**3.1 Characterization analysis of Cu/TiO2 film**

**Figure 3** shows SEM image of Cu/TiO2 film coated on netlike glass disc [28]. The SEM image was taken at 1500 times magnification. **Figure 4** shows EPMA image of *CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants DOI: http://dx.doi.org/10.5772/intechopen.93105*

Cu/TiO2 film coated on netlike glass disc [28]. EPMA analysis was carried out for SEM images taken by 1500 times magnification. In EPMA image, the concentration of each element in observation area is indicated by the different colors. Light colors, for example, white, pink, and red indicate that the amount of element is large, while dark colors like black and blue indicate that the amount of element is small.

From these figures, it can be observed that TiO2 film was coated on netlike glass fiber. During firing process, the temperature profile of TiO2 solution adhered on the netlike glass disc was not even due to the different thermal conductivities of Ti and SiO2. Their thermal conductivities of Ti and SiO2 at 600 K are 19.4 and 1.8 W/(mK) [30], respectively. Due to thermal expansion and shrinkage around netlike glass fiber, it can be considered that thermal crack is formed on the TiO2 film.

In addition, it is observed from **Figure 4** that nanosized Cu particles are loaded on TiO2 uniformly, resulted from that the pulse arc plasma method can emit nanosized Cu particles.

To evaluate the amount of loaded Cu within TiO2 film quantitatively, the observation area, which is the center of netlike glass disc, of diameter of 300 μm is analyzed by EPMA. The ratio of Cu to Ti is counted by averaging the data obtained in this area. As a result, the weight percentages of elements of Cu and Ti in the Cu/ TiO2 film are 0.6 and 99.4 wt%, respectively.

**Figure 3.** *SEM image of Cu/TiO2 film coated on netlike glass disc.*

**Figure 4.** *EPMA image of Cu/TiO2 film coated on netlike glass disc.*

**Figures 5** and **6** show TEM and EDX images of Cu/TiO2 film, respectively [27]. ESX analysis was carried out using TEM image taken by 150,000 times magnification. According to **Figure 6**, it is observed that Cu particles are distributed in TiO2 film. Though many Cu particles are loaded on the upside of TiO2 film, it is not confirmed that the Cu layer is formed.

**Figure 7** shows EELS spectra of Cu in Cu/TiO2 film [27]. From this figure, the peaks at around 932 and 952 eV can be observed. Compared to the report investigating peaks of Cu, Cu2O, and CuO [31], the EELS spectra of Cu2O matches with **Figure 7**. Therefore, Cu in Cu/TiO2 prepared in this study exists as Cu+ ion in Cu2O. It was reported that the heterojunctions between CuO and TiO2 contributed to the promotion of the photoactivity [32]. In addition, it was reported that Cu<sup>+</sup> was more active than Cu2+ [33]. Therefore, it is expected that Cu<sup>+</sup> would play a role to enhance the CO2 reduction performance in this study. **Figure 8** shows EELS spectra of TiO2 referred from EELS data base [34]. Comparing **Figure 8** with **Figure 7**, EELS spectra of TiO2 is very different from EELS spectra of Cu in Cu/TiO2.
