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The promising aspects of processing nanomaterials under mechanical stressing for physicochemical viewpoints
Advanced Powder Technology 21 (2010) 586–591
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Review paper
The promising aspects of processing nanomaterials under mechanical stressing for physicochemical viewpoints Mamoru Senna Technofarm AXESZ Co., Ltd., 3-45-4 Kamiishihara, Chofu, Tokyo 182-0035, Japan
a r t i c l e
i n f o
Keywords: Mechanochemical processes Ligand field distortion Coordination compounds Charge transfer Anion substitution
a b s t r a c t Solid-state mechanochemical processing of nanomaterials offers unique opportunities for the creation of value-added materials. The present review reexamines physicochemical aspects of these processes and identifies the merits of leveraging local deformation derived phenomena such as symmetry loss of ligand field, charge-transfer across the grain boundary, reaction-induced amorphization and the consequences of intimate mixing for the creation of new materials. Case studies are also presented that include ligand exchange of transition metal coordination compounds, anion substitution of metal oxides, organic synthesis and drug amorphization. All these studies provide promise for solid-state mechanochemical processing in the future as a cornerstone of powder technology. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deformation and excitation of solids under mechanical stressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Microplastic deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Change in the energy states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Off-symmetry ligand fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ligand exchange reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Ligand exchange in metal coordination compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Anion exchange reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other viewpoints of mechanochemical reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Homogenization during mechanical stressing for ceramic precursors. . . . . . . . . . . . . . . . . . . . . 4.2. Reaction-induced amorphization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Charge transfer complex formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Mechanochemical processes have acquired their popularity, particularly in last two decades. They are, however, not always too elegant and by no means ecologically benign. When fullerene powder is subjected to milling in oxygen atmosphere, for instance, oxygen oxidization could take place, which is beneficial for many purposes [1]. Fullerene oxide is, however, more rationally produced via a photochemical route [2]. Likewise, a number of mechanochemical processes could be replaced by some more elegant methods. It is therefore appropriate to reexamine whether and unE-mail address: [email protected]
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der what circumstances materials processing under mechanical stressing is genuinely the best way as compared to other competing processes including sonochemical, magnetochemical, hydrothermal or high-pressure processes. Exertion of mechanical stressing upon powder particles results not only in the size reduction, but also in the local and irreversible deformation of solids, particularly when the particle size become smaller than a few micrometers [3]. This immediately causes change in the symmetry of ligand fields and often of molecules as well, in case of molecular crystals [4]. Those microscopic changes may be associated with excitation in the electronic energy states [5,6]. Conversely, excitation by other external stimuli, among others by photons from electromagnetic waves, results in
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.06.010
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Fig. 1. (a) Change in energy of system and dihedral angle of anthracene with decreasing intermolecular distance calculated using AM1 method. The dihedral angle of anthracene at the state of activated complex is 150°, (b) The intermolecular HOMO–LUMO gap of (a) anthracene, (b) 9-methyl anthracene, (c) 9,10-dimethyl anthracene, (d) 9,10-bis(chloromethyl)anthracene, and (e) 9,10-bis(bromomethyl)anthracene.
Fig. 2. (a) Mössbauer spectra of FeCl24H2O before and after milling, (b) O1s XPS spectra of FeCl24H2O before and after milling.
the distortion of molecules or ligand fields. Genuine merit of mechanochemical process could, therefore, be found in the intrinsic nature and consequences of local deformation of solids and associated symmetry loss in the ligand field. In the present short overview, the author tries to answer the question, ‘‘Can mechanochemical technology survive as an industrial strategy in the era under heavy burden of energy cost, environmental pollution and serious economic crisis?”. The question will be answered from scientific point of view towards a breakthrough in important corner of powder technology to adapt upcoming demands. The unique feature of the solids is, therefore, discussed here from physicochemical viewpoints. Other phenomena imparted to a powder technological process under mechanical stressing, e.g. temporal alternation of contact points among dissimilar solids and associated homogenization of powder particles are also referred, in an attempt to persuade mechanochemical processes really attractive and affordable.
2. Deformation and excitation of solids under mechanical stressing Local deformation of the solid is the origin of most of the mechanochemical processes, which, in turn leads a number of practically beneficial phenomena.
2.1. Microplastic deformation Classical works of single particle stressing revealed unusual plastic deformation of a particle under compressive stress when the particle size goes down to a single micrometer regime [7]. Similar phenomena were found as a consequence of microscopic and local stressing on the massive solid [8,9]. They are common on the physico-mechanical basis, and are coined as microplasticity (MP) [3]. For the purpose of finest comminution, MP is to be avoided. By contrast, mechanical activation favors MP, since this is one of the most important origins of chemical changes in solids in conjunction with various structural defects. In a solid with MP, metastable states could be quenched and preserved. It is pity, however, that studies on MP are not always linked with a well established and well documented literatures on the behaviors of dislocations in the interests of solid-state mechanics, e.g. presented in an excellent recent review by Vitek and Paidar [10].
