 needles [136].
SEM images of (a) Flake shaped NiO prepared using 600 W of microwave power for 1 min. Adapted with permission from Ref. [134], Copyright (2015) The Royal Society of Chemistry; (b) Co3O4 nanocubes synthesized at 140 °C for 1 h. Adapted with permission from Ref. [135], Copyright (2013) The Royal Society of Chemistry; (c) needle-like morphology of α-Bi2O3 without pressure-assisted heat treatment. Adapted with permission from Ref. [136], Copyright (2015) American Chemical Society
4.3.1 CuO
Different morphologies of CuO were obtained by changing the copper source at the same synthesis conditions. For example, the use of Cu(CH3COO)2 or Cu(NO3)2 resulted in spherical particles or flower-like rod morphologies, respectively. The change in morphology also changed the catalytic activity, which was higher for the spheres [137]. Volanti and co-workers [69] demonstrated the stages involved in the formation of CuO with sea urchin-like morphology. In the first steps, a heterogeneous nucleation was promoted by hollow nanospheres; then the particles began to form triangular plates while they grew and sharp points became evident in the cores. These cores transformed into cross-like structures and lastly, 3D structures became evident with an open structure, which began to close until the sea urchin-like morphology was obtained with the growth of the plates over time. The whole process of formation of CuO urchin-like is seen in Fig. 11.
Stages of formation of CuO sea urchin-like synthesized by MAH method. (a) First CuO nanoparticle cores, (b–c) initial triangular points, (d) begin of the self-assembly process of CuO, (e–f) flower-like structures, (g–h) final 3D CuO sea urchin-like structure. Scale bars: (a–d) 60 nm; (e–f) 200 nm and (g–h) 1 mm. Reproduced with permission from Ref. [69], Copyright (2010) The Royal Society of Chemistry
Three distinct morphologies of CuO were prepared by changing the solvent or base in the synthesis. The urchin-like morphology was obtained using CuCl2 and NH4OH in aqueous medium, whereas the use of NaOH instead of NH4OH or ethanol instead of water changed the morphology to fiber-like and nanorod-like, respectively. The CuO urchin-like structures showed a superior catalytic activity for the catalytic dehydrogenation of ethanol in acetaldehyde [138].
4.3.2 NiO
NiO with nanoflake-like morphology has been produced for supercapacitor applications. The nanoflakes were synthesized by first preparing β-Ni(OH)2 by MAH followed by calcination at 300, 400 and 500 °C. Nanoflake-like hexagonal β-Ni(OH)2 was obtained after 5 min of microwave treatment, using Ni(NO3)2.6H2O, cetyltrimethyl ammonium bromide (CTAB) and ammonia. The calcination of β-Ni(OH)2 at 300 °C resulted to the formation of cubic NiO while preserving the nanoflake morphology. This phase exhibited the most promising results for supercapacitor applications, with maximum specific capacitance of 401 F g−1 at current density of 0.5 mA cm−2 [100].
Alternative preparation of NiO nanoflakes was reported by Babu and co-workers [134]. In this case, synthesis was performed by the reaction of Ni(NO3)2.6H2O and ammonia in the absence of surfactant, using a different range of microwave radiation intensities (300–900 W). Higher microwave power, 600 and 900 W, was demonstrated to result in well-defined flake-like morphology with low agglomeration. The flake shaped NiO produced with 600 W are shown in Fig. 10(a). These NiO samples exhibited enhanced specific capacitance.
4.3.3 Co3O4
Co3O4 nanocubes, with a lateral size of 20 nm, prepared by the microwave-assisted solvothermal method for 20 min, were used as gas sensors for detection of xylene and ethanol [139]. Chen et al. [140] demonstrated that MAH synthesis of Co3O4 with subsequent annealing promoted the formation of mesoporous nanoflakes. This mesoporous structure led to efficient catalysis of oxygen evolution as well as the catalytic stability of Co3O4.
Mulinari et al. [135] discussed the growth process of the crystalline Co3O4 nanocubes by MAH synthesis carried out for 10, 30 and 60 min at 140 °C in aqueous medium. Increasing the reaction time resulted into decreasing the poor crystallinity of the β-Co(OH)2 phase, while high crystalline nanocubes of Co3O4 became more evident. Figure 10(b) shows the nanocube morphology of Co3O4. The process could be explained by the two following steps: first, hydrolysis of cobalt salt used as precursor, leading to the formation of β-Co(OH)2; second, fast heating promoted the nucleation, dissolution and recrystallization for transforming β-Co(OH)2 into Co3O4.
4.3.4 Bi2O3
The photoluminescent property of α-Bi2O3 needles was evaluated for two different samples: the first was obtained by MAH synthesis (Fig. 10(c)), and the second by pressure-assisted heat treatment following MAH. The pressure treatment was found to enhance the angle between [BiO6]-[BiO6] clusters, promoting an increase in the material’s electronic density, which led to higher photoluminescence emission intensity. Theoretical studies were carried out to further understand the observed photoluminescence properties [136]. Liu et al. [141] reported the MAH preparation of cubic γ-Bi2O3 with nanorod structure, as well as the composite with reduced graphene oxide. The photocatalytic performance for both materials was evaluated, with degradation of methyl orange reported to be 78% for pure Bi2O3 vs 93% for the composite after 240 min under visible light.
4.4 Electroceramics
The preparation of electroceramics, by the MAH process including BaTiO3, SrTiO3, Sr0.5Ba0.5TiO3, PbTiO3, Pb(Zr0.52Ti0.48)O3, SrZrO3, BaZrO3, as well as pyrochlore phases with the Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 compositions, were first highlighted by the Komarneni group [7]. The number of electroceramic materials prepared by MAH/S has continued to increase. The syntheses of different electroceramics have recently been reported, including Ba1-xCaxTiO3 [142], BiFeO3 [143], and BiFexCr1 − xO3 [144]. Figure 12 gives some examples of morphologies achieved in the MAH/S synthesis of BaTiO3, CaTiO3, and BaZrO3. The process of obtaining each of these materials will be further detailed in the following sections.
(a) TEM image of BaTiO3 nanocubes prepared with 15 min of MW irradiation. Adapted with permission from Ref. [145], Copyright (2010) American Chemical Society. SEM images of: (b) CaTiO3 with box-shaped rectangular morphology. Adapted with permission from Ref. [146], Copyright (2013) The Royal Society of Chemistry; (c) BaZrO3 decaoctahedron obtained in 40 min of MAH treatment. Adapted with permission from Ref. [147], Copyright (2011) The Royal Society of Chemistry
4.4.1 BaTiO3
Different reaction conditions have been applied to produce barium titanate (BaTiO3) using the MAH method. Swaminathan et al. [145] studied the effect of MW irradiation time on BaTiO3 synthesis. They demonstrated a polymorphic change from cubic, for lower reaction times, to tetragonal structure after 15 min of MW irradiation. For longer reaction times, the nanocube morphology was preserved, but with increased particle size. Figure 12(a) shows the morphology of BaTiO3 produced within 15 min of MW irradiation. Moreira et al. [42] also studied the time dependence of BaTiO3 nanoparticle using MAH synthesis. They found that the tetragonal structure as well as the spherical morphology persisted for all reaction times.
