COMMENT pubs.acs.org/JPCC
Comment on “From Nanoparticles to Nanoplates: Preferential Oriented Connection of Ag Colloids during Electrophoretic Deposition” Libor Kvitek† and Jan Hrbac* Department of Physical Chemistry, Palacky University, tr. 17. listopadu 12, Olomouc, 77146, Czech Republic
T
he preparation of nanostructured materials possessing unique physicochemical characteristics represents one of the central tasks of incoming era of nanotechnology. After initial research focus in development of reproducible preparation methods of well separated and stable nanoparticles of inorganic and organic materials, the current research is oriented at preparation of nanoparticles' organized assemblies, most often nanostructured layers on the surface of a suitable solid substrate. Electrophoretic deposition d(EPD) of inorganic nanoparticulate materials onto electrode surface represents a simple yet relatively well reproducible method of preparation of nanostructured layers. Originally, the EPD was used for the preparation of nanostructured layers of oxidic or metallic materials to assemble advanced composites. The authors S. Yang, W. Cai, G. Liu, and H. Zeng used the EPD method to deposit silver nanoparticles prepared by laser ablation onto the surfaces of Si wafers.1 In this article the authors describe the used experimental procedure, which allows influence of the morphology of formed nanostructured layers by varying the intensity of current flowing through the EPD cell. At the same time, the authors attempted to offer the physicochemical description of this process. Despite indisputable and excellent practical results achieved in this work, substantial reservations can be raised concerning the validity of the proposed mechanism of the formation of the prepared nanostructured layers. The authors preclude that nanostructured layers on the surface of Si wafers are formed owing to the process of the oriented connection of primary silver nanoparticles, prepared by laser ablation. The nanoparticles are supposed to orient itself during the electrophoretic movement through the dispersion and then grow during mutual collisions by aggregation mechanism to form structures of defined geometry (e.g., nanoplates), which subsequently settle on the Si surface. This hypothesis can, however, be seriously questioned despite the fact that the article is missing some pieces of information such as a positive value of ζ potential of the primarily prepared silver nanoparticles and that formation of nanoplates on the cathode in the used electrophoretic system2 seems to give support for the proposal by the authors of the physical mechanism of nanostructured layer formation. What considerations lead us to assume that a different— chemical or better electrochemical—mechanism is more probable in this case? From the data concerning the concentration and size of silver nanoparticles composing the dispersion used for EPD process it can be deduced that 5.4 1012 nanoparticles of 30 nm mean size are present between the electrodes, which corresponds to a total surface area of 152.5 cm2. During the whole EPD experiment a charge of 3.78 C is passed through the electrodes. If this charge was transferred only by silver r 2011 American Chemical Society
nanoparticles present in the system, the surface charge density would amount approximately to 0.025 C/cm2 or 7 10-13 C per nanoparticle. An enormous surface potential of 5.35103 V (estimated as a charge on conducting sphere3) would correspond to the above value of surface charge density. This principially unreasonably high value of surface potential leads us to the assumption that the real mechanism of the formation of the observed nanostructures is based on chemical, rather than physical, processes. At sufficiently high voltage between the electrodes in the electrophoretic system used by the authors, an electrolysis of the studied system can occur. Here, due to high purity of the nanoparticle dispersion, only two processes are possible. The first one is a simple anodic dissolution of silver nanoparticles leading to silver ions, which are subsequently reduced at the cathode to yield observed nanostructures. This process was previously observed in a different experimental setup during electrolytic preparation of silver colloid particles.4,5 The second possible process is direct water electrolysis forming OH• and H• radicals, which