J. Phys. Chem. 1996, 100, 1833-1837
1833
Gold Particles Deposited on Electrodes in Salt Solutions under Different Potentials Da-Ling Lu and Ken-ichi Tanaka* The Institute for Solid State Physics, The UniVersity of Tokyo, 7-22-1, Roppongi, Minato-ku, Tokyo 106, Japan ReceiVed: July 31, 1995; In Final Form: October 19, 1995X
The crystal habit of gold particles formed on an amorphous carbon film at different electrode potentials in salt solutions was studied by means of transmission electron microscopy. It was found that the results of this experiment are consistent with the results of our previous experiments using acid solutions. That is, the gold particles grown at negative electrode potentials vs SCE are mainly decahedral and icosahedral, while the particles formed at positive electrode potentials vs SCE are normal fcc octahedral particles and polycrystalline particles. As the surface energy of the hexagonal structure is lower than that of the (1×1) structure under negative electrode potentials (SCE), the shape of fine gold particles is influenced by the negative electrode potential. We also found an interesting phenomenon of agglomeration of the gold particles at electrode potentials more negative than that of hydrogen evolution.
Introduction
Experimental Section
Crystal habits and structures of fine particles are sometimes quite different from those of the corresponding large crystals. Gold fine particles evaporated on NaCl under ultrahigh vacuum (UHV) conditions are pentagonal decahedral and hexagonal icosahedral in shape. These particles were designated as “multiply twinned particles” (MTP),1,2 being composed of five and 20 tetrahedrons, respectively. It was calculated3,4 that the total energies of the decahedral and the icosahedral gold particles are lower than that of tetrahedral gold particles with fcc structure. The structures of three low-index surfaces of gold singlecrystal electrodes have been investigated in recent years by the in situ electrochemical scanning tunneling microscope.5-8 It has been shown that the surface structure of the gold singlecrystal electrode undergoes reversible change with electrode potential. That is, at negative electrode potentials (SCE) a reconstruction occurs on the electrode surface and the (1×1) structure reappears after the electrode potential is raised from negative to positive potentials vs SCE. The reconstructed Au(100)-(5×20), Au(110)-(1×2) or -(1×3,) and Au(111)-(x3×23) surfaces have hexagonal structures, which are more dense structures than the (1×1) structures of these faces. Taking account of these results mentioned above, the gold particles grown from solution on an electrode at different electrode potentials may change in structure or habit depending on the electrode potential. On account of this consideration, we generated gold particles on a carbon film in acid solutions at different electrode potentials and observed these particles by transmission electron microscopy (TEM).9,10 We found that the majority of the gold particles formed at negative electrode potentials (SCE) are decahedral with a pentagonal profile and icosahedral with a hexagonal profile. On the other hand, the gold particles grown at positive electrode potentials (SCE) are either fcc octahedral single-crystal particles with square or hexagonal profiles or polycrystalline particles. In this paper we report the results of gold particles formed in neutral salt solutions at different electrode potentials.
Experimental details were described elsewhere.9,10 A conventional three-compartment electrochemical cell was used for the electrodeposition of gold particles from solution onto a working electrode. The working electrode was a gold mesh of the type used for TEM. The gold mesh was first covered with a collodion film on which an amorphous carbon film was evaporated and then was used as the working electrode. The counter electrode was a 0.5 mm platinum wire. A saturated calomel electrode (SCE) was used as a reference electrode. Solutions were prepared by using commercially available chemicals (CsClO4 and KClO4, Aldrich; HAuCl4, Kanto) and triple-distilled highly pure water. The concentrations of solutions were 50 mM CsClO4 + 1 mM HAuCl4 and 50 mM KClO4 + 1 mM HAuCl4, respectively. The electrolyte in the main compartment of the cell was deaerated by purified argon. The experiment was carried out at room temperature, and all potentials were measured and are reported vs SCE. The gold particles grown on the carbon film of the gold mesh in solutions at different electrode potentials were observed by a highresolution TEM (Hitachi 9000).