4.4.2 CaTiO3
Rectangular CaTiO3 particles were obtained by the MAH route using TiO2, CaCl2, urea and NaOH as reactants. The urea played an important role of shape-controlling, leading to rectangular prism-like morphology, as shown in the SEM image in Fig. 12(b) [146]. Mazzo et al. [147] verified the effect of two titanium precursors (titanium tetrachloride and titanium tetraisopropoxide) on the synthesis of CaTiO3. Micro-cube morphology was obtained for both precursors, but with the use of titanium tetraisopropoxide, CaCO3 impurity was detected and the material was found to be less crystalline. The mechanism of self-assembly of nanoplates was proposed for the formation of the cube-shape morphology.
4.4.3 BaZrO3
The process of BaZrO3 decaoctahedron growth was studied in detail by Moreira et al. [148]. In the early stages of the reaction (10 min), spherical-like structures were obtained, however BaCO3 and ZrO2 as secondary phases were still present. At 20 min, a pseudo core-shell morphology was obtained and the gradual growth of polyhedral was observed with longer times of reaction (> 40 min.). The SEM image in Fig. 12(c) highlights the decaoctahedron shape of BaZrO3 synthesized for 40 min. In each of these different stages of growth, the mechanisms of oriented attachment and Ostwald ripening were described. De Keukeleere et al. [149] studied the difference between microwave-assisted and conventional solvothermal /hydrothermal synthesis of BaZrO3 nanoparticles. For hydrothermal conditions, both methods resulted into mainly amorphous material, that required further heating at 700 °C. On the other hand, the microwave-assisted solvothermal (using water and isopropanol as solvents) synthesis led to the formation of crystalline BaZrO3 nanoparticles in a lower reaction time compared to the conventional solvothermal method.
4.5 Graphene
Graphene has attracted great interest due to its unique characteristics. MW has been used to synthesize graphene [150], to reduce graphene oxide (GO) to graphene or reduced graphene oxide (RGO) [151,152,153,154], and to prepare composites of metal oxides and graphene, discussed latter. Hu et al. [152] studied the capacity of microwave absorption by graphite oxide based on its degree of oxygenation, showing that the less oxidized samples interacted more with microwave radiation reaching higher temperatures. As a result, when a mixture of graphene and GO was irradiated by MW, the MW energy was transformed to heat, allowing the reduction of adjacent GO to graphene (Fig. 13).
Process of graphene formation: first, the graphene absorbs microwave irradiation and ignites, then an avalanche-like process of deoxygenation of GO occurs, leading to the formation of graphene. Reproduced with permission from Ref. [152], Copyright (2012) Elsevier
Microwave irradiation at a temperature range between 120 and 200 °C was used to prepare graphene sheets from expanded graphite using aqueous ammonia solution [150]. Ammonia plays the role of fabricating a more reduced graphene, since the synthesis carried out in pure water resulted in a high amount of oxygenated groups.
4.6 Composite materials
In this section, we exemplify three kinds of composite materials: n-p junction, metal-metal oxide semiconductor, and graphene-metal oxide semiconductor. Figure 14(a) shows a high resolution TEM image of an n-p junction, in which the heterojunction between p-type CuO and n-type TiO2 can be visualized through the lattice fringes of CuO and TiO2 polymorphs (anatase and rutile) [155]; see section 4.6.1 for a more detailed description of the CuO/TiO2 heterojunction. The TEM image in Fig. 14(b) shows the distribution of gold nanoparticles on the surface and in the framework of hollow ZnO nano-doughnuts, forming ZnO/Au nanocomposites [156] (section 4.6.2). A composite of In2O3-RGO synthesized using indium nitride and GO by MAH method [157] is shown in Fig. 14(c). The distribution of well-faceted In2O3 nanocubes on graphene sheets can be seen, generating a transparent film.
(a) High resolution TEM image of CuO/TiO2 junction, showing the lattice spacings of CuO and TiO2. Adapted with permission from Ref. [155], Copyright (2015) Wiley-VCH; (b) TEM image of hollow doughnut-like Au/ZnO nanocomposites. Adapted with permission from Ref. [156], Copyright (2012) American Chemical Society; (c) SEM image of In2O3 cube/RGO composites. Adapted with permission from Ref. [157], Copyright (2014) American Chemical Society
4.6.1 n-p junctions
Huang et al. [158] reported the synthesis of Bi2O3/Bi2WO6 heterostructures. They showed that at synthesis temperature of 180 °C only the orthorhombic phase of Bi2WO6 was obtained, while increasing the temperature enabled formation of higher content of cubic Bi2O3. Thus, a p-n junction of p-type Bi2O3 and n-type Bi2WO6 could be generated. NH4F was also used in the synthesis, fluoride ions could enter the Bi2WO6 lattice, resulting to formation of a Bi2O3/Bi2WO6 − xF2x heterostructure. The analysis showed that the p-n junction promoted higher photocatalytic activity with respect to photodegradation of rhodamine B.
The p-type-CuO/n-type-TiO2 heterojunction used for photocatalytic degradation of 2,4-dichlorophenoxyacetic acid was also reported [155]. The heterostructure was prepared from a mixture of TiO2 and Cu(CO2CH3)2.H2O in ethanol, heated under MAH treatment for 5 min at 150 °C. The formation of CuO particles was attributed to the Cu(OH)2 species, and a homogeneous dispersion of CuO promoted by the microwave method was demonstrated.
4.6.2 Metal-metal oxide junctions
The preparations of different junctions of metal and metal oxide by the microwave-assisted method have been reported in a series of studies. Rai et al. [159] described the synthesis of core-shell Ag@SnO2 nanoparticles (NPs), in which the Ag NPs were first prepared by the colloidal method, followed by deposition of SnO2 NPs on Ag NPs by the MAH method. The MAH treatment resulted in an excellent dispersion of Ag@SnO2 NPs, with no isolated Ag NPs in the sample. Moreover, the Ag surface plasmon was red-shifted after the SnO2 shell formation. Finally, it was discovered that the presence of Ag NPs improved the gas sensing detection of SnO2 for p-xylene.
Cu/Cu2O heterostructures were prepared by the MAH method, in which the change of reaction time led to different morphologies and the amount of Cu in the heterostructure increased with longer times. Obtaining Cu/Cu2O in a single-step was possible due to the reduction of Cu2O by ascorbic acid during synthesis. The hollow sphere obtained in 30 min of synthesis, which contained 10.5% wt of Cu, exhibited the highest photocatalytic degradation activity as well as the best gas sensing performance compared with the heterostructures synthesized with 5, 10, 20 and 60 min [76].
Geng et al. [156] reported the synthesis of hollow doughnut-like ZnO/Au by the one-step MAH method. Here, the formation of ZnO and reduction of Au occurred simultaneously, resulting to a dispersion of Au nanoparticles on the surface and in the structure of ZnO. This nanocomposite exhibited good biocompatibility, showing one more potential application for these materials.
4.6.3 Graphene-metal oxide junctions
The synthesis of composites of metal oxides and graphene (or graphene-based materials) by MAH has been extensively studied. Composites of graphene or RGO with SnO2 [74, 160,161,162], In2O3 [157], Mn3O4 [163, 164], TiO2 [165], CuO [166,167,168], Cu2O [14], Co3O4 [169], Fe2O3 [170], ZnO [171, 172], and WO3 [173, 174] have been reported.
