Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 www.elsevier.com/locate/pcrysgrow

Hydrothermal technology for nanotechnology K. Byrappa a,*, T. Adschiri b b

a University of Mysore, DOS in Geology, P.B. 21, Manasagangotri P.O., Mysore-570 006, India Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980 8577, Japan

Abstract The importance of hydrothermal technology in the preparation of nanomaterials has been discussed in detail with reference to the processing of advanced materials for nanotechnology. Hydrothermal technology in the 21st century is not just confined to the crystal growth or leaching of metals, but it is going to take a very broad shape covering several interdisciplinary branches of science. The role of supercritical water and supercritical fluids has been discussed with appropriate examples. The physical chemistry of hydrothermal processing of advanced materials and the instrumentation used in their preparation with respect to nanomaterials have been discussed. The synthesis of monodispersed nanoparticles of various metal oxides, metal sulphides, carbon nanoforms (including the carbon nanotubes), biomaterials, and some selected composites has been discussed. Recycling, waste treatment and alteration under hydrothermal supercritical conditions have been highlighted. The authors have discussed the perspectives of hydrothermal technology for the processing of advanced nanomaterials and composites. Ó 2007 Elsevier Ltd. All rights reserved. PACS: 82.Rx; 61.46.þw; 81.40.z; 81.10.Dn; 82.60.Lf; 35.Rh Keywords: A1. Nanostructures; A1. Morphology control; A2. Hydrothermal technology; A2. Solvothermal; A2. Supercritical fluid technology; A2. Nanoparticles fabrication

1. Introduction The hydrothermal technique is becoming one of the most important tools for advanced materials processing, particularly owing to its advantages in the processing of nanostructural * Corresponding author. Tel.: þ91 821 2419720; fax: þ91 821 2515346. E-mail addresses: [email protected] (K. Byrappa), [email protected] (T. Adschiri). 0960-8974/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pcrysgrow.2007.04.001

118

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

materials for a wide variety of technological applications such as electronics, optoelectronics, catalysis, ceramics, magnetic data storage, biomedical, biophotonics, etc. The hydrothermal technique not only helps in processing monodispersed and highly homogeneous nanoparticles, but also acts as one of the most attractive techniques for processing nano-hybrid and nanocomposite materials. The term ‘hydrothermal’ is purely of geological origin. It was first used by the British geologist, Sir Roderick Murchison (1792e1871) to describe the action of water at elevated temperature and pressure, in bringing about changes in the earth’s crust leading to the formation of various rocks and minerals. It is well known that the largest single crystal formed in nature (beryl crystal of >1000 g) and some of the large quantity of single crystals created by man in one experimental run (quartz crystals of several 1000s of g) are both of hydrothermal origin. Hydrothermal processing can be defined as any heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions to dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions. Definition for the word hydrothermal has undergone several changes from the original Greek meaning of the words ‘hydros’ meaning water and ‘thermos’ meaning heat. Recently, Byrappa and Yoshimura define hydrothermal as any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non-aqueous) above the room temperature and at pressure greater than 1 atm in a closed system [1]. However, there is still some confusion with regard to the very usage of the term hydrothermal. For example, chemists prefer to use a term, viz. solvothermal, meaning any chemical reaction in the presence of a non-aqueous solvent or solvent in supercritical or near supercritical conditions. Similarly there are several other terms like glycothermal, alcothermal, ammonothermal, and so on. Further, the chemists working in the supercritical region dealing with the materials synthesis, extraction, degradation, treatment, alteration, phase equilibria study, etc., prefer to use the term supercritical fluid technology. However, if we look into the history of hydrothermal research, the supercritical fluids were used to synthesize a variety of crystals and mineral species in the late 19th century and the early 20th century itself [1]. So, a majority of researchers now firmly believe that supercritical fluid technology is nothing but an extension of the hydrothermal technique. Hence, here the authors use only the term hydrothermal throughout the text to describe all the heterogeneous chemical reactions taking place in a closed system in the presence of a solvent, whether it is aqueous or non-aqueous. The term advanced material is referred to a chemical substance whether organic or inorganic or mixed in composition possessing desired physical and chemical properties. In the current context the term materials processing is used in a very broad sense to cover all sets of technologies and processes for a wide range of industrial sectors. Obviously, it refers to the preparation of materials with a desired application potential. Among various technologies available today in advanced materials processing, the hydrothermal technique occupies a unique place owing to its advantages over conventional technologies. It covers processes like hydrothermal synthesis, hydrothermal crystal growth leading to the preparation of fine to ultra fine crystals, bulk single crystals, hydrothermal transformation, hydrothermal sintering, hydrothermal decomposition, hydrothermal stabilization of structures, hydrothermal dehydration, hydrothermal extraction, hydrothermal treatment, hydrothermal phase equilibria, hydrothermal electrochemical reactions, hydrothermal recycling, hydrothermal microwave supported reactions, hydrothermal mechanochemical, hydrothermal sonochemical, hydrothermal electrochemical processes, hydrothermal fabrication, hot pressing, hydrothermal metal reduction, hydrothermal leaching, hydrothermal corrosion, and so on. The hydrothermal processing of advanced materials has