2.2. Change in the energy states Grey and Butler demonstrated the change in the electronic energy state under compression in a diamond anvil upon [CuCl4]2 anion [5]. This is a visible manifestation of excitation of solids under mechanical stressing, leading to piezochromism. Consequences of
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Fig. 3. HPLC profiles of the mixture comprising compounds A and D, before and after milling.
ligand field deformation play, however, much more diverse roles, including those on the change in the magnetic properties [11] and, more importantly, elevation of the reactivity of solids, as demonstrated below. Organic physical chemists tend to attribute a mechanochemical process to excitation of electronic energy state to reduce energy gap between HOMO and LUMO (HLG). Indeed, molecular distortion brings about energy excitation, understood under the concept of inverse Jahn–Teller effect, as Gilman pointed out [6]. This is just reverse to the photochemical process where photo induced excitation brings about molecular distortion. This is different from simple route via a radical formation, which mechanochemists frequently observe. When intermolecular HLG values for 4 derivatives of anthracene (AN) and benzoquinone (BQ) are plotted against the dihedral angle of the former, we recognize the decrease of HLG with increasing the extent of folding of AN derivatives, as shown in Fig. 1 [12]. Although the computation is based on the semi-empirical by using SPALTAN ’02, with relatively poor rigorousity, the general trend could be acceptable.
2.3. Off-symmetry ligand fields Microplasticity mentioned above is associated with the loss or break of symmetry of crystal field or ligand field. Beauty of crystal symmetry is in many cases associated with that of orbital symmetry of atoms. A typical example is that of 3d electrons in case of those compounds with transition metals. One of the most important consequences from the mechanochemical viewpoint is irreversible deformation leading to the ligand field distortion. When one of the simplest ferrous coordination compounds, FeCl24H2O, is solely milled, the ligand field around the central Fe2+ ion is irreversibly distorted. This was demonstrated by Mössbauer (Fig. 2(a)) and X-ray photoelectron spectra (Fig. 2(b)) [4].
Fig. 4. Enhanced ligand exchange by starting from preliminarily ground FeCl24H2O exhibited by higher intensity of MLCT band.
3. Ligand exchange reactions The concept of ligand exchange may often be interpreted in a broader sense, as most of the construction units of the transition metal oxide crystals are regarded as coordination units. 3.1. Ligand exchange in metal coordination compounds An orthodox, conventional ligand exchange reaction is demonstrated by milling a mixture of [Fe(phen)3](NCS)2H2O and
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Fig. 7. A scheme of deformed TiO6 octahedron by anion substitution.
Fig. 5. UV–Vis diffuse reflectance spectra. (A) Titania (anatase), milled for 1 h. (B–D) Titania + GLY, URA, and PTFE, respectively, milled for 3 h.
Fig. 6. Absorption in low energy side is apparently peaked at ca. 9000 cm 1 or 1200 nm in an observed spectrum (curve A). Deconvolution resulted into 3 subpeaks, i.e.: Peak 1 (1.0 eV): oxygen vacancy with two trapped electrons; Peak 2 (1.4 eV): oxygen vacancy with one trapped electron; Peak 3 (2.9 eV): d–d transition of Ti3+; curve E: adsorption edge.
