**3. Shapes of titanium dioxide nanopowders for dye solar cells**

Nowadays, most of the successful photovoltaic devices are assembled by semiconductor materials such as silicon (Si) [42]. In recent years, different alternatives to Si-based solar cells have become obtainable, and the large research is outstanding toward substantially decreasing the cost of electricity generation. Dye-sensitized solar cells (DSSCs) [43] are attractive alternative in comparison with others as they can be cheap, lightweight, portable, and flexible. On the other side, the attractive and extensive properties of titania (TiO<sup>2</sup> ) have led to its wide use in many industries, from traditional industries to high-technology industries [44]. The morphology and particle size of TiO<sup>2</sup> play critical roles in the photoelectric conversion efficiency of DSSCs [45]. For example, TiO<sup>2</sup> materials with different morphologies, such as nanoparticles [46], nanotubes [47], nanowires [48], and nanorods [49], have applied to fabricate the porous film electrodes. Nanostructured TiO<sup>2</sup> materials can be prepared by dry and wet processes as mentioned in the previous section. Because different reaction conditions, like reactants, reaction medium, temperature, and pH of solution, can be chosen in the wet processes, the crystallite size, crystal shape, and surface structure of the nanocrystals can be controlled more easily than that in the dry processes [50].

In particular, one-dimensional (1D) TiO<sup>2</sup> nanostructures, including nanorods (NRs) [49], nanowires (NWs) [48], and nanotubes (NTs) [47], have attracted large interest because of their unique microstructure and promising lineaments, like a high aspect ratio, high surface area, higher surface area/volume ratio, enhanced number of delocalized carriers, and increasing the charge transport afforded by dimensional anisotropy with the conventional properties [51]. Their remarkable properties have referred to their use in wide applications including dye-sensitized solar cells (DSSCs), photocatalysis, and photochromic devices [52]. For dyesensitized solar cell (DSSC) operation, 1D nanostructure-based photoanodes can contribute to rapid electron transport, ensuring efficient charge collection by the conducting substrate in competition with recombination. This point has promoted research on self-ordered, 1D photoanodes stretched on a substrate with enhanced electron transport properties due to their desirable features, namely, highly decreased intercrystalline contacts and a structure with a specified directionality. Kang et al. (2008) confirmed that TiO<sup>2</sup> nanorods (NRs) are believed to have exceptional properties and have been deemed an alternative to nanoparticles (NPs) as shown in **Figure 7** [53]. Furthermore, increasing the delocalization of carriers in rods, where they can move freely throughout the length of the NRs, is expected to decrease the e− /h<sup>+</sup> recombination probability. However, this is partially recompensed by the traps in the

calcined at low temperature. The method is also called combustion method, polymeric precursor method, and acid gel method (oxalate precursor, tartaric acid, lactic acid, and citrate precursor method). The process depends on complexation of metallic salts with aqueous solution of organic acid; the formed complex solution was evaporated at low temperature from 60 to 1000°C until viscous resin is formed; the formed polymer resin was dried and then calcined at low temperature from 200 to 1000°C for 1–4 h [40]. **Figure 6** shows TEM micrographs of titania

Nowadays, most of the successful photovoltaic devices are assembled by semiconductor materials such as silicon (Si) [42]. In recent years, different alternatives to Si-based solar cells have become obtainable, and the large research is outstanding toward substantially decreasing the cost of electricity generation. Dye-sensitized solar cells (DSSCs) [43] are attractive alternative in comparison with others as they can be cheap, lightweight, portable, and flex-

wide use in many industries, from traditional industries to high-technology industries [44].

nanoparticles [46], nanotubes [47], nanowires [48], and nanorods [49], have applied to fabri-

wet processes as mentioned in the previous section. Because different reaction conditions, like reactants, reaction medium, temperature, and pH of solution, can be chosen in the wet processes, the crystallite size, crystal shape, and surface structure of the nanocrystals can be

nanowires (NWs) [48], and nanotubes (NTs) [47], have attracted large interest because of their unique microstructure and promising lineaments, like a high aspect ratio, high surface area, higher surface area/volume ratio, enhanced number of delocalized carriers, and increasing the charge transport afforded by dimensional anisotropy with the conventional properties [51]. Their remarkable properties have referred to their use in wide applications including dye-sensitized solar cells (DSSCs), photocatalysis, and photochromic devices [52]. For dyesensitized solar cell (DSSC) operation, 1D nanostructure-based photoanodes can contribute to rapid electron transport, ensuring efficient charge collection by the conducting substrate in competition with recombination. This point has promoted research on self-ordered, 1D photoanodes stretched on a substrate with enhanced electron transport properties due to their desirable features, namely, highly decreased intercrystalline contacts and a structure

believed to have exceptional properties and have been deemed an alternative to nanoparticles (NPs) as shown in **Figure 7** [53]. Furthermore, increasing the delocalization of carriers in rods, where they can move freely throughout the length of the NRs, is expected to decrease the

recombination probability. However, this is partially recompensed by the traps in the

) have led to its

nanorods (NRs) are

play critical roles in the photoelectric conversion

nanostructures, including nanorods (NRs) [49],

materials with different morphologies, such as

materials can be prepared by dry and

**3. Shapes of titanium dioxide nanopowders for dye solar cells**

ible. On the other side, the attractive and extensive properties of titania (TiO<sup>2</sup>

nanopowders synthesized using organic acid precursor.

358 Titanium Dioxide - Material for a Sustainable Environment

The morphology and particle size of TiO<sup>2</sup>

In particular, one-dimensional (1D) TiO<sup>2</sup>

e− /h<sup>+</sup>

efficiency of DSSCs [45]. For example, TiO<sup>2</sup>

cate the porous film electrodes. Nanostructured TiO<sup>2</sup>

controlled more easily than that in the dry processes [50].

with a specified directionality. Kang et al. (2008) confirmed that TiO<sup>2</sup>

**Figure 7.** The TiO2 photoanodes consisting of the NPs and NRs in the configuration of DSSCs (Kang et al., 2008). From Ref. [53]. Reprinted with permission from Royal Society of Chemistry 2016.

surface sites to guarantee more efficient charge separation [53]. Moreover, NRs can potentially enhance the charge transport in the photoanodes of DSSCs. In addition, NRs offer direct electrical pathways for photo-generated electrons and can enhance the electron transport rate, which in turn may improve the performance of DSSCs.

In the DSSC, the photoanode encompassed of oriented attachment. TiO<sup>2</sup> NRs showed the following two main advantages: (1) confirmation of high surface area directly proportional to the light-harvesting yield (dye uptake) resulted from the NRs synthesized from the necking of truncated NPs by recovering the low surface area of the general TiO<sup>2</sup> NRs and (2) fast electron transport rate and degraded charge recombination from the decreased intercrystalline contacts between grain boundaries and specific directionality of NRs, bringing about the improved charge collection efficiency.