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

119

NanoTechnology

lots of advantages and can be used to give high product purity and homogeneity, crystal symmetry, metastable compounds with unique properties, narrow particle size distributions, a lower sintering temperature, a wide range of chemical compositions, single-step processes, dense sintered powders, sub-micron to nanoparticles with a narrow size distribution using simple equipment, lower energy requirements, fast reaction times, lowest residence time, as well as for the growth of crystals with polymorphic modifications, the growth of crystals with low to ultra low solubility, and a host of other applications. In the 21st century, hydrothermal technology, on the whole, will not be just limited to the crystal growth, or leaching of metals, but it is going to take a very broad shape covering several interdisciplinary branches of science. Therefore, it has to be viewed from a different perspective. Further, the growing interest in enhancing the hydrothermal reaction kinetics using microwave, ultrasonic, mechanical, and electrochemical reactions will be distinct [2]. Also, the duration of experiments is being reduced at least by 3e4 orders of magnitude, which will in turn, make the technique more economic. With an ever-increasing demand for composite nanostructures, the hydrothermal technique offers a unique method for coating of various compounds on metals, polymers and ceramics as well as for the fabrication of powders or bulk ceramic bodies. It has now emerged as a frontline technology for the processing of advanced materials for nanotechnology. On the whole, hydrothermal technology in the 21st century has altogether offered a new perspective which is illustrated in Fig. 1. It links all the important technologies like geotechnology, biotechnology, nanotechnology and advanced materials technology. Thus it is clear that the hydrothermal processing of advanced materials is a highly interdisciplinary subject and the technique is popularly used by physicists, chemists, ceramists, hydrometallurgists, materials scientists, engineers, biologists, geologists, technologists, and

Advanced Materials Technology

Hydrothermal Bio-Technology

Geo-Technology

Technology

Fig. 1. Hydrothermal technology in the 21st century.

120

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

Fig. 2. Hydrothermal tree showing different branches of science and technology.

so on. Fig. 2 shows various branches of science either emerging from the hydrothermal technique or closely linked with the hydrothermal technique. One could firmly say that this family tree will keep expanding its branches and roots in the years to come. The hydrothermal processing of materials is a part of solution processing and it can be described as super heated aqueous solution processing. Fig. 3 shows the PT map of various materials processing techniques [3]. According to this, the hydrothermal processing of advanced materials can be considered as environmentally benign. Besides, for processing nanomaterials, the hydrothermal technique offers special advantages because of the highly controlled diffusivity in a strong solvent media in a closed system. Nanomaterials require control over their physico-chemical characteristics, if they are to be used as functional materials. As the size is reduced to the nanometer range, the materials exhibit peculiar and interesting mechanical and physical properties: increased mechanical strength, enhanced diffusivity, higher specific heat and electrical resistivity compared to their conventional coarse grained counter-parts due to a quantization effect [4]. Hydrothermal technology as mentioned earlier in a strict sense also covers supercritical water or supercritical fluid technology, which is gaining momentum in the last 1½ decades owing to its enormous advantages in the yield and speed of production of nanoparticles and also in the disintegration, transformation, recycling and treatment of various substances including toxic organics, wastes, etc. In case of supercritical water technology, water is used as the solvent in the

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

121

Fig. 3. Pressure temperature map of materials processing techniques [3].

system, whereas supercritical fluid technology is a general term when solvents like CO2 and several other organic solvents are used, and because these solvents have lower critical temperature and pressure compared to water this greatly helps in processing the materials at much lower temperature and pressure conditions. Hence, chemists use the term green chemistry for materials processing using supercritical fluid technology. K. Arai, T. Adschiri, M. Goto (all from Japan) and V.J. Krukonis, J. Watkins, P. Savage, T. Brill (USA), M. Poliakoff (UK), M. Perrut, F. Cansell (France), Buxing Han (China), K.P. Yoo and Y.W. Lee (South Korea), etc., have done extensive studies in the area of supercritical fluid technology. Supercritical water (SCW) and supercritical fluids (SCF) provide an excellent reaction medium for hydrothermal processing of nanoparticles, since they allow varying the reaction rate and equilibrium by shifting the dielectric constant and solvent density with respect to pressure and temperature, thus giving higher reaction rates and smaller particles. The reaction products are to be stable in SCF leading to fine particle formation. The hydrothermal technique is ideal for the processing of very fine powders having high purity, controlled stoichiometry, high quality, narrow particle size distribution, controlled morphology, uniformity, less defects, dense particles, high crystallinity, excellent reproducibility, controlled microstructure, high reactivity with ease of sintering and so on. Further, the technique facilitates issues like energy saving, the use of larger volume equipment, better nucleation control, avoidance of pollution, higher dispersion, higher rates of reaction, better shape control, and lower temperature operations in the presence of the solvent. In nanotechnology, the hydrothermal technique has an edge over other materials processing techniques, since it is an ideal one for the processing of designer particulates. The term designer particulates refers to particles with high purity, high crystallinity, high quality, monodispersed and with controlled physical and chemical characteristics. Today such particles are in great demand in the industry. Fig. 4 shows the major differences in the products obtained by ball