[Fe(byp)3](NCS)23H2O (phen = 1,10-phenanthroline, byp = bipyridine). HPLC profile shown in Fig. 3 revealed that a mixed ligand states (B and C) appeared after milling by a planetary mill for 3 h [4]. This kind of ligand exchange can well be applied to a number of solvent-free syntheses of various coordination compounds. When FeCl24H2O is milled with phen, [Fe(phen)3]Cl2nH2O is obtained. This is a typical example of a solvent-free synthesis of ferrous coordination compound via a mechanochemical route. As shown in Fig. 4, the mechanochemical reaction proceeded much faster when one of the starting materials, FeCl24H2O, is preliminarily milled alone. This is explained by the ligand field distortion mentioned above (see Fig. 2(a) and (b)). Note that preliminary milling of phen alone did not change the rate of mechanochemical reaction at all. 3.2. Anion exchange reactions Concept of ligand exchange is by no means restricted to the coordination compounds in a traditional narrow definition. Metal oxides are not constructed by individual ionic species. They are consisting of the finite construction units like MO4 or MO6, where M stands for a metallic cation. Those oxides are, therefore, understood as a kind of coordination crystals. As we milled TiO2 (anatase) fine particles with different organic powders, i.e. poly tet-
rafluoroethylene (PTFE), glycine (GLY) or urea (URA), we observed significant change in the color and correspondingly in the diffuse reflectance spectra (DRS), as shown in Fig. 5 [13]. While the absorption peak at around 420 nm is attributed to the incorporation of nitrogen, a very broad peak extending to near infrared (NIR) region (see also Fig. 6) is associated with the introduction of oxygen vacancies, when titania is milled with F-containing organic powder like PTFE. The broadness of the Vis–NIR absorption band could be explained by the coexistence of various states of oxygen vacancies, including different numbers of electrons entrapped [14]. A closer look at the various changes imparted to the milled mixture of titania and organic compounds reveals that the mechanochemical change is much more sensitive to DRS than to the structural change observed by XRD. The latter told us a retardation of phase transformation to rutile when organic compounds were coexisting. All these changes are really typical for a mechanochemical process, which no other conventional solid-state processes could achieve. This could generally and integratively be explained by the distortion of the ligand field. While these are factually displayed in Fig. 2(a) and (b), it could further be speculated by the concept of autocatalytic reaction. What is important thereby is the enhanced distortion by the progress of anion substitution, as illustrated in Fig. 7. 4. Other viewpoints of mechanochemical reactions Phenomena occurring during milling of a powder mixture are really manifold. It is therefore essential to observe those possibly involved in these operations as widely as possible. 4.1. Homogenization during mechanical stressing for ceramic precursors Use of mills or similar apparata for the purpose of mechanical activation becomes so popular that we oftentimes overlook other functions of milling machine, notably homogenization of the powder mixture, which has been a routine operation in a ceramic fabrication process. By virtue of EPMA and using variation coefficient as a measure, homogeneity in a micrometer regime could be quantified [15]. This is by and large associated with the disintegration of aggregates usually contained in the starting mixture. Homogeneity in a smaller scale is analyzed by EDX coupled with TEM [16,17]. Decrease in the variation coefficient of the atomic ratio by increasing milling time is not only associated with the mechanical or granulometrical homogenization [18]. A much more important factor is related with the incipient chemical reaction across the boundary of dissimilar particles, so that it is far beyond the consequence of conventional intimate mixing. Indeed, a change in the electronic states has been verified by XPS [16] and is one of the most important factors of mechanochemical effects, which no other non-conventional methods could achieve.
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Fig. 8. Reactive amorphization of Mg(OH)2. M and MS stand for milling Mg(OH)2 alone and with SiO2, respectively. A number as an extension means milling time in hour.
Electroceramics are required to be well crystallized and finely divided. Since the driving forces of the two apparently different relaxation pathways, crystal growth and increase in the particle size, are much in common, it is from the beginning a big challenge to enhance one and suppress the other. The above-mentioned homogenization on the electronic basis serves as a very appropriate method for this purpose. A number of examples were published for various electroceramic compounds, e.g. for PMN-PT [16], PZNPT [19] or BBT [20]. 4.2. Reaction-induced amorphization Everybody recognizes that mechanical stressing degrades crystals degrade and brings about partial or, under circumstances, total amorphous states. To turn the solid states from crystalline to non-crystalline requires, however, a huge energy concentration when it is attempted via a mechanical route. Reaction-induced amorphization, in contrast, is a smart way of amorphization with-
Fig. 10. XPD profiles of (a) Hydroxypropyl methyl cellulose (HPMC), (b) Ibuprofen (IB) intact, (c) 20% IB physical mixture with HPMC, (d) 20% mixture milled for 6 h, (e) 20% xerogel film and (f) IB recrystallized in water/methanol solvent. Ref. [22] for details of (e) and (f).
out heating or rapid quenching. One rather traditional example is some hydroxides, like Ca(OH)2, whose mechanochemical amorphization is quite tough, while addition of silica, for instance, enables drastically faster amorphization, as shown in Fig. 8 [21]. Much more promising for our future technology is a reactioninduced amorphization of coordination compounds or molecular crystals. In the system, comprising FeCl24H2O and phen, amorphization was clearly observed as shown in Fig. 9 [4]. In the case of drugs, many efforts were paid to amorphize for the purpose of better biological absorption and hence, better bioavailability. With the coexistence of some derivatives of cellulose as an excipient, drugs could be amorphized quite easily, due to the recombination of hydrogen bonds from intracrystalline ones, which stabilizes the crystalline state, to those between the drug and excipients, as exemplified in Fig. 10 [22]. 4.3. Charge transfer complex formation Fig. 9. Reactive amorphization of a mixture comprising FeCl24H2O and phen by comilling.
