3.2 HR-TEM analysis

HR-TEM bright-field photomicrographs were used to determine the actual shape and size distribution of the as-synthesized samples UZ and DZ (1.0 mol% Ag) NPs. Therefore, it is important to know that the differences in the NP size and the dense distribution were used to explain these phenomena. Indeed, the larger size and highly dense distribution of Ag NPs mean that they could not preferentially attach to specific sites on the photoanode surface, resulting in a decrease of the catalytic activity [23]. Figures 3 and 4 depict the HR-TEM micrograph of the samples UZ and DZ (1.0 mol% Ag) NPs, respectively. As revealed in Figure 3A and 4A, the HR-TEM micrograph suggests that ZnO NPs are, well, crystallized with an intermediate or poor agglomeration. Moreover, as presented in Figure 3A and 4A, the HR-TEM images show the distribution of semispherical morphologies of ZnO NPs. The selected area electron diffraction (SAED) patterns of samples UZ and DZ (1.0 mol% Ag) NPs are illustrated in Figure 3B and 4B. This confirms the crystalline nature of the ZnO NPs. Five bright concentric diffraction rings in the SAED can be indexed as the (100), (002), (101), (102), and (110) planes of the wurtzite ZnO. The quantitative analysis of the size distribution for the ZnO NPs was achieved by plotting the histogram in Figure 3C and 4C. The size distribution of UZ and DZ (1.0 mol% Ag) NPs is given by mean standard deviation (SD), which can be produced from fitting the histogram using the normal function [solid brown line in Figures 3C and 4C]. The estimated values of D of the samples UZ and DZ (1.0 mol % Ag) NPs are 22 and 24 nm, respectively (Table 1). It is evident that the estimated D from HR-TEM images is in good agreement with the D estimated from the XRD patterns. The lattice fringe (Figures 3D and 4D) gives interlayer spacing of 0.22 and 0.24 nm, which corresponds to the d-spacing of (101) plane (d101 = 0.203 and 0.247 nm from XRD calculation for UZ and DZ (1.0 mol% Ag) NPs, respectively. Some small black dots are shown in Figure 4A, which can be indicated to the Ag NPs. The size distribution of Ag NPs was obtained from fitting the histogram using the normal function [solid red line in Figure 5]. The estimated value of Ag NPs size is 3.55 nm (quantum dos). The size and the agglomeration of Ag NPs can produce shift in the Fermi level, and therefore, the recombination of the accumulated electrons with the oxidized redox electrolyte and/or dye species will change [13]. LSPR is barely dependent on the NPL; near-field plasmonic enhancement increases significantly with decreasing particle size [13]. Metal plasmon must be 16 nm in size to escape recombination and produce enhanced photovoltaic parameters [15]. Another

Figure 3.

HR-TEM micrograph of sample UZ NPs (A) micrograph image, (B) diffraction patterns, (C) histograms, and (D) selected electron diffraction area.

Nanoplasmonic for Solar Energy Conversion Devices DOI: http://dx.doi.org/10.5772/intechopen.84953

Figure 4.

HR-TEM micrograph of sample DZ (1.0 Mol% Ag) NP (A) micrograph image, (B) diffraction patterns, (C) histograms, and (D) selected electron diffraction area.

Figure 5. Histogram of black dot sample DZ (1.0 Mol% Ag) NPs.

study has shown that smaller plasmonic Ag NPs, with an average diameter less than 25 nm in the intrinsic size regime, would exhibit an increased plasmon bandwidth [11]. Conversely, as the Ag NPs size became larger (32 nm), Ag NPs easily aggregated and formed islands. This could not preferentially attach to specific sites on the MO surface, resulting in the decrease of catalytic activity, thereby reducing the reaction rate between MO/dye [15]. In addition, as the thickness of the film is around 32 nm, the mixed films might result in blockage of sunlight which would prevent the incident light from reaching the MO NPs effectively, resulting in the decrease in the photocurrent density.