can participate on oxidation and backreduction (in conjunction with electrochemical reduction on the cathode) of primary silver nanoparticles, again yielding observed nanostructures. Additionally, the above-mentioned estimation of surface charge density on the studied silver nanoparticles corresponds to the concentration of monovalent ions to be adsorbed on the surface of the nanoparticles equal to the value of 2.5 10-3 mol/L. This concentration of monovalent ions is not supposable in the studied system, where laser ablation of a pure silver plate in the high purity water was realized. Moreover, the total concentration of silver in the studied system was only 4.6 10-4 mol/L; therefore the transferred charge of 3.78 C is sufficient to cause the electrochemical transformation of the silver nanoparticles present in the studied system. It is difficult to decide which mechanism is more probable on the basis of insufficient data about the experimental setup and process, especially if the voltage between the electrodes was not recorded over the time of the experiment and production of radicals during any part of the process was not studied. In any case, we believe that the proposed chemical mechanism of the formation of nanostructured layers from silver nanoparticles in the course of deposition experiment is more probable than the mechanism of oriented silver nanoparticle connection. The physical mechanism denoted as EPD based on aggregation of metal nanoparticles on the electrode surface yields only simple 2D structures (compact layer of nanoparticles on the electrode surface) for both unmodified metal nanoparticles6 and Received: July 2, 2010 Revised: February 2, 2011 Published: February 28, 2011 4980
dx.doi.org/10.1021/jp1061144 | J. Phys. Chem. C 2011, 115, 4980–4981
The Journal of Physical Chemistry C
COMMENT
nanoparticles modified by adsorbed surface modifiers.7-9 However, the EPD method is mainly applicable to deposit metal oxide materials such as ceramics,10,11 silica,12 or titania13 particles thta are chemically more stable in comparison to metal nanoparticles. Additionally, the surface potential of these particles commonly originates from dissociation of the surface groups (e.g., hydroxyl groups) and therefore adsorption of ions from solution or any other surface modification is not necessary for realization of electrophoretic motion of these particles. In this case the physical mechanism of oriented aggregation for EPD can undoubtedly be adopted for the emergence of some ordered structures in the electrophoretic system. However, the mechanism of oriented agglomeration during electrophoretic deposition should not be simply applied for the studied case reported in the article of Yang et al., in which highly ordered structures are formed from silver nanoparticles.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ420585634755. E-mail:
[email protected]. Fax: þ420585634461. Present Address †
Regional Centre of Advanced Technologies and Materials, Palacky University, Slechtitelu 11, 78371 Olomouc.
’ REFERENCES (1) Yang, S. K.; Cai, W. P.; Liu, G. Q.; Zeng, H. B. J. Phys. Chem. C 2009, 113, 7692–7696. (2) Information supplied by the Authors in their Reply to Comment, missing in the original article.Wang, S.; Cai, W. J. Phys. Chem. A 2011, 10.1021/jp109960q. (3) Stratton, J. A. Electromagnetic Theory; J. Wiley and Sons: Hoboken, NJ, 2007. (4) Jing, C. Y.; Fang, Y. J. Colloid Interface Sci. 2007, 314, 46–51. (5) Liu, G.; Cai, W.; Kong, L.; Duan, G.; L€u, F. J. Mater. Chem. 2010, 20, 767–772. (6) Zhao, S. Y.; Lei, S. B.; Chen, S. H.; Ma, H. Y.; Wang, S. Y. Colloid Polym. Sci. 2000, 278, 682–686. (7) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818–3829. (8) Zhang, G. D.; Chen, X.; Zhao, J. K.; Chai, Y. C.; Zhuang, W. C.; Wang, L. Y. Mater. Lett. 2006, 60, 2889–2892. (9) Zhao, S. Y.; Chen, S. H.; Li, D. G.; Zhou, M.; Ma, H. Y. Electrochem. Solid State Lett. 2007, 10, K43–K46. (10) Kaya, C.; Kaya, F.; Su, B.; Thomas, B.; Boccaccini, A. R. Surf. Coat. Technol. 2005, 191, 303–310. (11) Wei, M.; Ruys, A. J.; Milthorpe, B. K.; Sorrell, C. C.; Evans, J. H. J. Sol-Gel Sci. Technol. 2001, 21, 39–48. (12) Castro, Y.; Ferrari, B.; Moreno, R.; Duran, A. Surf. Coat. Technol. 2004, 182, 199–303. (13) Boccaccini, A. R.; Karapappas, P.; Marijuan, J. M.; Kaya, C. J. Mater. Sci. 2004, 39, 851–859.
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