COMMENT pubs.acs.org/JPCC
Reply to “Comment on ‘From Nanoparticles to Nanoplates: Preferential Oriented Connection of Ag Colloids during Electrophoretic Deposition’” Shikuan Yang and Weiping Cai* Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China.
e have reported1 fabrication of the films with tunable and controllable morphologies and structures, based on electrophoretic deposition (EPD) in the Ag colloidal solution, produced by laser ablation (LA) of a Ag flake in deionized water, under a constant current deposition mode, and found significant crystal growth during EPD. A mechanism was presented, on the basis of our experimental results, to describe formation of the regularly shaped single crystal, which was mainly attributed to the crystal growth induced by preferential oriented connection of the nanoparticles (NPs) in the colloidal solution and/or on the substrate during EPD. Recently, Kvitek and Hrbac oppugned our preferential oriented connection mechanism, based on the “unreasonable value of surface potential”, and presented chemical mechanism with two possible processes, (i) anodic dissolution of silver NPs and subsequent reduction at cathode and (ii) direct water electrolysis induced oxidation and back-reduction of primary silver NPs. They believed that the proposed chemical mechanism was more probable than the mechanism of oriented connection. Here are some replies.
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1. SOME INFORMATION The ζ potential of the initially prepared Ag colloidal solution was 29 mV and pH = 7. All the morphologies mentioned were formed on the surface of the cathode electrode (the polished silicon wafer). The voltage between electrodes was increased gradually under constant current mode. For 50 μA/cm2, the initial voltage was around 8 V and increased to 30 V after EPD for 10 h. 2. ABOUT ESTIMATION OF SURFACE POTENTIAL The surface potential deduced by Kvitek and Hrbac seems to be wrong. To our simple estimation from the data1 concerning the concentration and size of silver NPs composing the dispersion used for EPD process, the nanoparticle number (N) in the solution between the electrodes is 5.3 1012, instead of 5.3 1011; the total surface area (S) of particles in the solution is 150 cm2, instead of 15 cm2; the surface charge density (q) per nanoparticle is 0.025C/cm2, instead of 0.25C/cm2; and the surface potential (U) of the Ag particle is about 5000 V, instead of 5.4 1026V (the detailed estimation is seen in the Appendix of this Reply). Obviously, the estimated value of 5000 V for the surface potential is still too high. The exorbitant estimated value originates from the assumption that the charge was transferred r 2011 American Chemical Society
only by silver nanoparticles present in the system. In fact, it is unreasonable to suppose that all the charge during EPD process is transferred by silver nanoparticles in the colloidal solution. The water electrolysis could occur in our system, especially in the late stage of EPD when the voltage between the electrodes becomes higher, which consumed some charges. It means that the value 0.025 C/cm2 for the surface charge density of nanoparticles was overestimated.
3. ABOUT DIRECT WATER ELECTROLYSIS AND ANODIC DISSOLUTION As we know, Ag is insoluble in water. In our case, Ag NPs in water (pH = 7) should not dissolve. However, the water electrolysis could occur in our system, especially in the late stage of EPD when the voltage between the electrodes becomes higher. If anodic dissolution occurred during EPD, Agþ ions would be formed in the solution, and addition of Cl- ions would induce precipitate AgCl due to its very low solubility product. To confirm such possible anodic dissolution, further experiment has been performed by dropping Cl- ions into the solution during EPD. However, no change was found, indicating no detectable anodic dissolution. 4. REMARKS In our electrophoretic deposition, several processes, such as physical and chemical mechanisms, could take place and further work is needed. For formation of the regularly shaped single crystal on the cathode, however, we believe that the physical mechanism, proposed in ref 1 is more probable than the chemical mechanism; i.e., the regularly shaped single crystal is mainly induced by the preferential oriented connection of the Ag NPs in the colloidal solution and/or on the substrate during EPD (the details are seen in ref 1). ’ APPENDIX Some Basic Calculations 1. Ag Particle Number in the Colloidal Solution. Ag content (W) in the colloidal solution is 0.05 g/L, volume (V) of the colloidal solution is 16 mL, and Ag particle diameter (2R) is 30 nm.1 Letting Ag density (F) 10.5 g/cm3, we can get the Received: October 17, 2010 Revised: January 29, 2011 Published: February 28, 2011 4982
dx.doi.org/10.1021/jp109960q | J. Phys. Chem. C 2011, 115, 4982–4983
The Journal of Physical Chemistry C
COMMENT
particle number (N) in the colloidal solution, W V 3 0:05 0:016 N ¼4 3 ¼ 5:3 1012 ¼ 3 10:5 10-21 4π 15 3 πR F 3
2. Total Surface Area (S) of Particles in the Solution.
S ¼ N 3 4πR 2 ¼ 5:3 1012 4π 152 10-14 ¼ 150 cm2
3. Surface Charge Density (q) per Nanoparticle. EPD was carried out at a constant current density (i) of 50 μA/cm2 for time (t) of 14 h, and working area (s) of electrode is 1.5 cm2. On the assumption that the charge was transferred only by silver nanoparticles present in the system, we have
i s t 50 10-6 1:5 14 3600 ¼ 0:025 C=cm2 q¼ 3 3 ¼ 150 S
4. Surface Potential (U) of the Ag Particle. For a charge (Q) on conducting sphere, we have
U ¼
4πR 2 3 q R q Q ¼ ¼ 3 4πεr ε0 R εr ε0 4πεr ε0 R
where ε0 is the dielectric constant in vacuum (8.8538 10-12) and εr is the relative dielectric constant in dielectric (about 80 for water). So, if estimating as a charge on conducting sphere, we have U ¼
15 10-9 0:025 104 5000V 80 8:8538 10-12
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ86-551-5591434.
’ REFERENCES (1) Yang, S. K.; Cai, W. P.; Liu, G. Q.; Zeng, H. B. J. Phys. Chem. C 2009, 113, 7692.
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dx.doi.org/10.1021/jp109960q |J. Phys. Chem. C 2011, 115, 4982–4983