* To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-1833$12.00/0
Results and Discussion The gold particles deposited in 50 mM CsClO4 + 1 mM HAuCl4 and 50 mM KClO4 + 1 mM HAuCl4 at different electrode potentials are shown in Figure 1. It is clear that these results are identical with the results obtained in aqueous acid solutions.9,10 That is, at positive electrode potentials (SCE) the grown particles are of the fcc octahedral type with hexagonal profiles and polycrystalline particles with a polygonal appearance (Figure 1a,e). On the other hand, the gold particles formed at negative electrode potentials (SCE) include the decahedral ones with pentagonal profile and the icosahedral ones with hexagonal profile (Figure 1b,f). These results are open to the same explanation as that for acid solutions reported previously.9,10 That is, at positive electrode potentials (SCE) the (1×1) surface has a lower surface free energy,11 so that the Au atoms deposited from solution onto a carbon film favor growth of an ordinary bulk crystal in keeping with the (1×1) surfaces during the growth of the particles; thus, the particles grown in © 1996 American Chemical Society
1834 J. Phys. Chem., Vol. 100, No. 5, 1996
Lu and Tanaka
Figure 1. TEM images of gold particles grown for 60 s in 50 mM CsClO4 + 1 mM HAuCl4 at (a) 0.4 V, (b) -0.6 V, (c) -0.8 V, and (d) -1.2 V and in 50 mM KClO4 + 1 mM HAuCl4 at (e) 0.4 V, (f) -0.6 V, (g) -0.8 V, and (h) -1.2 V. The scale of the images is shown in (b) except as noted in (a) and (e).
Gold Particles Deposited on Electrodes
Figure 2. Diagram of the results of gold particles grown in acid and salt solutions taken by means of TEM comparing with the results5-7 on the surface structure of gold single crystals in 0.1 M HClO4 solution obtained by in situ electrochemical STM.
this case are fcc single-crystal particles. On the other hand, the hexagonal arrangement of the gold surface has a lower surface free energy at negative electrode potentials (SCE). Therefore, the deposited gold atoms will form particles in keeping with the hexagonal outermost stacking under negative electrode potentials (SCE), and then the particles grow into decahedra or icosahedra which may have higher densities than the fcc structure in a limited size. As the decahedral and the icosahedral structures are stable in a limited particle size under negative electrode potentials (SCE), these particles cannot grow larger than a limited size even if the deposition time is made longer. When a negative electrode potential (SCE) is applied for a longer time, the number of particles increases and the particles contact each other to coalesce and grow into short chains (see Figure 1b,f). From the results in aqueous acid solutions and in neutral salt solutions, we can conclude that the Au atoms deposited on a carbon film form fcc crystal particles at positive electrode potentials (SCE) and that the Au atoms on a carbon film prefer to keep the hexagonal arrangement for the outermost layer of the particles at negative electrode potentials (SCE). In Figure 2, our experimental results and the potential-induced reconstruction obtained by means of in situ electrochemical STM5-7 are compared with respect to the electrode potential. It is obvious that the potential-induced reconstruction and the effect of potential on the crystal habit are controlled by the same factors. In neutral salt solutions we can apply a more negative potential on the electrode than that in acid solutions because H2 gas evolution is suppressed. We found an interesting common phenomenon that the deposited gold particles in the two different salt solutions gave longer chains under more negative electrode potentials, below -0.7 V (SCE) (Figure 1c,d,g,h). It was observed even in the shorter deposition time. The more negative the electrode potential, the longer the chains of particles grown for an equal deposition time. Below about -0.7 V (SCE), H2 gas was bubbled at the gold mesh in these two solutions including 1 mM HAuCl4. The agglomeration of the gold particles into longer chains is not carried by the evolution of H2, because the gold particles grown at -0.6 V (SCE) in 0.1 M HClO4 + 1 mM HAuCl4 solution, where H2 gas is evolved, did not coalesce to form longer chains (Figure 3). Therefore, we can rule out the effect of H2 evolution and conclude that the gold particles make the longer chains by the agglomeration only in alkaline metal salt solution at a highly negative electrode potential (SCE). In the gas phase, it was shown that alkali metal atoms adsorbed on Au(111) surface induce local increases of negative charge density on the surface
J. Phys. Chem., Vol. 100, No. 5, 1996 1835
Figure 3. TEM image of gold particles grown in 0.1 M HClO4 + 1 mM HAuCl4 at -0.6 V for 60 s.
Figure 4. (a) TEM image of gold particles grown for 10 min in 50 mM CsClO4 + 1 mM HAuCl4 at -1.2 V. (b) The dark field image of (a). It can be seen clearly that the gold particles retain their original bulk structure after coalescence with each other.