122

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

Fig. 4. Difference in particle processing by hydrothermal and conventional techniques [5].

milling or sintering or firing and by the hydrothermal method [5]. In this respect hydrothermal technology has witnessed a seminal progress in the last decade in processing a great variety of nanomaterials ranging from microelectronics to micro-ceramics and composites. Here the authors discuss the progress made in the area of hydrothermal technology for the past one decade in the processing of advanced nanomaterials. These materials, when put into proper use, will have a profound impact on our economy and society at least in the early part of 21st century, comparable to that of semiconductor technology, information technology or cellular and molecular biology. It is widely speculated that the nanotechnology will lead to the next industrial revolution [6]. Though it is widely believed that commercial nanotechnology is still in its infancy, the rate of technology enablement is increasing in no small part, as substantial government mandated funds have been directed toward nanotechnology [7,8]. It is strongly believed that hydrothermal technology has a great prospect especially with respect to nanotechnology research. 2. History of nanomaterial processing using hydrothermal technology Gold nanoparticles have been around since Roman times. As per the literature data, Michael Faraday was the first scientist to seriously experiment with gold nanoparticles starting in the 1850s. They have recently become the focus of researchers interested in their electrical and optical properties. Similarly, the history of hydrothermal processing of nanomaterials is very interesting. It must have begun in 1845, when Schafthaul prepared fine powders of sub-microscopic

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

123

to nanosize quartz particles using a papin’s digester containing freshly precipitated silicic acid [9]. Majority of the early hydrothermal experiments carried out during the 1840s to the early 1900s mainly dealt with the nanocrystalline products, which were discarded as failures due to the lack of sophisticated electron microscopic techniques available during that time to observe such small sized products. Thus the whole focus was on the processing of bulk crystals or bulk materials. Many times when bulk crystals or single crystals were not obtained as products of several millimeter size the experiments were considered failures and the materials were washed away. Prior to Xray techniques, chemical techniques were mainly employed in identifying the products. It was only after the application of X-rays for crystal studies that the researchers slowly began to study the powder diffraction patterns of the resultant products and by the 1920s a systematic understanding of the products began. Before that the experiments were considered as failures. The experiments were concluded by stating that the solubility was not suitable for growing crystals. Until the works of Giorgio Spezia in 1900, hydrothermal technology did not gain much importance in the growth of bulk crystals, as the products in majority of the cases were very fine grained without any X-ray data [10]. Even the use of seeded growth was initiated by Spezia during that time. Morey [11] quotes in his classical work that the early hydrothermal experimenters used to have horrible experiences since sometimes experiments lasted for 3e6 months without any bearing on petrogenesis and phase equilibria, and ended up with very fine product whose status was not clear. The experiments were simply discarded as failures [11]. Gradually, from the late 1920s to the late 1950s, the products were being analyzed as fine crystalline materials. During this period a great variety of phosphates, silicates, germinates, sulphates, carbonates, oxides, etc., even without natural analogues, were prepared. However, no special significance was attached to such fine crystalline products except for the phase equilibria studies. In fact, the experimental duration was also enhanced in several cases to transform these fine crystalline products into small or bulk single crystals, whenever it was possible. Thus the interest on the growth of bulk crystals was revived during the 1960s and it survived until the 1990s. However, such attempts failed again because of the lack of knowledge on the hydrothermal solution chemistry. It was only during the 1950s and 1960s; some attempts were made to understand the hydrothermal solution chemistry and kinetics of the hydrothermal reactions. It was during the 1970s that some attempts were made to observe the hydrothermal reactions using sapphire windows in the autoclaves. However, owing to the extreme PT conditions these works were not encouraging and the in situ observation of the growth processes was later abandoned. But today, it has become one of the most attractive aspects of hydrothermal research technology. Combination of advanced hydrothermal reactor design with the new sophisticated analytical techniques like Laser Raman, FTIR, synchrotron, HR-SEM, etc. has greatly aided the observation of nucleation and materials processing in situ. With the availability of high resolution SEM from 1980 onwards hydrothermal researchers started observing such fine products which were earlier discarded as failures. The hydrothermal research in the 1990s marks the beginning of the work on the processing of fine to ultra fine particles with a controlled size and morphology. The advanced ceramic materials prepared during that time justify this statement. In the last two decades these sub-micron to nanosized crystalline products have created a revolution in science and technology under a new terminology, ‘Nanotechnology’. Today hydrothermal researchers are able to understand such nanosized materials and control their formation process, which in turn, give the desired properties to such nanomaterials. Thus hydrothermal technology and nanotechnology have a very close link ever since this hydrothermal technology was proposed.