Zhong et al. [161] developed rapid synthesis (20 min) of sandwiched SnO2 nanoparticles between graphene sheets by the microwave autoclave method. This method consisted of a two-step process: the first step involves the formation of SnO2 nanoparticles onto the graphite oxide, followed by reduction of graphite oxide to graphene as the second step. This method resulted in well dispersed SnO2 nanoparticles on graphene nanosheets, preventing restacking of the graphene sheets. The synthesis of RGO-SnO2 nanocomposites by a one-pot MAS method was also reported by Zito et al. [175]. The one-step method allowed the graphene oxide reduction and the growth of hollow SnO2 nanoparticles to occur concomitantly, as well it promoted a greater interconnectivity between RGO and SnO2. The evaluation of the gas sensing performance revealed that the nanocomposite exhibited a high ethanol response, which was larger than the response of pure hollow SnO2 nanoparticles, indicating the role of RGO in the gas-sensing performance.
Zhou et al. [166] reported a facile and rapid single-step MAH synthesis of graphene-wrapped CuO spheres. Samples with different content of CuO were studied with the graphene wrapped CuO spheres preventing their aggregation, with the material containing 16.9% of graphene showing the best electrochemical performance.
Hydrophobic ZnO nanoparticles were anchored onto graphene sheets by MAH at 160 °C, followed by annealing at 100 °C in order to transform Zn(OH)2 into ZnO. The hydrophobic character of ZnO nanoparticles allowed their dispersion on graphene sheets without chemical modification. This characteristic makes it desirable for solar cells application, since the hydrophobic ZnO-graphene nanocomposite could be mixed with polymer-fullerene blend (PCPDTBT:PCBM), thus improving the solar cell performance through better charge transport [171].
5 Applications of materials synthetized by MW-assisted hydrothermal/solvothermal method
5.1 Gas sensors
A growing number of materials synthesized by the microwave hydrothermal/solvothermal method are being examined for gas sensor applications. ZnO [176], WO3 [177], and In2O3-graphene [157] were used for NO2 detection; Pd/SnO2 [178], Au@TiO2 [179], and Au/SnO2 [180] for CO sensing; TiO2 [181], and Pd-GO [182] as H2 gas sensors; and α-Fe2O3/SnO2 [183], CuO [184, 185], and ZnO [120] for ethanol detection. Ag@SnO2 core-shell nanoparticles obtained by MAH method exhibited a higher gas sensor response for p-xylene compared to pure SnO2, showing that Ag NPs play an important role in the sensing mechanism due to their electronic and catalytic activity. The transformation of p-xylene into more active species by Ag NP catalyst after gas diffusion through SnO2 shells explained the improvement in Ag@SnO2 selectivity [158].
Hierarchical flower-like SnO2 structures were also applied as efficient ethanol sensors [186]. These structures exhibited excellent sensing properties that were attributed to the unique hierarchical morphology, facilitating the gas diffusion, but also to the surface defects and the high specific surface area promoted by the MAS method.
Volanti et al. [187] examined the influence of CuO morphology on H2 gas sensing performance. All three samples (urchin-like, fiber-like, and nanorods) exhibited good selectivity for H2 compared to CO and NO2 at different temperatures. However, at 200 °C, the H2 sensitivity of urchin-like specimens presented enhanced sensor signal compared to the other morphologies. The unique morphology of urchins providing parallel particle-particle contacts explained the better sensor response obtained for this material. Figure 15 gives an overview of the H2 selectivity of the gas sensor based on CuO with different morphologies.
(Left) Schematics of CuO particle-particle contacts and the SEM images for (a) urchin-like, (b) fiber-like, and (c) nanorods morphologies. The black and the orange regions correspond to bulk resistance and Debye-layer, respectively, while the gray region corresponds to the electrical contact. (Right) The selectivity for 50 ppm H2 in relation to CO (d) and (e) NO2 as function of temperature for the three CuO morphologies. The insets indicate the selectivity as function of operating temperatures for different gas concentration for CuO urchin-like structure. Adapted with permission from Ref. [187], Copyright (2012) Wiley-VCH
The combination of Cu2O nanorods and RGO led to a composite with remarkable response to NH3 at room temperature. The RGO was shown to play an important role in gas detection, since Cu2O by itself exhibited weak response even to 1000 ppm NH3 at elevated temperature (270 °C), whereas the Cu2O-RGO composites detected 100 ppm NH3 at room temperature. Furthermore, the composite sensor exhibited higher selectivity for NH3 with excellent repeatability [14].
5.2 Batteries
SnO2-RGO composites with graphene content of 33.3 wt% showed enhanced Li-cycling performance compared to pure SnO2, attributed to the graphene preserving the structural integrity of the composite. The cycling behavior study revealed that SnO2-RGO had a reversible discharge capacity of 590 mA h g−1 for up to 200 cycles, whereas for pure SnO2 this value only reached 78 mA h g−1 [161].
Lu and Wang [166] reported the growth of two different morphologies of CuO, nanosheets and fusiform, on graphene sheets, which was achieved by changing the reaction temperature. Both hybrid materials exhibited superior lithium-storage capacity than pure CuO and graphene because their layer-by-layer structure is likely to contain extra micro-pores and defects for Li storage. After 40 cycles, the charge capacities of the CuO nanosheets-graphene and fusiform CuO-graphene were 801 and 666 mA h g−1, respectively, while for pristine CuO and graphene these values were 173 and 631 mA h g−1.
Kong et al. [188] produced yolk-shell NiO nanospheres by synthesis of Ni-based metal-organic frameworks (Ni-MOF) using the MAH method, followed by calcination at 500 °C, seen in Fig. 16(a). This method produced a material with mesoporous structure (Fig. 16(b–c)), which was the reason for the large charge capacity of 1063 mA h g−1, at a current density of 0.2 A g−1. Moreover, the charge capacity of yolk-shell NiO nanospheres showed negligible decrease to 1060 mA h g−1 after 60 cycles (Fig. 16(d)). At larger current densities, the nanospheres showed lower charge capacity and more rapid capacity fading (Fig. 16(e)). The acronym N1.5 in the figures refers to the molar ratio Ni(NO3)2:benzene-1,3,5-tricarboxylic acid (BTC) equal to 1.5, which was the optimal ratio to obtain the yolk-shell NiO nanospheres.
(a) Formation of yolk–shell NiO Microspheres starting from MAH Synthesis of Ni-based Metal–Organic Framework; (b) SEM images of NiO microsphere (N1.5); (c) TEM image of NiO microsphere (N1.5); (d) Cycling performances of different NiO structures (N0, N1, N1.5, N2, and N2.5) at 0.2 A g−1; (e) Cycling performances of the yolk–shell NiO of N1.5 at different current densities. Adapted with permission from Ref. [188], Copyright (2015) American Chemical Society
5.3 Fuel cells
Chen et al. [189] prepared carbon supported Pt nanoparticles (Pt/C) using microwave irradiation and compared it with a commercial Pt/C catalyst. They demonstrated that Pt/C prepared by microwave irradiation showed a higher electrocatalytic performance for electrooxidation of methanol at room temperature. This result was mainly attributed to the uniformity of the small Pt particles, which was possible due to the microwave treatment. Mn2O3 and Cu-doped Mn2O3 nanoballs, as seen in Fig. 17(a), were synthesized by MAH and their potential as catalysts for alkaline fuel cells was also studied [190]. Both materials (Cu-doped and pure Mn2O3) exhibited great stability for oxygen reduction and evolution reaction in alkaline medium, evaluated by chronoamperometry (Fig. 17(b–c)). The materials showed distinct long-term choronoamperometic stability, necessary for alkaline fuel cells. In addition, the Cu presence improved the electrocatalytic activity.
