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M.H. Wakamatsu*, R. Salomão*
Ceramic Nanoparticles: What Else Do We Have to Know? THE AUTHORS Mitzi Has Wakamatsu has received a Master Degree in Nanosciences and Advanced Materials from the Federal University of the ABC Region (UFABC, Santo André, Brazil). Her PhD project aims an interdisciplinary evaluation on the risks and opportunities associated with Nanosciences & Nanotechnology.
Prof. Dr. Rafael Salomão is a Materi-als Engineer and received his PhD in Materials Science and Engineering from Federal University of São Carlos (Bra-zil) in 2005. Since 2007 he is a lecturer and researcher at the Federal University of the ABC Region (UFABC, Santo André, Brazil). He has authored/co-authored more than 50 technical papers. Prof. Salomão’s research lines include ceramic-polymer nanocomposites, nanoporous ceramics and refractories.
ABSTRACT
KEYWORDS
The development of products and processes containing ceramic nanoparticles has generated novel and fascinating applications of these materials in the past decades. In addition to these exciting findings, ceramic nanoparticles tend to be highly stable. Their routes of synthesis are well known and relatively cheap. The combination of technical advantages and profuse investment in research and development increased the number of patents and publications in this area. Even more recently (since 2002), research programs based on toxicology, eco-toxicology, ethics and public perception of nanotechnologies have pointed out potential risks and impacts associated with nanotechnologies. Because of their wide employment, the ceramic nanoparticles extensively have been studied by means of these new approaches and several unexpected hazardous effects such as high toxicity and environmental persistency were observed. This paper aims to report on a critical review of some ceramic nanoparticles used as raw materials, on their synthesis, properties, applications, potentially dangerous effects as well as on the demand for regulation.
1 Introduction “Nanoparticle” is a general term employed to designate any solid portion of matter in which at least one of its dimensions is smaller than 100 nm. This value is arbitrary and represents the size limit whose properties and characteristics distinct from those observed in a bulk form due to the prfesence of surface effects. These surface effects are related to the facts that firstly the surface of the material presents differences in the manner in which atoms and molecules are bonded together, and secondly in nanoparticles, a large portion of the material behaves as a surface [1–3]. If concepts were not familiar to you, don’t worry. Clear understanding of these effects is one of the aims of this work. It is important to establish a critical view on the development and use of nanoparticles. Thus, these developments and uses will be
* Universidade Federal do ABC (UFABC), Rua da Catequese, 242 09090-400 Santo André, SP–Brazil; Contact:
[email protected],
[email protected]
described in more detail in the following sections. Nanoparticles based products and processes received profuse investments for research and development during the past decade. Despite the difficulties in compiling data from government and private institutions
ceramic nanoparticles, nanotechnologies, innovation, toxicity, environmental degradation. Interceram 59 (2010) [1]
for research and development, the market for nanotechnology based products is estimated to be in the range of 10–20 billion dollars for the year 2010 [4–7]. The use of nanoparticles also is growing as illustrated by the number of worldwide patents registered between 1990 and 2009 (Figure 1; key-
1
Fig. 1 • Evolution of the worldwide number of patents for nanomaterials (for 2009, the result presented is an estimation for the whole year based on the first six months)
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Table 1 • Applications of some ceramic nanoparticles (CNP) [8-13]
Oxide based
Ceramic raw material
Application
Al2O3, Al(OH)3
200–10
Ultra-polishing, mechanical reinforcement in polymeric composites, ceramic binder for refractory applications, catalysis, foams stabilizers
MgO / Mg(OH)2
200–80
Ethanol hydrogenation, anti-flame agent in polymers, carbon nanotubes catalytic support
SiO2, microsilica
300–50
Binder agent for paper fibers and refractory castables, water clarifying, anti-static agent in polymeric films, construction (filler),
TiO2 (rutile), ZnO
100–5
Pigment (white), sun blocker lotion
TiO2 (anatase)
200–10
Semiconductor, catalysis, self-cleaning surfaces, hydrophobic and bactericide coatings
ZrO2, CeO2
100–20
Fuel and hydrogen cells
Clays (montmorillonite and hydrotalcite) Non-oxide based
Particle size / nm
300–100 ⫻ 1–10
Carbon black Expanded graphite Graphene Nanotubes and fullerenes
