The X-Ray Diffraction Analyses

Figure 3-18a shows the XRD patterns of ZnO/TiO2 core-shell nanowires with various sputtering power, time as TiO2 deposition. Figure 3-18b shows the XRD patterns of the structure with various sputtering power, time after annealing 400℃ 1 hour in air. Figure 3-18c shows the XRD patterns of the structure with various sputtering power, time after annealing 500℃ 1 hour in air.

From Figure 3-18a, it only show the peak at ZnO(100), ZnO(101) and ZnO(002). It means that ZnO nanowires are wurtizite and no other phase appear and TiO2 shell on the ZnO nanowires is an amorphous type which are consistent with TEM analyses. From Figure 3-18b-c, it still did not appear any TiO2 peak after annealing. Because of the lattice mismatch of wurtizite ZnO between Rtile and Aatase TiO2, TiO2 is hard to form any crystal structure on single crystalline ZnO nanowires. After the TiO2 deposition, the ZnO(002) peak is somewhat a little shift form 34.300º to 34.502º. It means that the lattice constant of ZnO nanowres are changed. The small deviation of the lattice constants can be due to the lattice mismatch between TiO2 and ZnO.

Figure 3-18. The XRD patterns of ZnO/TiO2 core-shell structure with various sputtering power, time (a) as TiO2 deposition, (b) after annealing 400 1 hour ℃ in air, (c) after annealing 500 1 hour in air.℃

3.3.4 The Optical Properties

Figure 3-19 shows the the cathodeluminescence (CL) spectra of ZnO nanowires and ZnO/TiO2 core-shell structures as deposited. The ultraviolet (UV) emission peak (374 nm) of ZnO is generally attributed to the exciton-related activity, and the visible light region (450-700 nm) may be due to the transitions of native defects such as Zn vacancies, Zn interstitials, oxygen interstitials and single ionized oxygen vacancies. The imperfect boundaries and stacking faults of ZnO nanowires would cause the unstable surface states to trap impurities and further damage the optical property. Figure 3-19 show that the UV peak of ZnO/TiO2 core-shell nanowires was stronger than the peak of ZnO nanowires and the visible light region emission of ZnO/TiO2 core-shell structures became very weaker. It may be because that coating an amorphous TiO2 film can repair the surface-related defects. Thus, the visible light region emission decreased so that the UV emission become stronger. In conclusion, coating TiO2 film on ZnO is very important to improve the optical properties of ZnO.

In order to get better optical characteristic of ZnO, we put the ZnO/TiO2

core-shell nanowires in the oven for thermal annealing 400 and 500 1 hour ℃ ℃ under the air. Figure 3-20 show the CL spectras of ZnO/TiO2 core-shell

visible light region emission may be because the nonstoichiometry TiO2

attracted the oxygen in the ZnO nanowires in the thermal process. It will produce many single ionized oxygen vacancies (VO*) in the ZnO nanowires.

Another reason is that after annealing Ti may diffuse into the ZnO nanowires.

This phenomenon is resulted Zn vacancies (VZn), Zn interstitials (Zni), oxygen interstitials (Oi) and single ionized oxygen vacancies (VO*) in the ZnO nanwires. But the UV peak did not shift clearly and the ZnO(002) peak of XRD analyses is only a little shift. So that Ti may not diffuse into ZnO nanowires.

Therefore, the main reason may be the nonstoichiometry TiO2. In conclusion, ZnO/TiO2 core-shell nanowires as deposited had best optical property.

Figure 3-19. CL spectra of ZnO nanowires and ZnO/TiO2 core-shell nanowires as deposited.

a

b

3.3.5 The Field Emission Properties

The field emission characteristics of ZnO/TiO2 core-shell nanowires are indicated in Figure 3-22 and Table 3-3. Comparing with ZnO nanowries, ZnO/TiO2 core-shell nanowires had higher Eon and lower β value. In addition, as the sputtered power and time increased, the turn on field increased. The reason may be that the nanowires with thicker TiO2 film had lower aspect ratio.

So that it is due to worse field emission properties. Another reason is that there is a barrier high at the interface between TiO2 and ZnO as show in the Figure 3-21. The inter fields will let surface electron toward ZnO. It will let electrons pass through hardly so that nanowires needed a higher electric field to emit the electron. This phenomenon will reduce the field emission current and lower the β value. In other word, this phenomenon is an advantage of ZnO/TiO2

core-shell nanowries which are used in DSSCs. The surface fields within each nanowire can be used to enforce charge separation and thereby ensure that faster transport results in a longer diffusion length in the nanowires.

Figure 3-21.Schematic energy level diagram of a ZnO/TiO2 core-shell structure.

a

b

ZnO

Table 3-3. The detail data of field emission property of ZnO/TiO2 core-shell nanowires as deposited.

The field emission characteristics of ZnO/TiO2 core-shell nanowires after thermal treatment are indicated in Figure 3-23. The detail data of field emission properties of ZnO/TiO2 core-shell nanowires after annealing are indicated in Table 3-4. As the figures showed, the turn on fields and the field enhance factor of different sputtering conditions became similar. So aspect ratio is not the critical reason at this case. It is know from the CL analyses that there are many defects in the ZnO nanowires after annealing such as VZn, Zni, Oi and VO* etc.

The defects will catch the electrons and holes. Therefore, there is a large recombination current in the ZnO nanowires. This current will reduce the field emission current and the decrease the β value. So after thermal treatment the field emission characteristics of ZnO/TiO2 core-shell nanowires are poorer.

a

b

60w 30min Electrode Area 0.00709(cm²) 0.00709(cm²) 0.00709(cm²) Work Function 4.5 eV

Table 3-4. The detail data of field emission property of ZnO/TiO2 core-shell nanowires after annealing.