(a) SEM image of Cu-Mn2O3 nanoballs. Chronoamperometric curves for the: (b) oxygen reduction reaction, (c) oxygen evolution reaction, showing distinct long-term choronoamperometic stability. Adapted with permission from Ref. [190], Copyright (2015) The Royal Society of Chemistry
5.4 Photocatalysis
ZnS@ZnO core-shell structures, prepared in one-step process, were shown to exhibit increased photocatalytic activity for p-chlorophenol compared to pure ZnO or ZnS. Moreover, no crystallographic phase change was observed after the photocatalytic reaction. Furthermore, photocorrosion observed for pure ZnO was overcome, showing good stability as photocatalyst [89]. Different morphologies of ZnO (plates, rounded plates, brush-like, and 3-D flower-like) were prepared by MAH/S method, changing the precursor concentration, reaction time, temperature, and solvent [191]. The photocatalytic activity was studied under UV and visible illumination. The rounded plates and flower-like structures presented enhanced photocatalytic activity, which was related to the oxygen vacancies created in the crystal growth process.
Yang et al. [126] demonstrated the photocatalytic efficiency of nanoparticulate TiO2 microspheres synthesized by MAH. Figure 18(a) shows the porosity of the microspheres. When the photocatalyst was applied for the decontamination of a mixture of Cr(VI) with methyl orange, higher reduction of Cr(VI) and oxidation of methyl orange was observed (Fig. 18(b–c), curve 4), while for pure Cr(VI) or methyl orange the efficiency was not so enhanced (Fig. 18(b–c), curve 3). In addition to this synergistic effect, the photocatalyst could be easily recovered with its efficiency nearly constant after 3 cycles. Figure 18(d) illustrates the synergistic effect of reduction of Cr(VI) with the simultaneous oxidation of methyl orange.
(a) TEM image of TiO2 showing the pores of the microsphere. Conversion of (b) Cr(VI) and (c) methyl orange. C0 is the original concentration of Cr(VI) or methyl orange, C is concentration of the remaining Cr(VI) or methyl orange at a specific time t. Curve 1– the photocatalyst is absent in the single systems, 2- photocatalyst is absent in the mixed system with Cr(VI) and methyl orange, 3- TiO2 in the Cr(VI) or methyl orange single system, 4- TiO2 in the mixed system of Cr(VI) and methyl orange, respectively. (d) Conversion mechanism of Cr(VI) and methyl orange by TiO2 under UV light irradiation, in which ROSs are the reactive oxidative species. Adapted with permission from Ref. [126], Copyright (2014) American Chemical Society
Cu/Cu2O hollow spheres demonstrated good photocatalytic activity because of their porous structure and heterojunctions. In 30 min, 92.1% of methyl orange was degraded. However, after the third cycle, a decrease of 23% in photocatalytic activity was reported, attributed to the photo-oxidation to CuO [76].
5.5 Solar cells
TiO2 nanorods were grown on substrates of fluorine-doped tin oxide (FTO) by MAH. Figure 19(a) displays the TEM image of a typical TiO2 nanorod. The FTO substrates were coated with a thin layer of TiO2 nanoparticles, which helped to promote heterogeneous nucleation of the nanorods. Following synthesis, the nanorods were also immersed in TiCl4 solution. The results showed an energy conversion efficiency of 1.31% for the TiO2 based dye-sensitized solar cell (DSSC) without TiCl4 treatment, while after treatment with TiCl4 for 2 h, this value increased to 3.7%, probably due to the quantity of dye adsorbed on the coating layer. Furthermore, the diffusion coefficient and carrier life time of DSSCs were analyzed and it was found that increasing the treatment time with TCl4, the diffusion coefficient decreased (Fig. 19(b)), but the lifetime remained unchanged except for the nanorod without treatment (Fig. 19(c)), which was attributed to the backward electron transfer from the FTO to the electrolyte [192].
(a) TEM image of TiO2 nanorod, and the selected area diffraction pattern in the inset. (b) Diffusion coefficient and (c) lifetime of DSSCs for pure nanorods, nanorods treated with TiCl4 for different times, rutile nanoparticles grown from TiCl4 solution. Adapted with permission from Ref. [192], Copyright (2011) American Chemical Society
Liu et al. [193] described synthesis of ZnO nanowire arrays on FTO substrates by MAH from 1 to 5 h. With increasing reaction time, the length of the nanowires increased from 3.4 to 17.2 μm, which led to the increase of internal surface area along the enhancement of the energy conversion efficiency when the ZnO nanowires were used as DSSC photoanodes. The lowest efficiency was obtained (0.35%) for ZnO nanowires with length of 3.4 μm, whereas maximum efficiency of 1.25% was achieved for nanowires of 17.2 μm, due to the highest quantity of dye absorbed on the ZnO as a result of the larger internal surface area of the array.
Four distinct morphologies of SnO2 were obtained by a microwave-assisted solvothermal process by using four bases NaOH, KOH, TMAOH, and NH4OH. All these materials were deposited on a FTO substrate, and then tested as photoanodes for DSSC. The energy conversion efficiency was better than 1.5% for all morphologies of SnO2, except for the one prepared with TMAOH which exhibited the highest efficiency of 3.6%. In addition, the four samples of SnO2 photoanodes showed an enhanced efficiency during the first 15 days, and after which the efficiency remained unchanged for up to 50 days [194].
6 Conclusions and outlook
This work provided an overview of MAH/S discussing microwave heating theory and means for achieving fine crystal morphology control. This method holds great promise for the preparation of different classes of materials such as metals, semiconductors, electroceramics, composites, etc. from solution via nucleation/growth mechanisms for technological applications relevant to advanced gas sensors, batteries, fuel cells, photocatalysts, and solar cells. Some research trends and directions are expected including: (1) achieving a detailed understanding of crystal shape by chemical reaction, nucleation, and growth of inorganic, organic or composite materials in liquid phase under microwave irradiation; (2) clarifying controversial non-thermal or specific microwave effects under microwave irradiation; (3) generating theoretical simulations of microwave effects in crystal morphology modeling; (4) developing sustainable, clean and green microwave-assisted synthesis methods at low cost and high efficiency; and (5) studying the correlations between structure−property relations and new advanced applications of microwave-prepared nanostructured materials. The major challenges of this method are in interfacing ultrasound, spray pyrolysis, in flow systems especially on scaling-up for industrial applications. Progress towards these goals will greatly advance the MAH/S method and accelerate its practical applications.
Furthermore, the key advantages obtained by the application of MAH/S in materials preparation over conventional conductive heating, specifically are improved higher yields, lower processing time, selective dielectric solvents and/or reagents heating, avoiding direct contact with the MW source, enhanced reproducibility, temperature gradient absence and efficient heating distribution throughout the sample. It was shown that it is possible to obtain porous materials with high surface area that helped decisively in improved application in sensors, catalysts, etc. Also, the control of the size-selective growth assists in the selectivity of these materials. In addition, it is possible to obtain various classes of composites with fine composition which are often difficult to obtain by other synthesis routes. However, the benefits of the use of microwaves in MAH/S remain very broad and deserve to be more explored in the future.