200–50 300–100 ⫻ 1–50 300–100 ⫻ 0.5 1,000 ⫻ 5, 1
Diamond and SiC
words used for this patent research: nanoparticle, nanorod, nanowire, nanocrystal, nanotube or carbon nanotube). Amongst the several types of nanoparticles, those based on ceramic materials (CNP) have received particularly significant attention. Compared to other classes of nanoparticles, the ceramic nanoparticles tend to be highly stable in comparison to metallic ones. The synthesis routes of ceramic nanoparticles are well known and relatively cheap [3]. In the past decades, the development of products and processes containing ceramic nanoparticles has generated novel applications for these materials. Some examples are presented in Table 1 [8–13]. Even more recently (nearly since 2002, especially in European Community and Japan), research programs based on toxicology, eco-toxicology, ethics and public perception of science and technology expressed concerns regarding the potential benefits that some of the applications of nanoparticles claimed to provide. For instance, benefits discovered under laboratory conditions may not be realized on a commercial scale. Equally, concerns have been observed about the abusive use of the word “nano” in advertising and requesting of research funds. Finally, the huge potential environmental and human health impacts of these materials have been pointed out. Because of their wide employment, carbon nanoparticles have been extensively studied under these new approaches and several unexpected hazardous effects were ob-
500–50
Catalysis, drug delivery, nanocomposites, inks, perforation, barrier agent in polymeric films Pigment (black), reinforcement in rubber composites, inks Pigment (graphite), high temperature lubricant, source of graphene Semiconductor, mechanical reinforcement Conductors, semiconductor, lubricants, mechanical reinforcement, drug delivery, sensors Ultra-polisher, surface coating
served, such as high toxicity and environmental persistency [5, 7, 14]. Despite the fact that similar concerns are recurrent in the history of the technological development, such as those found for nuclear power, computers and genetically modified organisms, the growing use of nanoparticles in general (and ceramic nanoparticles in particular) requires special attention, mainly due two aspects: Firstly, unlike from nuclear or genomic technology, the production of nanoparticles can be carried out using cheap and commercially available reactants, employing well known and extensively published processes and techniques. Secondly, similar rates of development (publications, books, patents, products and processes), due to the profuse government and private investments and due to the computational and new characterization techniques support have never been seen before. In some other fields (mostly related to health care), an open discussion on the potential benefits and risks associated with nanotechnologies already has begun. The present work aims to start this debate amongst the ceramic community presenting a critical review of some ceramic nanoparticles used as raw materials, their synthesis, properties, applications, potentially dangerous effects and the need of regulation. 2 Fundamental concepts on ceramic nanoparticles Initially, it is important to mention that the properties of materials may change (in some
cases, it can be tuned or engineered) when their size oscillates toward values close to 100 nm. This arbitrary value was chosen because in this range the first signs of “surface effects” and the unusual properties encountered in nanoparticles attributed to them appear. These effects are directly related to the small size of these particles, which will be described in details in the following section. 2.1 The surface effects The surface of materials can be understood as its largest and most important defect. It represents a sudden interruption on the regularity of the crystalline arrangement, causing reorganization of the atoms due to a lack of nearest neighbours or smaller coordination numbers. These atoms or molecules present dangling or unsatisfied bonds, and are under inwardly directed forces reducing the interatomic or intermolecular bond energy and distances in comparison to those found between bulk atoms and molecules. The surface effects affect the first dozens of atomic layers beneath the surface, the socalled “surface region”, as schematically shown in Figure 2 [1, 2, 8]. Their consequences for the characteristics of the materials involve higher atomic density, lower activation energy for physical-chemical reactions (melting, vaporization, dissolution, diffusion and oxidation), changes in the surface energy (therefore, in wettability) and distinct thermo-mechanical, electromagnetic and optical properties.
