Page 176 - Multifunctional Photocatalytic Materials for Energy

162 Multifunctional Photocatalytic Materials for Energy

NH + H O « NH + + OH - (8.2)
3 2 4
2OH + Zn 2+ ® ZnOs () + HO (8.3)
-
2
Liquid phase synthesis processes can be separated into five general categories [29]:
(1) the zinc acetate hydrate (ZAH)-derived nano-colloidal sol-gel route [30], (2) ZAH
treated by sodium hydroxide (NaOH) or tetra methyl ammonium hydroxide (TMAH)
in alcoholic solutions [31], (3) sacrificial template-assisted fabrication of 1D ZnO NRs
[32], (4) spray pyrolysis for growth of thin films [33], and (5) electrophoresis [34].
Unlike liquid phase synthesis, gas phase synthesis is performed in closed chambers
under a gaseous environment. Normally, the synthesis process is carried out at high tem-
peratures ranging from 500°C to 1500°C. Some popular gas phase methods are (1) vapor
phase transport, which includes vapor solid (VS) and vapor liquid solid (VLS) growth
[35], (2) physical vapor deposition (PVD) [36], (3) chemical vapor deposition (CVD)
[37], (4) metal organic chemical vapor deposition (MOCVD) [38], (5) thermal oxidation
of pure Zn and condensation, and (6) microwave-assisted thermal decomposition [39].
Fig. 8.7 shows different SEM images of ZnO NRs grown in different patterns [27]. In
addition, some groups have reported that electrochemical fabrication is an easy and cost-
effective method for fabricating ZnO NR arrays, such as in electrochemical deposition.

8.2.2.3 Fabrication of ZnO NTs

Unlike NWs and NRs, NTs with a hollow cavity structure possess high porosity and
may offer a larger surface area for adsorption at the active layer. The synthesis meth-
ods for the aqueous growth of ZnO NTs at low temperatures are normally one-step
and two-step hydrothermal methods. Fig. 8.8 shows the typical SEM and TEM images
of ZnO NTs fabricated by using a hydrothermal method at low temperatures. In this
process, a 100 mL zinc nitrate (Zn(NO 3 ) 2 ) aqueous solution and a 100 mL hexameth-
ylenetetramine (HMT) aqueous solution with equal concentration (0.1 M) were mixed
together and kept under mild magnetic stirring for several minutes [40]. Then the mixed
solution was transferred into a flask or autoclave and heated at 60°C to 90°C for several
6
hours. When the ZnO NTs with a diameter of ~500 nm and a density of ~5.4 × 10 per
square centimeter were functioning as photoanode material for DSSCs, a 2.3% PCE was
achieved [39]. ZnO NTs can also be prepared by coating anodic aluminum oxide (AAO)
templates via atomic layer deposition (ALD). However, it yields a relatively low PCE of
1.6%, primarily because of the modest roughness factor of commercial membranes [41].

8.2.3 Niobium pentoxide (Nb 2 O 5 )
Nb 2 O 5 is another commonly used nanomaterial for solar cells that could be func-
tional as both the photoanode and the cathode. Nanostructures of Nb 2 O 5 have been
prepared by various methods, such as hydrothermal, sol-gel, electrodeposition, PVD,
thermal oxidation, and so on. However, Nb 2 O 5 is insoluble in water and hardly sol-
uble in acid, so the fabrication of Nb 2 O 5 costs more than the previously mentioned
other materials. In addition, Nb power is toxic in the air. And like TiO 2 or ZnO, the
typical nanostructures of Nb 2 O 5 are NPs, NRs, NWs, and NTs. All of the previously