References
F. Gao, Q.Y. Lu, S. Komarneni, Chem. Mater. 17, 856 (2005)
J. M. Kong, C. V. Wong, Z. Q. Gao, X. T. Chen, Synth. React. Inorganic, Met. Nano-Metal Chem. (2008), pp. 186–188
A.M.R. Galletti, C. Antonetti, A.M. Venezia, G. Giambastiani, Appl. Catal. A Gen. 386, 124 (2010)
S.-W. Lee, L.M. Lozano-Sánchez, V. Rodríguez-González, J. Hazard. Mater. 263 Pt 1, 20 (2013)
V. Moghimifar, A. Raisi, A. Aroujalian, N.B. Bandpey, Adv. Mater. Res. 829, 846 (2013)
W. Cao, L. Chen, Z. Qi, J. Mol. Catal. A Chem. 401, 81 (2015)
S. Komarneni, Q. Li, K.M. Stefansson, R. Roy, J. Mater. Res. 8, 3176 (1993)
J.P. Cheng, Mater. Res. Innov. 1, 44 (1997)
S.F. Liu, I.R. Abothu, S. Komarneni, Mater. Lett. 38, 344 (1999)
A. Dias, V.S.T. Ciminelli, Chem. Mater. 15, 1344 (2003)
P. Wan, W. Yang, X. Wang, J. Hu, H. Zhang, Sensors Actuators B Chem. 214, 36 (2015)
S. Bai, C. Chen, R. Luo, A. Chen, D. Li, Sensors Actuators B Chem. 216, 113 (2015)
F. Ren, G. Zhu, P. Ren, K. Wang, X. Cui, X. Yan, Appl. Surf. Sci. 351, 40 (2015)
H. Meng, W. Yang, K. Ding, L. Feng, Y. Guan, J. Mater. Chem. A 3, 1174 (2015)
S. Khamlich, T. Mokrani, M.S. Dhlamini, B.M. Mothudi, M. Maaza, J. Colloid Interface Sci. 461, 154 (2016)
S. Komarneni, Y.D. Noh, J.Y. Kim, S.H. Kim, H. Katsuki, Z. Naturforsch. Sect. B. J. Chem. Sci. 65, 1033 (2010)
F.-H. Ko, Y.-C. Hsu, M.-T. Wang, G.S. Huang, Microelectron. Eng. 84, 1300 (2007)
C.D. Madhusoodana, R.N. Das, Y. Kameshima, K. Okada, J. Mater. Sci. 41, 1481 (2006)
J. Wang, G. Du, R. Zeng, B. Niu, Z. Chen, Z. Guo, S. Dou, Electrochim. Acta 55, 4805 (2010)
R.C. Lima, L.R. Macario, J.W.M. Espinosa, V.M. Longo, R. Erlo, N.L. Marana, J.R. Sambrano, M.L. dos Santos, A.P. Moura, P.S. Pizani, J. Andrés, E. Longo, J.A. Varela, J. Phys. Chem. A 112, 8970 (2008)
D. Zhang, G. Li, X. Yang, and J. C. Yu, Chem. Commun. (Camb). 4381 (2009)
M.S. Anwar, S. Kumar, F. Ahmed, G.W. Kim, B.H. Koo, J. Nanosci. Nanotechnol. 12, 5523 (2012)
Z. Zhu, Y. Zhang, Y. Zhang, H. Liu, C. Zhu, Y. Wu, Ceram. Int. 39, 2567 (2013)
D.E. Motaung, G.H. Mhlongo, S.S. Nkosi, G.F. Malgas, B.W. Mwakikunga, E. Coetsee, H.C. Swart, H.M.I. Abdallah, T. Moyo, S.S. Ray, ACS Appl. Mater. Interfaces 6, 8981 (2014)
M. Poupon, N. Barrier, S. Petit, S. Clevers, V. Dupray, Inorg. Chem. 54, 5660 (2015)
L. Wang, Y. Huang, X. Sun, H. Huang, P. Liu, M. Zong, Y. Wang, Nanoscale 6, 3157 (2014)
S.H. Jhung, J.-H. Lee, P.M. Forster, G. Férey, A.K. Cheetham, J.-S. Chang, Chemistry 12, 7899 (2006)
W. Liu, L. Ye, X. Liu, L. Yuan, X. Lu, J. Jiang, Inorg. Chem. Commun. 11, 1250 (2008)
Y. Tian, B. Chen, R. Hua, N. Yu, B. Liu, J. Sun, L. Cheng, H. Zhong, X. Li, J. Zhang, B. Tian, H. Zhong, CrystEngComm 14, 1760 (2012)
J. Huang, C. Xia, L. Cao, X. Zeng, Mater. Sci. Eng. B 150, 187 (2008)
J. Ji, L.L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang, R.S. Ruoff, ACS Nano 7, 6237 (2013)
L. Xu, Y.-S. Ding, C.-H. Chen, L. Zhao, C. Rimkus, R. Joesten, S.L. Suib, Chem. Mater. 20, 308 (2008)
Y. Liu, L. Xia, Y. Lu, S. Dai, M. Takeguchi, H. Hong, Z. Pan, J. Colloid Interface Sci. 381, 24 (2012)
C. Yang, F. Xiao, J. Wang, X. Su, Sensors Actuators B Chem. 207, 177 (2015)
C.-Y. Wu, D.S. Raja, C.-C. Yang, C.-T. Yeh, Y.-R. Chen, C.-Y. Li, B.-T. Ko, C.-H. Lin, CrystEngComm 16, 9308 (2014)
M. Zhou, Y. Hu, Y. Liu, W. Yang, H. Qian, CrystEngComm 14, 7686 (2012)
J.K. Vaishnav, S.S. Arbuj, S.B. Rane, D.P. Amalnerkar, RSC Adv. 4, 47637 (2014)
T. Cetinkaya, U. Tocoglu, M. Uysal, M.O. Guler, H. Akbulut, Microelectron. Eng. 126, 54 (2014)
Z. Cui, Y. Zhang, S. Li, S. Ge, Catal. Commun. 72, 97 (2015)
O. Mendiuk, M. Nawrocki, L. Kepinski, Ceram. Int. 42, 1998 (2016)
R. Adhikari, G. Gyawali, T.H. Kim, T. Sekino, S.W. Lee, Mater. Lett. 91, 294 (2013)
M.L. Moreira, G.P. Mambrini, D.P. Volanti, E.R. Leite, M.O. Orlandi, P.S. Pizani, V.R. Mastelaro, C.O. Paiva-Santos, E. Longo, J.A. Varela, Chem. Mater. 20, 5381 (2008)
A. Rizzuti, M. Dassisti, P. Mastrorilli, M.C. Sportelli, N. Cioffi, R.A. Picca, E. Agostinelli, G. Varvaro, R. Caliandro, J. Nanopart. Res. 17, 408 (2015)
K. Byrappa, T. Ohachi, Crystal Growth Technology (William Andrew Pub. ;Springer, Norwich, N.Y. Berlin; New York, 2003)
K. Byrappa, M. Yoshimura, Handbook of Hydrothermal Technology: A Technology for Crystal Growth and Materials Processing, 1st ed (Noyes Publications, Park Ridge, 2001)
M. Yoshimura, K. Byrappa, J. Mater. Sci. 43, 2085 (2008)
K. Byrappa, T. Adschiri, Prog. Cryst. Growth Charact. Mater. 53, 117 (2007)
S. Diodati, P. Dolcet, M. Casarin, S. Gross, Chem. Rev. 115(20), 11449 (2015)