Charge transfer (CT) can take place across the interparticulate boundary at room temperature, even without giving any severe
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Acknowledgments The author thanks Prof. K. Becker, Prof. V. Sepelak and Dr. J. Shi in T.U. Braunschweig for their experimental cooperation as well as valuable discussions for the contents in 4.2. He also thanks his coauthors in the majority of references cited in this manuscript for their cooperation. A part of the experimental work was supported by Alexander von Humboldt Foundation.
References
Fig. 11. Products comparison in view of molecular symmetry.
Fig. 12. Effects of molecular catalysis on the rate of mechanochemical Diels–Alder reaction.
external stimuli. This kind of CT complexes could be formed more easily from the combination of less symmetrical reactants, as shown in Fig. 11 [23]. Melting points of such complexes are often lower than room temperature to give autogenous liquefaction during the formation of a CT complex. Here, milling only serves to enhance contacts among dissimilar particles, so that the intensity of mechanical stressing may be very weak. It is worth mentioning that this kind of CT complex formation can be utilized as a molecular catalysis, when one of the reactants for mechanochemical organic synthesis could form an autogenously CT complex with an appropriate additive. An example is given in Fig. 12 [24]. 5. Concluding remarks The rational design of solid-state mechanochemical processes using a fundamental framework of the physicochemical processes that occur during local deformation of the particles under stressing is anticipated to lead to the creation of advanced materials. In this age of energy burden and economic crisis, these processes offer opportunities for the development of superior materials for a variety of important applications including visible light responsive photocatalysts, solar cells, and electrode materials for lithium ion batteries. Although, mechanochemical technology offers immense benefits for producing materials with unique properties, caution should be applied since the method is notoriously energy inefficient. This could be surmounted, if partly, by taking the components suggested above into account.
[1] H. Watanabe, E. Matsui, Y. Ishiyama, M. Senna, Solvent free mechanochemical oxidation of fullerene under oxygen atmosphere, Tetrahedron Lett. 48 (2007) 8132–8137. [2] J. Averdung, G. Torres-Garcia, H. Luftmann, I. Schlachter, J. Mattay, Progress in fullerene chemistry: from exohedral functionalization to heterofullerenes, Fullerene Sci. Technol. 4 (1996) 633–654. [3] A.G. Smekal, The microplasticity of hard substances, Plansee Proc. (1956) 28– 40. [4] T. Ohshita, D. Nakajima, A. Tsukamoto, N. Tsuchiya, M. Senna, N. Yoshioka, H. Inoue, Role of molecular strain on the solid-state synthesis of coordination compounds from iron (II) chloride tetrahydrate and 1,10-phenanthroline, Ann. Chem. Sci. Mater. 27 (2002) 91–101. [5] J.K. Grey, I.S. Butler, Effects of high external pressures on the electronic spectra of coordination compounds, Coord. Chem. 219–221 (2001) 713–759. [6] J.J. Gilman, Mechanochemistry, Science 274 (1996) 65. [7] W. Hess, T.U. Karusruhe, Dissertation (1981) 154. [8] S. Chida, B. Uchino, M. Senna, A topochemical study on the microplastic deformation and preferred dissolution of pure silicon single crystal, Solid State Ionics 39 (1990) 263–272. [9] Y. Saito, T. Isobe, M. Senna, Incipient chemical reaction on the scratched silicon {1 1 1} surface with ethoxy and hydroxy groups, J. Solid State Chem. 120 (1995) 96–100. [10] V. Vitek, V. Paidar, in: J.P. Hirth (Ed.), Dislocation in Solids, Elsevier, Amsterdam, 2008, p. 493 (Chapter 87). [11] K.L. Bray, H.G. Drickamer, Pressure-induced isomerizations in five-coordinate solid-state copper(II) complexes, J. Phys. Chem. 95 (2010) 559–562. [12] M.F. Pradipta, H. Watanabe, M. Senna, Semiempirical computation of the solid phase Diels–Alder reaction between anthracene derivatives and p-benzoquinone via molecular distortion, Solid State Ionics 172 (2004) 169– 172. [13] M. Senna, V. Sepelak, J.M. Shi, B. Bauer, K.D. Becker, A. Feldhoff, Preparation and properties of anion-modified TiO2 nanoparticles via a mechanochemical route with organic fine particles, in: Proceedings of the World Congress Particle Technology, 2010 in Nürnberg, Adv. Powder Technol., this issue. [14] A.A. Lisachenko, V.N. Kuznetsov, M.N. Zakharov, R.V. Michailov, The interaction of O2, NO and N2O with surface defects of dispersed titanium dioxide, Kinet. Catal. 45 (2004) 189–197. [15] C. Ando, R. Yanagawa, H. Chazono, H. Kishi, M. Senna, Nuclei-growth optimization for fine-grained BaTiO3 by precision-controlled mechanical pretreatment of starting powder mixture, J. Mater. Res. 19 (2004) 3592– 3599. [16] J.G. Baek, K. Gomi, T. Isobe, M. Senna, Flat dielectric properties of capacitor (1 x)Pb(Mg1/3Nb2/3)O3 – xPbTiO3 by controlled concentration gradient, Mater. Sci. Eng. B49 (1997) 46–52. [17] S. Komatsubara, T. Isobe, M. Senna, Effect of preliminary mechanical treatment on the microhomogenization during heating of hydrous gels as precursors for lead titanate, J. Am. Ceram. Soc. 77 (1994) 278–282. [18] R. Yanagawa, C. Ando, H. Chazono, H. Kishi, M. Senna, Well-crystallized tetragonal BaTiO3 micro particles via a solid-state reaction preceded by agglomeration-free mechanical activation, J. Am. Ceram. Soc. 90 (2007) 809– 814. [19] S. Shinohara, J.G. Baek, T. Isobe, M. Senna, Synthesis of phase pure Pb(ZnxMg1 x)1/3Nb2/3O3 up to x = 0.7 from a single mixture via a softmechanochemical route, J. Am. Ceram. Soc. 83 (2000) 3208–3210. [20] M. Senna, T. Kinoshita, Y. Abe, H. Kishi, C. Ando, Y. Doshida, B. Stojanovic, Smart soft-mechanochemical syntheses of well-crystallized pure phase fine particulates of mixed oxides for electroceramics, J. Eur. Ceram. Soc. 27 (2007) 4301–4306. [21] J.F. Liao, M. Senna, Enhanced dehydration and amorphization of Mg(OH)2 in the presence of ultrafine SiO2 under mechanochemical conditions, Thermochim. Acta 210 (1992) 89–102. [22] S. Nakayama, K. Ihara, M. Senna, Structure and properties of ibuprofen– hydroxypropyl methylcellulose nanocomposite gel, Powder Technol. 