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2
3
Fig. 2 • Schematic representation of the atomic arrangement near to the surface of a crystalline material
Fig. 3 • Relationships employed to calculate the volumetric percent of surface region (⭋Sup) (a-b) and the specific surface area (SSA) (c) as a function of the particle size (considering cubic elements)
For a certain portion of matter, the volumetric percent of surface region ( Sup) can be calculated as shown in Figs. 3a and 3b. Figure 4 depicts the dependence of Sup on the particle size. In a macro-micro-particle, the surface effects are not relevant because the parameter Sup is very small, and other effects such as gravity are more pronounced. However, when size of the particles is reduced to nanoscale (the 100 nm value currently is adopted as a practical approach), the volume occupied by a dozen of atomic layers become relevant and the Sup values become significant (nearly 90 vol.-%, see Figure 4). Nanoparticles combine the high Sup values with another important geometric aspect: their huge specific surface area (SSA). The specific surface area can be defined as the total surface area (usually given in squared meter) that a certain portion of material (mass, grams, or volume, cm3) presents. The specific surface area can be calculated by means of the mean particle size and real density assuming that particles are perfectly spherical or measured by adsorption methods such as BET (designated according [15]). Nanoparticles can present impressive values of 10–1,000 m2/g depending on their size and shape. As most of chemical reactions occur at surfaces, nanoparticles can be much more reactive than a similar mass of material in bulk form [3]. The dependence of the specific surface area on the particle size easily can be demonstrated assuming some considerations: a) Particles surface is smooth and flawless.
b) All particles have the same shape and size. c) They are symmetrical (cubes or spheres) (Figure 3). The smallest specific surface area that a certain amount of material with a certain volume can assume is a single sphere. If this volume V were divided in two smaller spheres, each one with a volume of V/2, the sum of the specific surface area of these two spheres will be larger than the specific surface area of the single sphere. Figure 5 depicts the dependence of the specific surface area on the particle size based on the expressions presented in Fig. 3c. It is important to mention that real ceramic particles usually present flaws such as pores and cracks on their surface. Thus, ceramic particles may be highly asymmetric. Besides the geometric effect of size reduction, these defects can increase the specific surface area even more. 2.2 Synthesis methods The characteristics of the ceramic nanoparticles such as shape, particle size distribution, crystal habit, state of agglomeration or dispersion are defined in the course of their synthesis. Therefore, understanding the methods that can be used to synthesize ceramic nanoparticles and the processes that result in ceramic nanoparticles as a byproduct (such as carbon based pollutants) are the key to understanding the potential environmental and toxic risks of these materials. There is a wide range of methods to produce ceramic nanoparticles. They are usually di-
vided in two main groups: Firstly, top-down, occurring when a single large portion of matter is reduced to many smaller unities; Secondly, bottom-up, occurring when atoms and molecules are assembled in a controlled manner in order to form particles [3]. One of the mainly used operations in ceramic processing, known as milling, is the principal example of a top-down method and has been described as unsuitable to produce nanoparticles. Even when high energetic processes such as jet-milling are employed, particles sizes below 200 nm are rarely obtained despite the great consumption of time and energy. This occurs, because more and more energy is required for comminution as the particle size is reduced. Therefore, the efficiency of this process exponentially drops with the time. This kind of process usually results in particles with sharp edges, a broad size distribution and with an inherently high concentration of defects (cracks and impurities). On the contrary, the bottom-up techniques – based on chemical reactions – generate particles with different and engineered shapes, crystal habit, and size distribution. Additionally, these particles are almost free of flaws. Examples of both processes are presented in Table 2. The concepts of stabilized suspensions, colloidal suspensions, particles packing and dispersion play a major role in the synthesis of ceramic nanoparticles, since most of them are carried out in a liquid medium. Due to their high surface area and thermodynamic instability, ceramic nanoparticles
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4
Fig. 4 • Dependence of the surface region volumetric region (⭋Sup) on the mean particle size
5
Fig. 5 • Examples of ceramic particles, their typical mean size and estimated number contained in a volume of 1 cm3 and the correspondent specific surface area (SSA)
Table 2 • Examples of the most used methods for synthesis of ceramic nanoparticles [3, 10, 12, 13]
Bottom-up
Top-down
Method (and principle)
Material
Particles characteristics
High energy milling, jet-mill (mechanical fracture)