I. Bilecka, M. Niederberger, Nanoscale 2, 1358 (2010)
R.S. Varma, Green Chem. 1, 43 (1999)
V. Polshettiwar, R.S. Varma, Acc. Chem. Res. 41, 629 (2008)
V. Polshettiwar, M.N. Nadagouda, R.S. Varma, Aust. J. Chem. 62, 16 (2009)
M.N. Nadagouda, R.S. Varma, Cryst. Growth Des. 8, 291 (2008)
V. Polshettiwar, R.S. Varma, Green Chem. 12, 743 (2010)
V. Polshettiwar, R.S. Varma, Chem. Soc. Rev. 37, 1546 (2008)
V. Polshettiwar, R.S. Varma, Curr. Opin. Drug Discov. Dev. 10, 723 (2007)
B. Baruwati, V. Polshettiwar, R.S. Varma, Green Chem. 11, 926 (2009)
C. Gabriel, S. Gabriel, E.H. Grant, B.S.J. Halstead, D.M.P. Mingos, Chem. Soc. Rev. 27, 213 (1998)
D.M.P. Mingos, D.R. Baghurst, Chem. Soc. Rev. 20, 1 (1991)
B. L. Hayes, Microwave Synthesis: Chemistry at the Speed of Light (CEM Publishing, 2002)
M.B. Gawande, S.N. Shelke, R. Zboril, R.S. Varma, Acc. Chem. Res. 47, 1338 (2014)
J.-S. Schanche, Mol. Divers. 7, 291 (2003)
A. de la Hoz, A. Diaz-Ortiz, A. Moreno, Chem. Soc. Rev. 34, 164 (2005)
K. Huang, X. Yang, W. Hua, G. Jia, L. Yang, New J. Chem. 33, 1486 (2009)
C. Antonio, R.T. Deam, Phys. Chem. Chem. Phys. 9, 2976 (2007)
J. Robinson, S. Kingman, D. Irvine, P. Licence, A. Smith, G. Dimitrakis, D. Obermayer, C.O. Kappe, Phys. Chem. Chem. Phys. 12, 4750 (2010)
M.R. Rosana, Y. Tao, A.E. Stiegman, G.B. Dudley, Chem. Sci. 3, 1240 (2012)
J.-Y. Li, S. Xiong, J. Pan, Y. Qian, J. Phys. Chem. C (2010)
D.P. Volanti, M.O. Orlandi, J. Andres, E. Longo, CrystEngComm 12, 1696 (2010)
L. Qin, J. Xu, X. Dong, Q. Pan, Z. Cheng, Q. Xiang, F. Li, Nanotechnology 19, 185705 (2008)
A. Birkel, F. Reuter, D. Koll, S. Frank, R. Branscheid, M. Panthofer, E. Rentschler, W. Tremel, CrystEngComm 13, 2487 (2011)
S. Xuan, Y.-X.J. Wang, J.C. Yu, K. Cham-Fai Leung, Chem. Mater. 21, 5079 (2009)
H. Zhang, J. Feng, T. Fei, S. Liu, T. Zhang, Sensors Actuators B Chem. 190, 472 (2014)
G. Neri, S.G. Leonardi, M. Latino, N. Donato, S. Baek, D.E. Conte, P.A. Russo, N. Pinna, Sensors Actuators B Chem. 179, 61 (2013)
Z. Ai, L. Zhang, S. Lee, W. Ho, J. Phys. Chem. C 113, 20896 (2009)
X. Zou, H. Fan, Y. Tian, M. Zhang, X. Yan, Dalton Trans. 44, 7811 (2015)
P. Rai, W.-K. Kwak, Y.-T. Yu, ACS Appl. Mater. Interfaces 5, 3026 (2013)
R. Krishnapriya, S. Praneetha, A. Vadivel Murugan, CrystEngComm 17, 8353 (2015)
J. Chen, H. Bin Yang, J. Miao, H.-Y. Wang, B. Liu, J. Am. Chem. Soc. 136, 15310 (2014)
K. Manseki, Y. Kondo, T. Ban, T. Sugiura, T. Yoshida, Dalton Trans. 42, 3295 (2013)
G. Wulff, Z. Kryst. Mineral 34, 449 (1901)
H. Zhang, M. Jin, Y. Xiong, B. Lim, Y. Xia, Acc. Chem. Res. 46, 1783 (2013)
J. Andrés, L. Gracia, A.F. Gouveia, M.M. Ferrer, E. Longo, Nanotechnology 26, 405703 (2015)
H. Cölfen, Mesocrystals and Nonclassical Crystallization, 1st ed (John Wiley & Sons Ltd, Chichester, 2008)
J.W. Mullin, Crystallization, 4th ed (Butterworth-Heinemann, Oxford, 2001)
H. Cölfen, S. Mann, Angew. Chemie Int. Ed. 42, 2350 (2003)
W. Ostwald, Z. Phys. Chem. Stochiometrie Verwandtschaftslehre 34 (1900)
Z. Wu, S. Yang, W. Wu, Nanoscale 8, 1237 (2016)
W. Chen, H. Ruan, Y. Hu, D. Li, Z. Chen, J. Xian, J. Chen, X. Fu, Y. Shao, Y. Zheng, CrystEngComm 14, 6295 (2012)
Z. Kozakova, I. Kuritka, N.E. Kazantseva, V. Babayan, M. Pastorek, M. Machovsky, P. Bazant, P. Saha, Dalton Trans. 44, 21099 (2015)
A.P. Moura, L.S. Cavalcante, J.C. Sczancoski, D.G. Stroppa, E.C. Paris, A.J. Ramirez, J.A. Varela, E. Longo, Adv. Powder Technol. 21, 197 (2010)
G.J. Wilson, A.S. Matijasevich, D.R.G. Mitchell, J.C. Schulz, G.D. Will, Langmuir 22, 2016 (2006)
R.L. Penn, J.F. Banfield, Science (80-. ) 281, 969 (1998)
M. Niederberger, H. Cölfen, Phys. Chem. Chem. Phys. 8, 3271 (2006)
N.T.K. Thanh, N. Maclean, S. Mahiddine, Chem. Rev. 114, 7610 (2014)
R.L. Penn, J.A. Soltis, CrystEngComm 16, 1409 (2014)
M. Distaso, M. Mačković, E. Spiecker, W. Peukert, Chemistry 20, 8199 (2014)
S.-W. Cao, Y.-J. Zhu, Nanoscale Res. Lett. 6, 1 (2010)
C.-Y. Cao, Z.-M. Cui, C.-Q. Chen, W.-G. Song, W. Cai, J. Phys. Chem. C 114, 9865 (2010)
S. Vijayakumar, S. Nagamuthu, G. Muralidharan, ACS Appl. Mater. Interfaces 5, 2188 (2013)
T.M. Perfecto, C.A. de Zito, D.P. Volanti, RSC Adv. 6, 105171 (2016)
S. Komarneni, R. Roy, Q.H. Li, Mater. Res. Bull. 27, 1393 (1992)
M. Baghbanzadeh, L. Carbone, P.D. Cozzoli, C.O. Kappe, Angew. Chem. Int. Ed. 50, 11312 (2011)
C.O. Kappe, D. Dallinger, Nat. Rev. Drug Discov. 5, 51 (2006)
C.O. Kappe, Chem. Soc. Rev. 37, 1127 (2008)
D. Dallinger, C.O. Kappe, Chem. Rev. 107, 2563 (2007)
S. Komarneni, D.S. Li, B. Newalkar, H. Katsuki, A.S. Bhalla, Langmuir 18, 5959 (2002)
