2.3.1 SEM and EDS Characterization
After AgNO3(aq) was added to a stirring solution containing CTAC(aq) and HNO3(aq), the mixture turned white and opaque immediately. This indicated the formation of suspended AgCl colloids. To this mixture, an SPC electrode with a piece
Figure 2.1 Optical images of the commercial SPC electrode (A) before and (B) after the Ag NW growth.
of Cu foil attached to its contact was immersed. The exposed electrode surface turned gray gradually (Figure 2.1). To avoid oxidation, the as-prepared Ag NWs electrodes were stored in a N2 filled glove box to prevent surface oxidation. In Figure 2.2A, a SEM image shows that the electrode surface is covered by a lot of urchin-like NWs.
Based on the EDS measurement displayed in Figure 2.2A (inset), we conclude that the NWs are composed of Ag only. The C signal is assigned to the SPC substrate. From the high magnification image displayed in Figure 2.2B, some branching of the NWs can be observed. The diameters of the NWs are estimated to be about 100 nm while the lengths are found to be in the range 3 – 10 µm. With different growth conditions, the diameters can vary from 80 to 120 nm while the lengths can differ from 1 to 10 µm. An individual cluster of NWs formed initially at 2 h (Figure 2.3) show that many
NWs protruded from a surface on the substrate to form the urchin-like morphology.
The side-view image is shown in Figure 2.2C. We can notice that whole urchin-like Ag NWs arise from the electrode surface. The image displays that in each urchin-like structure, the NWs radiate from an apparent common nucleus. Other growth conditions of different Ag samples are shown in appendix.
(A)
(B) (A)
(B)
Figure 2.2 SEM studies of urchin-like Ag NWs on a SPC electrode. (A) Low magnification surface image (inset, EDS of an area in (A)), (B) high magnification image, and (C) high magnification side-view image.
Figure 2.3 A cluster of urchin-like Ag NWs formed initially on a SPC electrode at 2 h.
(C)
(B) (A)
(C) (C)
(B) (B) (A)
(A)
Figure 2.4 XRD pattern of Ag NWs on a SPC electrode.
2.3.2 XRD Analysis
The XRD patterns are shown in Figure 2.4. The peaks at 2θ=38.1°, 44.3°, 64.5°, 77.4° are assigned to Ag (111), (200), (220), and (311) reflections, respectively (JCPDS file 04-0783).23 The broad band around 54° comes from the SPC electrode.
From the patterns, the lattice parameter a was estimated to be 0.409 nm, consistent with the value reported for Ag.23
2.3.3 TEM Characterization
TEM studies of a group of urchin-like NWs are shown in Figure 2.5. The image in Figure 2.5A reveals an overall morphology closely related to the ones presented in Figure 2.2C and Figure 2.3. Extension of the NWs from an apparent initial growth point and branching of some NWs are observed. The SAED patterns of the tip and the root of a NW, which branches from another NW stem, are shown in Figure 2.5B and 2.5C, respectively. Interestingly, they display the same set of dot patterns revealing their single crystalline nature. They both correspond to the [001] crystallographic zone axis of an fcc structure with the lattice parameter a calculated to be 0.41 nm.23
SPC electrode
Ag 3C
(JCPDS 04-0783) Ag NWs on a SPC electrode
Intensity (a.u.)
Figure 2.5 (A) TEM image of urchin-like Ag NWs. SAED patterns from (B) the white-dashed and (C) the yellow-dashed circles in (A).
Figure 2.6 Effect of lengths of growth time. SEM images of urchin-like Ag NWs grown for (A) 1 h, (B) 3 h, and (C) 6 h.
The NW growth direction is determined to be along the [-1-10] direction. The data suggest that the overall branched Ag NW structure is a single crystal.
2.3.4 Reaction Time Influence
SEM images of urchin-like Ag NWs grown from different lengths of time were shown in Figure 2.6. At 1 h, there were few urchin-like NWs with lengths less than 1
(A)
Figure 2.7 Proposed mechanism of urchin-like Ag nanowire.
µm on the SPC electrode (Figure 2.6A). At 3 h, more urchin-like clusters with longer
NWs were observed, as shown in Figure 2.6B. When the growth was extended to 6 h, more coverage of longer NWs on the electrode surface was found in Figure 2.6C.