124

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

Table 1 Current trends in hydrothermal technology [5] Compound

Authora

Earlier work 

R:MVO4, R ¼ Nd, Eu, Tm; M ¼ Y, Gd

T ¼ 500e700 C P ¼ 500e1500 bars T ¼ 450  C P ¼ 1000 bars T ¼ 700e900  C P ¼ 2000e3000 bars Melting point >1800  C

LaPO4

Synthesized at >1200  C

Li2B4O7 Li3B5O8(OH)2 NaR(WO4)2, R ¼ La, Ce, Nd

a

T ¼ 240  C P ¼ 200 m2 g1 was obtained. Adschiri and co-workers [48e53] have worked out in detail a continuous synthesis of fine metal oxide particles using supercritical water as the reacting medium. They have shown that fine metal oxide particles are formed when a variety of metal nitrates are contacted with supercritical water in a flow system. They postulated that the fine particles were produced because supercritical water causes the metal hydroxides to rapidly dehydrate before significant growth takes place. The two overall reactions that lead from metal salts to metal oxides are hydrolysis and dehydration: MðNO3 Þ2 þ xH2 O/MðOHÞx þ xHNO3 1 MðOHÞx /MOx=2 þ xH2 O 2 Processing in SCW increases the rate of dehydration such that this step occurs while the particle size is small and the reaction rate is less affected by diffusion through the particle. Furthermore, the gas-like viscosity and diffusivity of water in the critical region lead to a negligible mass transfer limitation. The net effect is that the overall synthesis rate is very large. The high temperature also contributes to the high reaction rate. Several metal oxides including a-Fe2O3, Fe3O4, Co3O4, NiO, ZrO2, CeO2, LiCoO2, a-NiFe2O4, Ce1xZrxO2, etc. have been prepared through this technique. Fig. 18aec shows the nanoparticles prepared by Adschiri and co-workers. Reverchon and Adami have reviewed the preparation of these metal oxide nanoparticles under SCF conditions [29]. 5.3. Hydrothermal processing of TiO2 and ZnO nanoparticles The processing of TiO2 and ZnO nanoparticles occupies a unique place in hydrothermal processing of advanced materials owing to their importance as photocatalysts. There are more than

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

137

Fig. 17. FESEM and TEM photographs of Fe3O4 nanorods (photos: courtesy Prof. Y.T. Qian).

1000 articles dealing with the processing of these materials under hydrothermal and SCW conditions, and their properties. The hydrothermal processing of TiO2 has been carried out by a large number of workers [54e61]. It is the most important material being studied extensively in the last few years owing to its unique properties. TiO2 shows maximum light scattering with virtually no absorption. It is non-toxic and chemically inert. This has been employed extensively in studies of heterogeneous photocatalysis and has been accepted as one of the best photocatalysts for the degradation of environmental contaminants. The process involves the absorption of a photon by TiO2, leading to the promotion of an electron from the valence band to the conduction band and thus producing an electron hole. The electron in the conduction band is then removed by reaction with O2 in the outer system; the hole in the valence band can react with OH or H2O species, which are absorbed on the surface of the TiO2 to give the hydroxyl radical. This hydroxyl radical initiates

138

K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166

the photocatalytic oxidation, a pollution control technology or detoxification technology, which destroys the organic chemical contaminants in air, water, and soil. It can be used to treat polluted water (both surface and ground water, similarly waste and drinking water) and soil. The technique can be used as an industrial pollution management technique for cleaning up gaseous and aqueous waste streams containing organic compounds. The photocatalytic activity of TiO2 depends upon its crystal structure (anatase, or rutile), surface area, size distribution, porosity, and presence of dopants, surface hydroxyl group density, etc. These factors influence directly the production of electronehole pairs, the surface adsorption and desorption process and the redox process. TiO2 is also used as a photoanode in photoelectrochemical solar cells. The hydrothermal method has many advantages e a highly homogeneous crystalline product can be obtained directly at a relatively lower reaction temperature (