Almost every ceramic raw material
Usually, not bellow 200 nm, broad size distribution, significant contamination on defects generation. Time energy consuming
Pirolisys (incomplete combustion of gases)
Carbon black, carbon nanotubes
High output. 10–200 nm, broad size distribution, smaller degree of morphology control, and, in some cases, not intentionally produced (combustion engines, for example)
Controlled precipitation (chemical equilibrium shifting)
Alumina, silica, boehmite (AlO(OH)) Mg(OH)2, Al(OH)3
Medium output, 2–100 nm, excellent morphology control (size distribution, crystalline habit), high purity CNP can be obtained
Chemical vapor deposition (solid-gas reaction in controlled atmosphere)
Carbon nanotubes, silica
Low output, 1–100 nm, good morphology control
Sol-gel method (decomposition of a polymeric molecule containing metallic atoms)
Alumina, ZrO2, SnO2, TiO2, ZnO
Medium to low output, 1–100 nm, good morphology control, medium purity CNP
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Table 3 • Effects of different ceramic nanoparticles on a population of aquatic microorganisms (results from reference [18]) Compound TiO2 (rutile)
ZnO
CuO
Particle size
LC50 / mg/L
NOEC / mg/L
3 μm
~ 20,000
> 20,000
30 nm
~ 20,000
> 20,000
2 μm
9
1.5
50 nm
3
0.5
10 μm
165
50
20 nm
3
0.5
arise and exert sophisticated control over molecular level organization ever more, the morphology of materials increasingly becomes important. There are some classical examples of the dependence of properties on size and shape. Gold is inert in a bulk form. Due to its particle size of 2–5 nm it becomes highly reactive and finds important application in catalysis [16]. Considering ceramic materials: Firstly, in microscopic scale (river sand, for example), silica usually is highly crystalline, inert and almost insoluble in most chemicals; on the other hand, the colloidal silica with a diameter between 10 and 100 nm is a powerful binder for refractory systems and paper fibres due to its ability to form amorphous gels [17]; secondly, known as one of the softest materials (explaining why it is used as lubricant), graphite originates the single sheeted compound known as graphene with excellent mechanical properties, when reduced to nanoscale [12]; thirdly, phase segregation: due to the fact that nanoparticles easily form single crystals and present low activation energy for diffusion, impurities and defects are readily to be repelled from the interior to their surface, which make doping operations more difficult, but favours purification [3]. In all these cases, it can be observed that the chemical composition of these materials is identical; the different sizes and physical state (bulk materials or nanoparticles) accounts for their novel chemical properties. These observations suggest that the understanding of how ceramic nanoparticles behave is more important than simply to know their size and shape. Under consideration of many other aspects such as size and composition, the way a certain type of particle behaves on different environments is known as functionality [7]. Amongst other aspects, the functionality first of all deals with some properties that could be evaluated by conventional laboratory techniques (specific surface area, electric charges distribution on its surface or Zeta potential, their tendency
to agglomerate or dissolve at different pH values, ionic conductivity), and, secondly, other characteristics that would require a more careful and long term analysis (toxicity = are these particles deleterious to a certain group of organisms?; persistency = could these particles be trapped in the environment or in an organism?; bioavailability = once these particles have been introduced in an organism, is it inert or can it be assimilated by tissues and organs?) An interesting example for the analysis of the functionality of ceramic nanoparticles is presented in reference [18]. This paper describes the impact of different ceramic nanoparticles on a very common crustaceous microorganism, Daphnia magna, frequently employed in toxicity tests. For this study, ceramic particles and nanoparticles of ZnO, CuO and TiO2 in the modification of rutile were chosen due to their long history of use in commercial products, such as hand creams and sunblockers. They were introduced as aqueous suspensions in a healthy population of microorganisms. After a certain period of time, two parameters were evaluated: a) LC50 (mg/L), the lethal concentration for 50 % of the individuals, and b) NOEC (mg/L), the maximum number of observed effect concentration. In both cases, the lower the parameter value, the more intense the biocide effect of the compound evaluated. The papers results are depicted in Table 3. It can be observed that a variation of the particle size did not produce any significant effect in the case of TiO2. For the systems containing ZnO and CuO, the presence of nanoparticles reduced the LC50, respectively, 3 times and 10 times. Similar results can be observed for NOEC. This indicates a significant increase in the biocide behaviour of these materials. A superficial analysis would indicate a size reduction as the main cause for this change, since the chemical composition of these particles remained unaltered. However, this conclusion is not valid be-