O.V. Belousov, N.V. Belousova, A.V. Sirotina, L.A. Solovyov, A.M. Zhyzhaev, S.M. Zharkov, Y.L. Mikhlin, Langmuir 27, 11697 (2011)
H.L. Nguyen, L.E.M. Howard, S.R. Giblin, B.K. Tanner, I. Terry, A.K. Hughes, I.M. Ross, A. Serres, H. Bürckstümmer, J.S.O. Evans, J. Mater. Chem. 15, 5136 (2005)
P.N. Njoki, L.V. Solomon, W. Wu, R. Alam, M.M. Maye, Chem. Commun. (Camb.) 47, 10079 (2011)
W. Wu, P.N. Njoki, H. Han, H. Zhao, E.A. Schiff, P.S. Lutz, L. Solomon, S. Matthews, M.M. Maye, J. Phys. Chem. C 115, 9933 (2011)
Y. Wang, J. Tian, C. Fei, L. Lv, X. Liu, Z. Zhao, G. Cao, J. Phys. Chem. C 118, 25931 (2014)
Z. Wang, X. Zhou, Z. Li, Y. Zhuo, Y. Gao, Q. Yang, X. Li, G. Lu, RSC Adv. 4, 23281 (2014)
A. Pimentel, D. Nunes, P. Duarte, J. Rodrigues, F.M. Costa, T. Monteiro, R. Martins, E. Fortunato, J. Phys. Chem. C 118, 14629 (2014)
X. Li, S. Yao, J. Liu, P. Sun, Y. Sun, Y. Gao, G. Lu, Sensors Actuators B Chem. 220, 68 (2015)
K. Manseki, T. Sugiura, T. Yoshida, New J. Chem. 38, 598 (2014)
A. Phuruangrat, D.J. Ham, S.J. Hong, S. Thongtem, J.S. Lee, J. Mater. Chem. 20, 1683 (2010)
J. Sungpanich, T. Thongtem, S. Thongtem, Ceram. Int. 38, 1051 (2012)
T.M. Perfecto, C.A. Zito, D.P. Volanti, CrystEngComm 19, 2733 (2017)
P. Rai, H.-M. Song, Y.-S. Kim, M.-K. Song, P.-R. Oh, J.-M. Yoon, Y.-T. Yu, Mater. Lett. 68, 90 (2012)
R.A. Silva, M.O. Orlandi, J. Nanomater., 4054058 (2016)
F.V. Motta, R.C. Lima, A.P.A. Marques, E.R. Leite, J.A. Varela, E. Longo, Mater. Res. Bull. 45, 1703 (2010)
K. Chen, Y. Dong Noh, W. Huang, J. Ma, S. Komarneni, D. Xue, Ceram. Int. 40, 2877 (2014)
M. Song, P. Rai, K.-J. Ko, S.-H. Jeon, B.-S. Chon, C.-H. Lee, Y.-T. Yu, RSC Adv. 4, 3529 (2014)
P.-S. Shen, Y.-C. Tai, P. Chen, Y.-C. Wu, J. Power Sources 247, 444 (2014)
S. Yoon, E.-S. Lee, A. Manthiram, Inorg. Chem. 51, 3505 (2012)
Y. Yang, G. Wang, Q. Deng, D.H.L. Ng, H. Zhao, ACS Appl. Mater. Interfaces 6, 3008 (2014)
K.F. Moura, J. Maul, A.R. Albuquerque, G.P. Casali, E. Longo, D. Keyson, A.G. Souza, J.R. Sambrano, I.M.G. Santos, J. Solid State Chem. 210, 171 (2014)
A.A. Al-Ghamdi, F. Al-Hazmi, O.A. Al-Hartomy, F. El-Tantawy, F. Yakuphanoglu, J. Sol-Gel Sci. Technol. 63, 187 (2012)
V. Polshettiwar, B. Baruwati, R.S. Varma, ACS Nano 3, 728 (2009)
Z. Moorhead-Rosenberg, K.L. Harrison, T. Turner, A. Manthiram, Inorg. Chem. 52, 13087 (2013)
S. Ghosh, P. Kar, N. Bhandary, S. Basu, S. Sardar, T. Maiyalagan, D. Majumdar, S. K. Bhattacharya, A. Bhaumik, P. Lemmens, S. K. Pal, Catal. Sci. Technol. (2016)
J. Sodtipinta, H.-K. Kim, S.-W. Lee, S.M. Smith, P. Pakawatpanurut, K.-B. Kim, J. Electroceram. 35, 111 (2015)
G. Anandha Babu, G. Ravi, T. Mahalingam, M. Kumaresavanji, Y. Hayakawa, Dalton Trans. 44, 4485 (2015)
T.A. Mulinari, F.A. La Porta, J. Andrés, M. Cilense, J.A. Varela, E. Longo, CrystEngComm 15, 7443 (2013)
S. Schmidt, E.T. Kubaski, D.P. Volanti, T. Sequinel, V.D.N. Bezzon, A. Beltrán, S.M. Tebcherani, J.A. Varela, Inorg. Chem. (2015)
G. Qiu, S. Dharmarathna, Y. Zhang, N. Opembe, H. Huang, S.L. Suib, J. Phys. Chem. C 116, 468 (2012)
D.P. Volanti, A.G. Sato, M.O. Orlandi, J.M.C. Bueno, E. Longo, J. Andres, ChemCatChem 3, 839 (2011)
C. Sun, X. Su, F. Xiao, C. Niu, J. Wang, Sensors Actuators B Chem. 157, 681 (2011)
S. Chen, Y. Zhao, B. Sun, Z. Ao, X. Xie, Y. Wei, G. Wang, ACS Appl. Mater. Interfaces 7, 3306 (2015)
X. Liu, L. Pan, T. Lv, Z. Sun, C.Q. Sun, J. Colloid Interface Sci. 408, 145 (2013)
A.E. Souza, S.R. Teixeira, C.M. -Santos, W.H. Schreiner, P.N. Lisboa Filho, E. Longo, J. Mater. Chem. C 2, 7056 (2014)
I. Velasco-Davalos, F. Ambriz-Vargas, G. Kolhatkar, R. Thomas, A. Ruediger, AIP Adv. 6, 65117 (2016)
G. Kolhatkar, F. Ambriz-Vargas, R. Thomas, A. Ruediger, Cryst. Growth Des. 17, 5697 (2017)
V. Swaminathan, S.S. Pramana, T.J. White, L. Chen, R. Chukka, R.V. Ramanujan, ACS Appl. Mater. Interfaces 2, 3037 (2010)
L.M. Lozano-Sánchez, S.-W. Lee, T. Sekino, V. Rodríguez-González, CrystEngComm 15, 2359 (2013)
T.M. Mazzo, G.S. Do Nascimento Libanori, M.L. Moreira, W. Avansi, V.R. Mastelaro, J.A. Varela, E. Longo, J. Lumin. 165, 130 (2015)
M.L. Moreira, J. Andrés, V.R. Mastelaro, J.A. Varela, E. Longo, CrystEngComm 13, 5818 (2011)
K. De Keukeleere, J. Feys, M. Meire, J. De Roo, K. De Buysser, P. Lommens, I. Van Driessche, J. Nanopart. Res. 15, 2074 (2013)
I. Janowska, K. Chizari, O. Ersen, S. Zafeiratos, D. Soubane, V. da Costa, V. Speisser, C. Boeglin, M. Houllé, D. Begin, D. Plee, M.J. Ledoux, C. Pham-Huu, Nano Res. 3, 126 (2010)
S. Vadahanambi, J.H. Jung, I.K. Oh, Carbon N. Y. 49, 4449 (2011)
