The absorption spectra of the different-ratio Zn-CIS quantum dots employed in the present study are shown in Figure 5.2(a). These particles exhibit absorption in the visible with an onset corresponding to different Cu/Zn ratios. The shift of the onset absorption to lower wavelengths with decreasing n value represents the Zn doping effect in these particles. By comparing the excitonic transition (660, 570, 530 nm for n
= 3, 2, and 1, respectively.) with the absorption curve reported by Nakamura and co-workers, [75] the average particle diameter of these samples was identified as 4.4, 4.5, and 3.5 nm respectively. It is noticeable that the absorption and photoluminescence (not shown here) properties of Zn-CIS nanocrystal are not only dominated by its particle size but also the concentration of Zn doping. The Cu site in the lattice is preferentially substituted by Zn and the substitution is found to be more energetically favorable to prevent anti-site defects, which will affect the absorption spectrum. Therefore, by increasing the Cu/Zn ratio (or decreasing Zn concentration) in the reaction solution, the absorption edge shifted from ca. 400 to ca. 700 nm toward the longer wavelength. Figure 5.2(b) shows the similar absorption spectra of hydrophilic QDs after ligand exchange, which means that particle size and optical property of the QDs were not changed after surface modification compared to original QDs (in toluene). Exchange of TOP by thiols at the surface of Zn-CIS nanocrystals was accompanied by a slight blue shift of absorption spectrum. Similar phenomena were also reported for CdSe ad CdTe QDs. [5,119] The blue shift was due to a redistribution of electronic density and an increase in confinement energy induced by the stronger bond between the thiol and the Cu atoms at the Zn-CIS nanocrystal surface. These quantum dots were then deposited on ZnO nanowires for spectroscopic and photoelectrochemical investigation.
Figure 5.3(a) shows a cross-sectional scanning electron micrography of typical ZnO nanowires used as photoanodes. The nanowires are approximately 50-60 nm in diameter and ~5 μm length, indicating a relatively large surface area to accommodate high concentration of sensitizer molecules (and higher concentration of the QD deposition). All the samples used in this study displayed good stoichiometric composition and crystallinity according to the XRD and XPS analyses (not shown here).
400 500 600 700 800 900 in water
(b) n=2 n=3
n=1
Absorbance (a.u.)
Wavelength/nm
in toluene
(a)
n=3 n=1 n=2
Figure 5.2 Absorption spectra of Zn-CIS quantum dots with various Cu/Zn ratios from n=1, n=2, and n=3 in (a) toluene and (b) water.
Figure 5.3(b) shows a high-resolution transmission electron micrograph (HRTEM) of a MPA-capped Zn-CIS QD isolated from a colloidal suspension. TEM images indicate that the QDs are single crystals with diameters ranging from 3 to 4 nm, i.e., 3.5 nm in average. Such MPA-capped Zn-CIS QDs could attach to the ZnO surfaces via carboxylate groups. A bright-field TEM image of a ZnO nanowire largely covered with an ensemble of Zn-CIS QDs was shown in Figure 5.3(c). The QDs are visible as the circular dark spots on the surface of the nanowire and EDX confirms the Zn-CIS composition. However, for a sample deposited with high concentration QDs, there will be a certain degree of aggregation during the QD deposition. In this condition, a size larger than 3.5 nm is possibly observed in Figure 5.3(c). The HRTEM image of the nanowire edge (Figure 5.3(d)) provides more compelling evidence that QDs are attached to the nanowire surface. An abrupt transition is observed between the (1010) lattice planes of the ZnO nanowire and the (112) lattice planes of the Zn-CIS QD, indicating an intimate contact between the QD and the nanowire within an approximate distance of 0.29 nm, or one ZnO (1010) lattice spacing.
2 nm (b)
5 50 nm
(c) (d)
5 nm 1μm
(a)
Figure 5.3 (a) Cross-sectional scanning electron micrograph of ZnO nanowires. (b) High-resolution transmission electron micrograph of a Zn-CIS quantum dot capped with MPA. (c) Bright-field transmission electron micrograph of a ZnO nanowire decorated with Zn-CIS quantum dots. (d) High-resolution transmission electron micrograph of Zn-CIS quantum dots attached to a ZnO nanowire. Some quantum dots have been outlined.
Figure 5.4(a) shows the appearance of three solutions with different ratios of Zn-CIS quantum dots dispersed in water, and then deposited to OTE/ZnO nanowires (Figure 5.4(b)), respectively. The photographs show the electrodes of different appearance of, yellow, red, and black, corresponding to the deposition of the Zn-CIS QDs of 3.5, 4.5, and 4.4 nm size, [75] respectively. However, since prolonged immersion in the Zn-CIS solution does not further enhance the absorption ability of the Zn-CIS, it was then assumed that the coverage of Zn-CIS nanoparticles on ZnO nanowire surface is spaciously saturated as a monolayer deposition. This can be
further verified by the fact that the QD surface was first organically modified by MPA, wherein the surface interaction between ZnO nanowire and Zn-CIS QD resulted from a chemical anchorage of the acidic group to ZnO nanowire and mercapto group to Zn-CIS QD. A multilayer deposition may be sterically inhibited as a result of repulsive interactions of MPA molecules between two approaching QD nanoparticles.
The optical absorption properties of the Zn-CIS-coated ZnO nanowires are presented in Figure 5.4(c). It is evident that different-sized Zn-CIS particles exhibit excitonic transitions at 660, 570, and 530 nm. These excitonic peaks are similar to those observed for the QDs in the solution (Figure 5.2) and thus this confirms the binding of Zn-CIS QDs of varying sizes to the ZnO surface. Thus, the coloration of the ZnO nanowires offers an opportunity to selectively harvest the incident light. (The reflectance spectrum of an uncoated F-doped SnO2 substrate was used as reference.)