2.3.5 Proposed Growth Mechanism
We suggest that our previously proposed growth pathways for one-dimensional Cu, Ag, and Au nanostructures are applicable for the urchin-like Ag NWs too.13,24-26 The preparation of Ag NW is dependent on the presence of CTAC and HNO3. All the growth process is presented in Figure 2.7. At the beginning, there are 7.5 mM Ag+(aq) ions in this reaction solution. Some Ag+(aq) ions (2.1 mM) would reduce by Cu(s) to form Ag(s) clusters through the galvanic reduction, 2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+(aq) E° = 0.46 V.27 The clusters enlarged as more Ag(s) reduced to become urchin nucleus on the electrode. At the same time, the other Ag+(aq) ions (5.4 mM) combined with
Cl-(aq) anions (5.4 mM) from CTAC molecules to form AgCl(s) colloids which covered by a shell of CTAC molecules. Since the reaction 2AgCl(s) + Cu(s) → 2Ag(s) + Cu2+(aq)
+ 2Cl-(aq) E° = -0.12 V27 is not thermodynamically favored, the source of Ag cannot obtained via this route. The growth cannot just proceed via the reaction 2Ag+(aq) + Cu(s)
→ 2Ag(s) + Cu2+(aq) without CTAC either because this reaction alone did not produce the urchin-like NWs structure. As the literature reports, it is known that AgCl(s) can be reduced to Ag(s) by light.28 The Ag(s) clusters reduced by light could be seeds randomly adsorbed on the urchin nucleus and initiated the growth of Ag NWs assisted by CTAC as the surface capping reagent, which may selectively adsorbed on low-index facets to form a bilayer interface structure.29,30 The presence of NO3
-(aq) ions in the growth solution is another determinant for controlling the crystal shape in the system. The function of the ions may oxidize Ag(s) into Ag+(aq) ions. During the crystal growth, the less stable facets might be oxidized by NO3-(aq) easily, leaving the more stable facets exposed for further developments. As a result, all these factors function cooperatively in the reaction to direct the crystals to grow into urchin-like Ag NWs.
2.3.6 Extensive Application on Electrochemical Deposition
In our reaction, reduction of Ag+(aq) ions were contributed by galvanic reductions.
However, we discovered that Ag NWs could be grown direct on Au or Pt seeding layers on Indium Tin Oxide (ITO) substrate via a simple two-electrode electrochemical deposition method in the same reaction solution at 293 K. The results are shown in Figure 2.8. Figure 2.8A to 2.8D display that there are some Ag NWs on the ITO substrates. The diameters are about 60 nm, which are smaller than the ones react by galvanic reductions. The lengths are found to be in the range 1 – 5 µm. Based on the EDS measurement displayed in Figure 2.8A (inset) and 2.8C (inset), we
Figure 2.8 SEM images of Ag NWs on ITO substrates with different kinds seeding laeyer. On Au seeding layer (A) Low magnification surface image (inset, EDS of an area in (A)), (B) high magnification surface image. On Pt seeding layer (C) Low magnification surface image (inset, EDS of an area in (C)), (D) high magnification surface image. (E) TEM image of Ag NWs in (C) (inset, [001] zone SAED of the Ag NW) (F) HRTEM image of the red marked circle region in (E).
conclude that the NWs are composed of Ag only. TEM characterizations are displayed in Figure 2.8E and 2.8F. The SAED pattern in Figure 2.8E (inset) shows a spot pattern, which can be indexed to be [001] zone axis of Ag reveals that the Ag NW is single crystalline. From the pattern, the growth direction of Ag NW is determined to be along [110] direction, which is identical to the one reacted by galvanic reduction.
Figure 2.8F presents an HRTEM image from the red marked circle region in Figure 2.8E. The dihedral angle between [220] and [200] is 134.8°, which is close to the theoretical value of an fcc structure. The [220] and [200] d-spacing are measured to be
(A)
(B)
(C)
(D)
(E)
(F)
0.14 nm and 0.20 nm, respectively. Both the value are close to the previous report of Ag, 0.144 nm and 0.204 nm.23