190 (2009) 221–224. [23] R. Hiraoka, H. Watanabe, M. Senna, A solvent-free organic synthesis from solid-state reactants through autogenous fusion due to formation of molecular complexes and increasing alcohol nucleophilicity, Tetrahedron Lett. 47 (2006) 3111–3114. [24] H. Watanabe, M. Senna, A Diels–Alder reaction catalyzed by eutectic complexes autogenously formed from solid state phenols and quinones, Tetrahedron Lett. 47 (2006) 4481–4484.
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Review paper
The promising aspects of processing nanomaterials under mechanical stressing for physicochemical viewpoints Mamoru Senna Technofarm AXESZ Co., Ltd., 3-45-4 Kamiishihara, Chofu, Tokyo 182-0035, Japan
a r t i c l e
i n f o
Keywords: Mechanochemical processes Ligand field distortion Coordination compounds Charge transfer Anion substitution
a b s t r a c t Solid-state mechanochemical processing of nanomaterials offers unique opportunities for the creation of value-added materials. The present review reexamines physicochemical aspects of these processes and identifies the merits of leveraging local deformation derived phenomena such as symmetry loss of ligand field, charge-transfer across the grain boundary, reaction-induced amorphization and the consequences of intimate mixing for the creation of new materials. Case studies are also presented that include ligand exchange of transition metal coordination compounds, anion substitution of metal oxides, organic synthesis and drug amorphization. All these studies provide promise for solid-state mechanochemical processing in the future as a cornerstone of powder technology. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Contents 1. 2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deformation and excitation of solids under mechanical stressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Microplastic deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Change in the energy states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Off-symmetry ligand fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ligand exchange reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Ligand exchange in metal coordination compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Anion exchange reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other viewpoints of mechanochemical reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Homogenization during mechanical stressing for ceramic precursors. . . . . . . . . . . . . . . . . . . . . 4.2. Reaction-induced amorphization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Charge transfer complex formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Mechanochemical processes have acquired their popularity, particularly in last two decades. They are, however, not always too elegant and by no means ecologically benign. When fullerene powder is subjected to milling in oxygen atmosphere, for instance, oxygen oxidization could take place, which is beneficial for many purposes [1]. Fullerene oxide is, however, more rationally produced via a photochemical route [2]. Likewise, a number of mechanochemical processes could be replaced by some more elegant methods. It is therefore appropriate to reexamine whether and unE-mail address: [email protected]
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der what circumstances materials processing under mechanical stressing is genuinely the best way as compared to other competing processes including sonochemical, magnetochemical, hydrothermal or high-pressure processes. Exertion of mechanical stressing upon powder particles results not only in the size reduction, but also in the local and irreversible deformation of solids, particularly when the particle size become smaller than a few micrometers [3]. This immediately causes change in the symmetry of ligand fields and often of molecules as well, in case of molecular crystals [4]. Those microscopic changes may be associated with excitation in the electronic energy states [5,6]. Conversely, excitation by other external stimuli, among others by photons from electromagnetic waves, results in
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.06.010
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Fig. 1. (a) Change in energy of system and dihedral angle of anthracene with decreasing intermolecular distance calculated using AM1 method. The dihedral angle of anthracene at the state of activated complex is 150°, (b) The intermolecular HOMO–LUMO gap of (a) anthracene, (b) 9-methyl anthracene, (c) 9,10-dimethyl anthracene, (d) 9,10-bis(chloromethyl)anthracene, and (e) 9,10-bis(bromomethyl)anthracene.