cause the same effect was not observed for the TiO2 particles. Otherwise, on the basis of the functionality of the particles, the authors point out that for ZnO and CuO the particle size reduction caused modifications in the Zeta potential and in the solubility while increasing their biocide activity. Other examples of the effects of the functionality of ceramic nanoparticles (since these effects are not observed in microscopic particles of the same materials) involve [19–29]: • carbon nanotubes: risks of bioaccumulation in soft tissues, including lungs, heart, kidney, reproductive organs and brain, DNA damage in lung cells; • ZnO and TiO2: allergenic reaction due to inhalation and skin exposure; • carbon black: cholesterol clogging formation [19–28]. 4 The urgent need of regulation Regulation can be defined as a list of rules and protocols created in order to promote a safe use and development of ceramic nanoparticles and other nanotechnologies plus a detailed description of the organs and associations responsible for its implementation, supervision and updating [14]. Many unanswered technical questions have to deal with: • In order to support the conclusions of the authors, these eco-toxicological experiments were conducted in controlled environments. What would have happened if this situation had occurred in a real environment such as a lake or river where water continously depicts changes of the pH value, composition and temperature? Would measurements of properties such as Zeta potential and particle size in laboratory conditions be enough to predict the functionality of the particles? • Several ceramic particles (microparticles and nanoparticles) have been used as a raw material for many applications (such as pigments, thixotropic agents for viscosity correction in foods, cosmetics and medicines and lubricants) which involve intentional (or unintentional) contact with living organisms. During their use, are these particles being introduced in living organisms and/or the environment? Is the absence of immediate effects evidence of no effect at all? • At low concentrations, most of the conventional techniques of characterization such as XRD, MEV, TGA or FTIR hardly can detect nanoparticles; nevertheless, even at very small dosages, they are able to produce nocive effects as seen earlier. How accurate are the current methods for
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HIGH-PERFORMANCE CERAMICS the detection and quantification of pollutants? How could the life cycle of nanoparticles efficiently be traced? Other more subjective and interdisciplinary questions which require a combination of the knowledge of different fields such as ethics, ecology and biology besides materials sciences and have to be answered: • Is it correct to induce a costumer to pay more for a certain product that advertises the benefits of nanotechnology, even if they are not noteworthy or if they were not asked for? • Would it be ethic not to tell the costumers that a certain product contains ceramic nanoparticles of which functionality was not fully understood? Is the current regulation regarding ceramic nanoparticles comprehensive enough to prevent future consequences for users? Who could be responsible for the disposal of these ceramic nanoparticles in the environment after the use of the product: the costumer or the producer? Why is it urgent? There are two good reasons. Firstly, the number of patents and the volume of products based on nanotechnology commercialized grew exponentially in the last five years (since nearly 2004) [3–7, 14]; secondly, like other technologies that are now known to have deleterious side effects (e.g. asbestos and silicosis, combustion engines and air pollution), nanotechnology is reaching a point of no return in its use that can cause the Technology Control Dilemma proposed by David Collingridge in 1980: In the early stages of a new technology, not enough is known to create a suitable control on the potential risks involved; on the other hand, when the problems emerge, the benefits of the technology are too wellestablished to be changed without major disruptions [7, 30]. Despite the complexity of the subject, solutions for this dilemma concerning nanotechnologies regulation must be locally considered and brought into account legislation, culture and values of each country. The presentation of definitive answers unfortunately is well beyond the scope of this paper, but certainly, the suggestions below can be useful: • Encouragement of public discussions involving risks and benefits of products and processes involving nanotechnologies.
• Ceramic nanoparticles should not be handled, stored and disposed simply in the same way as their micrometric equivalents. Be sure that the safety instructions of products and protection equipments are being followed when dealing with ceramic nanoparticles (or any other nanoparticles or chemicals). • Be conscientious on the products and raw materials containing ceramic nanoparticles you, your company or university purchase. Information request is an obligation and a right. • Inquire with political authorities, managers and directors about the organization politics on novel technologies and their potential implications on health, ethics, economy and environment. Acknowledgments The authors thank Universidade Federal do ABC (Brazil) and the Brazilian Research Foundation CNPq for supporting this work.
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Received: 09.01.2010