H. Hu, Z. Zhao, Q. Zhou, Y. Gogotsi, J. Qiu, Carbon N. Y. 50, 3267 (2012)
J. Long, M. Fang, G. Chen, J. Mater. Chem. 21, 10421 (2011)
M. M. Viana, M. C. F. S. Lima, J. C. Forsythe, M. Cho, Y. Cheng, G. G. Silva, and M. S. Wong, 26, 978 (2015)
S.J.A. Moniz, J. Tang, ChemCatChem 7, 1595 (2015)
J. Geng, G.-H. Song, X.-D. Jia, F.-F. Cheng, J.-J. Zhu, J. Phys. Chem. C 116, 4517 (2012)
W. Yang, P. Wan, X. Zhou, J. Hu, Y. Guan, L. Feng, ACS Appl. Mater. Interfaces 6, 21093 (2014)
J. Huang, G. Tan, H. Ren, W. Yang, C. Xu, C. Zhao, A. Xia, ACS Appl. Mater. Interfaces 6, 21041 (2014)
P. Rai, S.M. Majhi, Y.-T. Yu, J.-H. Lee, RSC Adv. 5, 17653 (2015)
N. Garino, A. Sacco, M. Castellino, J.A. Muñoz, A. Chiodoni, V. Agostino, V. Margaria, M. Gerosa, G. Massaglia, M. Quaglio, ACS Appl. Mater. Interfaces (2016)
D. Wang, X. Li, J. Wang, J. Yang, D. Geng, R. Li, M. Cai, T.-K. Sham, X. Sun, J. Phys. Chem. C 116, 22149 (2012)
C. Zhong, J. Wang, Z. Chen, H. Liu, J. Phys. Chem. C 115, 25115 (2011)
L. Li, K.H. Seng, H. Liu, I.P. Nevirkovets, Z. Guo, Electrochim. Acta 87, 801 (2013)
C.-L. Liu, K.-H. Chang, C.-C. Hu, W.-C. Wen, J. Power Sources 217, 184 (2012)
Q. Xiang, J. Yu, M. Jaroniec, Nano 3, 3670 (2011)
L.Q. Lu, Y. Wang, J. Mater. Chem. 21, 17916 (2011)
W. Zhou, F. Zhang, S. Liu, J. Wang, X. Du, D. Yin, L. Wang, RSC Adv. 4, 51362 (2014)
Z. Wang, Y. Xiao, X. Cui, P. Cheng, B. Wang, Y. Gao, X. Li, T. Yang, T. Zhang, G. Lu, ACS Appl. Mater. Interfaces 6, 3888 (2014)
S.-H. Park, H.-K. Kim, K.C. Roh, K.-B. Kim, Electron. Mater. Lett. 11, 282 (2015)
Y. Zou, J. Kan, Y. Wang, J. Phys. Chem. C 115, 20747 (2011)
L. Kashinath, K. Namratha, K. Byrappa, Appl. Surf. Sci. 357, 1849 (2015)
R. Sharma, F. Alam, A.K. Sharma, V. Dutta, S.K. Dhawan, J. Mater. Chem. C 2, 8142 (2014)
Y. Gui, Z. Liu, S. Fang, J. Tian, F. Gong, J. Mater. Sci. Mater. Electron. (2015)
Y. Gui, J. Zhao, W. Wang, J. Tian, M. Zhao, Mater. Lett. 155, 4 (2015)
C.A. Zito, T.M. Perfecto, D.P. Volanti, Sensors Actuators B Chem. 244, 466 (2017)
M. Chen, Z. Wang, D. Han, F. Gu, G. Guo, J. Phys. Chem. C 115, 12763 (2011)
Z. Wang, P. Sun, T. Yang, Y. Gao, X. Li, G. Lu, Y. Du, Sensors Actuators B Chem. 186, 734 (2013)
Q. Wang, C. Wang, H. Sun, P. Sun, Y. Wang, J. Lin, G. Lu, Sensors Actuators B Chem. 222, 257 (2016)
Y.-S. Kim, P. Rai, Y.-T. Yu, Sensors Actuators B Chem. 186, 633 (2013)
T. Yanagimoto, Y.-T. Yu, K. Kaneko, Sens. Actuators B 166–167, 31 (2012)
L.M. Sikhwivhilu, S. Mpelane, B.W. Mwakikunga, S. Sinha Ray, ACS Appl. Mater. Interfaces 4, 1656 (2012)
R.D. Martínez-Orozco, R. Antaño-López, V. Rodríguez-González, New J. Chem. 39, 8044 (2015)
P. Sun, C. Wang, J. Liu, X. Zhou, X. Li, X. Hu, G. Lu, ACS Appl. Mater. Interfaces 7, 19119 (2015)
C. Yang, X. Su, J. Wang, X. Cao, S. Wang, L. Zhang, Sensors Actuators B Chem. 185, 159 (2013)
C. Yang, X. Su, F. Xiao, J. Jian, J. Wang, Sensors Actuators B Chem. 158, 299 (2011)
C.A. Zito, T.M. Perfecto, D.P. Volanti, Adv. Mater. Interfaces 4, 1700847 (2017)
D.P. Volanti, A.A. Felix, M.O. Orlandi, G. Whitfield, D.-J. Yang, E. Longo, H.L. Tuller, J.A. Varela, Adv. Funct. Mater. 23, 1759 (2013)
S. Kong, R. Dai, H. Li, W. Sun, Y. Wang, ACS Sustain. Chem. Eng. 3, 1830 (2015)
W.X. Chen, J.Y. Lee, Z.L. Liu, Chem. Commun. 2588 (2002)
S. Ghosh, P. Kar, N. Bhandary, S. Basu, S. Sardar, T. Maiyalagan, D. Majumdar, S. K. Bhattacharya, A. Bhaumik, P. Lemmens, S. K. Pal, Cat. Sci. Technol. (2016)
G. Byzynski, A.P. Pereira, D.P. Volanti, C. Ribeiro, E. Longo, J. Photochem. Photobiol. A Chem. 353, 358 (2018)
M. Yang, B. Ding, S. Lee, J.-K. Lee, J. Phys. Chem. C 115, 14534 (2011)
L. Liu, K. Hong, X. Ge, D. Liu, M. Xu, J. Phys. Chem. C 118, 15551 (2014)
A. Birkel, Y.-G. Lee, D. Koll, X. Van Meerbeek, S. Frank, M.J. Choi, Y.S. Kang, K. Char, W. Tremel, Energy Environ. Sci. 5, 5392 (2012)
Acknowledgements
The authors acknowledge São Paulo Research Foundation (FAPESP), grants #2015/50526-4, #2015/05916-9 and #2014/17343-0; National Council for Scientific and Technological Development – CNPQ, grants #447760/2014-9, #444926/2014-3 and #443138/2016-8, and the Coordination for the Improvement of Higher Education Personnel – CAPES.
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Zito, C.A., Orlandi, M.O. & Volanti, D.P. Accelerated microwave-assisted hydrothermal/solvothermal processing: Fundamentals, morphologies, and applications. J Electroceram 40, 271–292 (2018). https://doi.org/10.1007/s10832-018-0128-z
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DOI: https://doi.org/10.1007/s10832-018-0128-z