Fig. 2. (a) Mössbauer spectra of FeCl24H2O before and after milling, (b) O1s XPS spectra of FeCl24H2O before and after milling.
the distortion of molecules or ligand fields. Genuine merit of mechanochemical process could, therefore, be found in the intrinsic nature and consequences of local deformation of solids and associated symmetry loss in the ligand field. In the present short overview, the author tries to answer the question, ‘‘Can mechanochemical technology survive as an industrial strategy in the era under heavy burden of energy cost, environmental pollution and serious economic crisis?”. The question will be answered from scientific point of view towards a breakthrough in important corner of powder technology to adapt upcoming demands. The unique feature of the solids is, therefore, discussed here from physicochemical viewpoints. Other phenomena imparted to a powder technological process under mechanical stressing, e.g. temporal alternation of contact points among dissimilar solids and associated homogenization of powder particles are also referred, in an attempt to persuade mechanochemical processes really attractive and affordable.
2. Deformation and excitation of solids under mechanical stressing Local deformation of the solid is the origin of most of the mechanochemical processes, which, in turn leads a number of practically beneficial phenomena.
2.1. Microplastic deformation Classical works of single particle stressing revealed unusual plastic deformation of a particle under compressive stress when the particle size goes down to a single micrometer regime [7]. Similar phenomena were found as a consequence of microscopic and local stressing on the massive solid [8,9]. They are common on the physico-mechanical basis, and are coined as microplasticity (MP) [3]. For the purpose of finest comminution, MP is to be avoided. By contrast, mechanical activation favors MP, since this is one of the most important origins of chemical changes in solids in conjunction with various structural defects. In a solid with MP, metastable states could be quenched and preserved. It is pity, however, that studies on MP are not always linked with a well established and well documented literatures on the behaviors of dislocations in the interests of solid-state mechanics, e.g. presented in an excellent recent review by Vitek and Paidar [10].
2.2. Change in the energy states Grey and Butler demonstrated the change in the electronic energy state under compression in a diamond anvil upon [CuCl4]2 anion [5]. This is a visible manifestation of excitation of solids under mechanical stressing, leading to piezochromism. Consequences of
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Fig. 3. HPLC profiles of the mixture comprising compounds A and D, before and after milling.
ligand field deformation play, however, much more diverse roles, including those on the change in the magnetic properties [11] and, more importantly, elevation of the reactivity of solids, as demonstrated below. Organic physical chemists tend to attribute a mechanochemical process to excitation of electronic energy state to reduce energy gap between HOMO and LUMO (HLG). Indeed, molecular distortion brings about energy excitation, understood under the concept of inverse Jahn–Teller effect, as Gilman pointed out [6]. This is just reverse to the photochemical process where photo induced excitation brings about molecular distortion. This is different from simple route via a radical formation, which mechanochemists frequently observe. When intermolecular HLG values for 4 derivatives of anthracene (AN) and benzoquinone (BQ) are plotted against the dihedral angle of the former, we recognize the decrease of HLG with increasing the extent of folding of AN derivatives, as shown in Fig. 1 [12]. Although the computation is based on the semi-empirical by using SPALTAN ’02, with relatively poor rigorousity, the general trend could be acceptable.