which weaken the bonding of Au atoms between the topmost and the second layer of Au(111).11 As a result, the contraction of the surface layer is induced by the adsorption of alkali metals. The gold electrode as well as gold particles at negative electrode potentials (SCE) may have negative charge density on the surface so that the gold atoms on the surface tend to form a more dense arrangement under negative electrode potentials (SCE) for the same reason. When small particles under negative electrode potentials (SCE) contact each other, the surface atoms which may be a little loose can diffuse quite quickly for coalescing. As the hexagonal structure of the fine particles might be stable at the negative electrode potentials (SCE), the coalescing only increases the outermost hexagonal surface of the particles, but the bulk structure of the particles may not
1836 J. Phys. Chem., Vol. 100, No. 5, 1996
Lu and Tanaka It is also interesting that some stronger spots and weaker diffraction rings which do not belong to the Debye-Scherrer rings of gold appeared inside the {111} Debye-Scherrer ring and between the other high-index Debye-Scherrer rings of gold particles which were formed at -1.2 V (SCE) in the potassium and cesium solutions (Figure 5). However, no such diffraction spots or rings were observed at less negative electrode potentials even when the deposition time was longer. The elemental analysis of the particles formed at -1.2 V (SCE) in 50 mM CsClO4 + 1 mM HAuCl4 solution was performed by scanning electron microscopy (SEM). It can be seen (Figure 6) that in addition to gold there were cesium and chlorine in the products. Taking account of the fact that the anions were common in these two solutions and the distances L from direct spot to diffraction spots and rings are different for these two solutions (L of spots with strong intensity inside the {111} Debye-Scherrer ring of gold was 5.8 mm for CsClO4 solution and 6.4 mm for KClO4 solution, respectively), it can be supposed that some products including Cs or K might be coproduced with gold particles from these two solutions. It should be pointed out that the reversible Nernst potentials for the deposition of Cs and K are -2.923 V for Cs and -2.925 V for K vs NHE, respectively. Generally speaking, Cs+ or K+ cannot deposit on the surface of the electrode at -1.2 V vs SCE (about -1.44 V vs NHE), which is much higher than the UPD potential of Cs+ or K+. Further study to investigate this phenomenon is needed. Conclusion
Figure 5. TEM selected-area electron diffraction patterns of grown particles at -1.2 V for 10 min in (a) 50 mM CsClO4 + 1 mM HAuCl4 and (b) 50 mM KClO4 + 1 mM HAuCl4. The diffraction spots inside the {111} Debye-Scherrer ring of gold particles do not belong to the diffraction pattern of the gold particles.
change (Figure 4). Thus, the coalesced small particles form a long chain. We found a similar agglomeration phenomenon in vacuum during the observation of the particles by TEM. Due to the effect of the electron beam, two small particles linked and the surface area to volume ratio was decreased so as to lower the total energy.
We investigated the structure and habit of gold particles grown at different electrode potentials in neutral salt solutions. The shape of gold particles formed at positive and negative electrode potentials (SCE) is the same as the shape of gold particles formed at the identical electrode potentials in acid solutions reported previously. That is, the gold particles grown at positive electrode potentials (SCE) are fcc structure octahedral single-crystal particles and polycrystalline particles. On the other hand, at negative electrode potentials (SCE) the particles formed are mainly decahedra and icosahedra, which may have more dense structures than the fcc structure. At more negative electrode potentials the gold particles grow to form longer chains, which are not observed at less negative electrode
Figure 6. Element analysis of particles formed in CsClO4 + 1 mM HAuCl4 at -1.2 V for 10 min obtained by SEM.
Gold Particles Deposited on Electrodes potentials. We also observed the Cs and K compounds formed at certain more negative electrode potentials. Acknowledgment. We gratefully acknowledge Prof. A. Aramata of Hokkaido University for stimulating discussions. We also thank Mr. K. Suzuki and Mr. M. Ichihara of ISSP for their help in the TEM experiment. This work was supported by a Grant-in-Aid for Science Research (05403011) of the Ministry of Education, Science and Culture of Japan. References and Notes (1) Ino, S. J. Phys. Soc. Jpn. 1966, 21, 346. (2) Allpress, J. G.; Sanders, J. V. Surf. Sci. 1967, 7, 1.
J. Phys. Chem., Vol. 100, No. 5, 1996 1837 (3) Ino, S. J. Phys. Soc. Jpn. 1969, 27, 941. (4) Fukano, Y.; Wayman, C. M. J. Appl. Phys. 1969, 40, 1656. (5) Gao, X.; Hamelin, A.; Weaver, M. J. Phys. ReV. Lett. 1991, 67, 618. (6) Gao, X.; Hamelin, A.; Weaver, M. J. Phys. ReV. B 1991, 44, 10983. (7) Gao, X.; Hamelin, A.; Weaver, M. J. J. Chem. Phys. 1991, 95, 6993. (8) Magnussen, O. M.; Hotlos, J.; Behm, R. J.; Batina, N.; Kolb, D. M. Surf. Sci. 1993, 296, 310. (9) Lu, D.; Okawa, Y.; Suzuki, K.; Tanaka, K. Surf. Sci. 1995, 325, L397. (10) Lu, D.; Okawa, Y.; Ichihara, M.; Aramata, A.; Tanaka, K. To be published in J. Electroanal. Chem. (11) Barth, J. V.; Behm, R. J.; Ertl, G. Surf. Sci. Lett. 1994, 302, L319.
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