2.3. Off-symmetry ligand fields Microplasticity mentioned above is associated with the loss or break of symmetry of crystal field or ligand field. Beauty of crystal symmetry is in many cases associated with that of orbital symmetry of atoms. A typical example is that of 3d electrons in case of those compounds with transition metals. One of the most important consequences from the mechanochemical viewpoint is irreversible deformation leading to the ligand field distortion. When one of the simplest ferrous coordination compounds, FeCl24H2O, is solely milled, the ligand field around the central Fe2+ ion is irreversibly distorted. This was demonstrated by Mössbauer (Fig. 2(a)) and X-ray photoelectron spectra (Fig. 2(b)) [4].
Fig. 4. Enhanced ligand exchange by starting from preliminarily ground FeCl24H2O exhibited by higher intensity of MLCT band.
3. Ligand exchange reactions The concept of ligand exchange may often be interpreted in a broader sense, as most of the construction units of the transition metal oxide crystals are regarded as coordination units. 3.1. Ligand exchange in metal coordination compounds An orthodox, conventional ligand exchange reaction is demonstrated by milling a mixture of [Fe(phen)3](NCS)2H2O and
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Fig. 7. A scheme of deformed TiO6 octahedron by anion substitution.
Fig. 5. UV–Vis diffuse reflectance spectra. (A) Titania (anatase), milled for 1 h. (B–D) Titania + GLY, URA, and PTFE, respectively, milled for 3 h.
Fig. 6. Absorption in low energy side is apparently peaked at ca. 9000 cm 1 or 1200 nm in an observed spectrum (curve A). Deconvolution resulted into 3 subpeaks, i.e.: Peak 1 (1.0 eV): oxygen vacancy with two trapped electrons; Peak 2 (1.4 eV): oxygen vacancy with one trapped electron; Peak 3 (2.9 eV): d–d transition of Ti3+; curve E: adsorption edge.
[Fe(byp)3](NCS)23H2O (phen = 1,10-phenanthroline, byp = bipyridine). HPLC profile shown in Fig. 3 revealed that a mixed ligand states (B and C) appeared after milling by a planetary mill for 3 h [4]. This kind of ligand exchange can well be applied to a number of solvent-free syntheses of various coordination compounds. When FeCl24H2O is milled with phen, [Fe(phen)3]Cl2nH2O is obtained. This is a typical example of a solvent-free synthesis of ferrous coordination compound via a mechanochemical route. As shown in Fig. 4, the mechanochemical reaction proceeded much faster when one of the starting materials, FeCl24H2O, is preliminarily milled alone. This is explained by the ligand field distortion mentioned above (see Fig. 2(a) and (b)). Note that preliminary milling of phen alone did not change the rate of mechanochemical reaction at all. 3.2. Anion exchange reactions Concept of ligand exchange is by no means restricted to the coordination compounds in a traditional narrow definition. Metal oxides are not constructed by individual ionic species. They are consisting of the finite construction units like MO4 or MO6, where M stands for a metallic cation. Those oxides are, therefore, understood as a kind of coordination crystals. As we milled TiO2 (anatase) fine particles with different organic powders, i.e. poly tet-
rafluoroethylene (PTFE), glycine (GLY) or urea (URA), we observed significant change in the color and correspondingly in the diffuse reflectance spectra (DRS), as shown in Fig. 5 [13]. While the absorption peak at around 420 nm is attributed to the incorporation of nitrogen, a very broad peak extending to near infrared (NIR) region (see also Fig. 6) is associated with the introduction of oxygen vacancies, when titania is milled with F-containing organic powder like PTFE. The broadness of the Vis–NIR absorption band could be explained by the coexistence of various states of oxygen vacancies, including different numbers of electrons entrapped [14]. A closer look at the various changes imparted to the milled mixture of titania and organic compounds reveals that the mechanochemical change is much more sensitive to DRS than to the structural change observed by XRD. The latter told us a retardation of phase transformation to rutile when organic compounds were coexisting. All these changes are really typical for a mechanochemical process, which no other conventional solid-state processes could achieve. This could generally and integratively be explained by the distortion of the ligand field. While these are factually displayed in Fig. 2(a) and (b), it could further be speculated by the concept of autocatalytic reaction. What is important thereby is the enhanced distortion by the progress of anion substitution, as illustrated in Fig. 7. 4. Other viewpoints of mechanochemical reactions Phenomena occurring during milling of a powder mixture are really manifold. It is therefore essential to observe those possibly involved in these operations as widely as possible. 4.1. Homogenization during mechanical stressing for ceramic precursors Use of mills or similar apparata for the purpose of mechanical activation becomes so popular that we oftentimes overlook other functions of milling machine, notably homogenization of the powder mixture, which has been a routine operation in a ceramic fabrication process. By virtue of EPMA and using variation coefficient as a measure, homogeneity in a micrometer regime could be quantified [15]. This is by and large associated with the disintegration of aggregates usually contained in the starting mixture. Homogeneity in a smaller scale is analyzed by EDX coupled with TEM [16,17]. Decrease in the variation coefficient of the atomic ratio by increasing milling time is not only associated with the mechanical or granulometrical homogenization [18]. A much more important factor is related with the incipient chemical reaction across the boundary of dissimilar particles, so that it is far beyond the consequence of conventional intimate mixing. Indeed, a change in the electronic states has been verified by XPS [16] and is one of the most important factors of mechanochemical effects, which no other non-conventional methods could achieve.
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Fig. 8. Reactive amorphization of Mg(OH)2. M and MS stand for milling Mg(OH)2 alone and with SiO2, respectively. A number as an extension means milling time in hour.
Electroceramics are required to be well crystallized and finely divided. Since the driving forces of the two apparently different relaxation pathways, crystal growth and increase in the particle size, are much in common, it is from the beginning a big challenge to enhance one and suppress the other. The above-mentioned homogenization on the electronic basis serves as a very appropriate method for this purpose. A number of examples were published for various electroceramic compounds, e.g. for PMN-PT [16], PZNPT [19] or BBT [20]. 4.2. Reaction-induced amorphization Everybody recognizes that mechanical stressing degrades crystals degrade and brings about partial or, under circumstances, total amorphous states. To turn the solid states from crystalline to non-crystalline requires, however, a huge energy concentration when it is attempted via a mechanical route. Reaction-induced amorphization, in contrast, is a smart way of amorphization with-
Fig. 10. XPD profiles of (a) Hydroxypropyl methyl cellulose (HPMC), (b) Ibuprofen (IB) intact, (c) 20% IB physical mixture with HPMC, (d) 20% mixture milled for 6 h, (e) 20% xerogel film and (f) IB recrystallized in water/methanol solvent. Ref. [22] for details of (e) and (f).
out heating or rapid quenching. One rather traditional example is some hydroxides, like Ca(OH)2, whose mechanochemical amorphization is quite tough, while addition of silica, for instance, enables drastically faster amorphization, as shown in Fig. 8 [21]. Much more promising for our future technology is a reactioninduced amorphization of coordination compounds or molecular crystals. In the system, comprising FeCl24H2O and phen, amorphization was clearly observed as shown in Fig. 9 [4]. In the case of drugs, many efforts were paid to amorphize for the purpose of better biological absorption and hence, better bioavailability. With the coexistence of some derivatives of cellulose as an excipient, drugs could be amorphized quite easily, due to the recombination of hydrogen bonds from intracrystalline ones, which stabilizes the crystalline state, to those between the drug and excipients, as exemplified in Fig. 10 [22]. 4.3. Charge transfer complex formation Fig. 9. Reactive amorphization of a mixture comprising FeCl24H2O and phen by comilling.
Charge transfer (CT) can take place across the interparticulate boundary at room temperature, even without giving any severe
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Acknowledgments The author thanks Prof. K. Becker, Prof. V. Sepelak and Dr. J. Shi in T.U. Braunschweig for their experimental cooperation as well as valuable discussions for the contents in 4.2. He also thanks his coauthors in the majority of references cited in this manuscript for their cooperation. A part of the experimental work was supported by Alexander von Humboldt Foundation.
References
Fig. 11. Products comparison in view of molecular symmetry.
Fig. 12. Effects of molecular catalysis on the rate of mechanochemical Diels–Alder reaction.
external stimuli. This kind of CT complexes could be formed more easily from the combination of less symmetrical reactants, as shown in Fig. 11 [23]. Melting points of such complexes are often lower than room temperature to give autogenous liquefaction during the formation of a CT complex. Here, milling only serves to enhance contacts among dissimilar particles, so that the intensity of mechanical stressing may be very weak. It is worth mentioning that this kind of CT complex formation can be utilized as a molecular catalysis, when one of the reactants for mechanochemical organic synthesis could form an autogenously CT complex with an appropriate additive. An example is given in Fig. 12 [24]. 5. Concluding remarks The rational design of solid-state mechanochemical processes using a fundamental framework of the physicochemical processes that occur during local deformation of the particles under stressing is anticipated to lead to the creation of advanced materials. In this age of energy burden and economic crisis, these processes offer opportunities for the development of superior materials for a variety of important applications including visible light responsive photocatalysts, solar cells, and electrode materials for lithium ion batteries. Although, mechanochemical technology offers immense benefits for producing materials with unique properties, caution should be applied since the method is notoriously energy inefficient. This could be surmounted, if partly, by taking the components suggested above into account.
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