Cost‐effective dye‐sensitized solar cells based on rutile‐phase three‐dimensional TiO2 hierarchical nanostructures

Specifically engineered three‐dimensional (3D) and 1D morphologies are expected to play significant roles in the development of next‐generation dye‐sensitized solar cells. In this study, using a hydrothermal approach without a surfactant or template, we attempted to synthesize a 3D hierarchical rutile titanium dioxide (TiO2) architecture by varying the growth temperature and time. X‐ray diffraction patterns of the synthesized TiO2 correlated well with rutile TiO2. Scanning electron microscopy images exhibited different nanostructures, such as nanorods, aggregated nanorods, and 3D TiO2 microflowers comprised of nanorods at 100°C, 130°C, and 160°C, respectively, after growth for 6 h. A significantly improved efficiency was observed for the TiO2 microflowers. The TiO2 microflowers exhibited an efficiency of 1.16%, short‐circuit current density of 12.8 mA cm−2, open‐circuit voltage of 0.692 V, and fill factor of 0.67.


| INTRODUCTION
Dye-sensitized solar cells (DSSCs) are promising candidates for photovoltaic devices owing to their simple fabrication and cost-effectiveness compared to commercially available solar cells (Anand et al., 2022;Bhullar et al., 2022;Nam & Boo, 2022;Shobana et al., 2022;Wang et al., 2022;Yadav et al., 2022). The performances of DSSCs are primarily determined by the porosity of the photoanode, optical properties, redox reaction, and photochemical properties of molecular sensitizers (Dong et al., 2014;Ganesh et al., 2019;Ganesh et al., 2020;Singh et al., 2022;Xin et al., 2012;Zouhri et al., 2022). TiO 2 is a promising candidate owing to its low cost, nontoxicity, chemical stability, and band gap in the field of solar cells for industrial photocatalytic and sensing applications (Mahadik et al., 2022;Ullah et al., 2022). The TiO 2 photoanode is a vital aspect influencing DSSCs performance. The structure and morphologies of the photoanode influence the energy conversion efficiency, dye adsorption, electrolyte diffusion, and electron transport of DSSCs. Extensive studies have been conducted on controlling the morphology of the photoanode and TiO 2 structure (Javed et al., 2021;Kandasamy et al., 2022;Zamiri et al., 2021;Zhang et al., 2022). One-dimensional (1D) nanostructures exhibit faster electron transport without electron-hole recombination compared to nanoparticle morphologies. Moreover, they improve the electron transport rate and efficiency of the device (Chen et al., 2021;Jian et al., 2023;Li et al., 2021;Li et al., 2022;Ma et al., 2023;Peighambardoust et al., 2019;Yun et al., 2020). 3D hierarchical structures are of particular interest as several of these architectures possess lattice imperfections, grain boundaries, and excellent photon scattering compared to simple nanocrystalline structures (Chen et al., 2020;Ghani et al., 2019;Ma et al., 2017;Rezaei et al., 2018;Zhao et al., 2015). 3D nanostructures of ZnO, SiO 2 , and SnO 2 have been synthesized using wet chemical methods Du et al., 2014;George et al., 2022;Liu et al., 2021;Xiang et al., 2021;Zhao et al., 2022). However, it is challenging to synthesize TiO 2 3D nanostructures using the above process owing to the high hydrolysis rate of TiO 2 . Tuning the morphology of TiO 2 nanostructures can influence the hydrolysis rate and aggregation of particles (Ali et al., 2018). Thus simulate us to work TiO 2 3D structure to study its properties.
In this study, we attempted to modify the TiO 2 morphology by varying the growth time and temperature. Controlled nanostructures of rutile 3D TiO 2 microflowers were successfully synthesized by a hydrothermal process. In addition, we explained the growth mechanism of 3D TiO 2 nanostructures at different times and temperatures with the influence of hydrochloric acid Finally, we demonstrated DSSC applications using different nanostructures.

| EXPERIMENTAL METHODS
First, 1 mL titanium butoxide was added dropwise into a mixture of hydrochloric acid and distilled water (1:8:8) and stirred for 1 h. The solution was then poured into an autoclave, and the temperature was maintained at 100 C for 6 h. The autoclave was cooled to room temperature, and the reaction products were filtered, washed with deionized water, and dried at 80 C. To understand the growth behaviors of the TiO 2 nanostructures, different growth temperatures (100 C, 130 C, and 160 C) were maintained for the same growth period. To further modify the morphology of the TiO 2 nanostructures at these growth temperatures, the solution temperatures were maintained for 6, 12, and 18 h as shown in Figure 1. The fabrication of DSSC photoanodes is described in our previous study (Ganesh et al., 2020).       combines with the R(CH 2 CH 2 CH 2 CH 3 ) group. Finally, a TiO 2 nanopowder is formed after washing and calcination. The presence of Cl À ions restricts the growth of the (110) faces, improving the growth along the (001) direction. The SEM images show that different TiO 2 nanostructure morphologies were obtained by tuning the growth temperature and time. The growth period of 6 h exhibited a significant influence on the morphology and uniformity in the nanostructure. The formation mechanism under these synthesis conditions has been described (Xin et al., 2012).

| RESULTS AND DISCUSSION
The rapid hydrolysis of the titanium precursor leads to the formation of Ti (IV) complex ions. The dehydration of titanium complex ions led to different TiO 2 nanostructures at different temperatures and times. Initially, the anatase nucleates formed as a result of precursors reactions and then the growth of the nanostructure depends on the interfacial energy of the material. The anatase and rutile phases consist of TiO 6 octahedra ( Figure 5b) as a basic unit. The rutile phase is the most stable phase. The arrangement of octahedra by face sharing initiates the anatase phase, whereas edge sharing leads to the rutile phase. In a highly acidic medium under hydrothermal conditions, the formation of the rutile phase by the "edge sharing" mode can be accelerated (Hu et al., 2013;Kumar & Rao, 2014;Wu et al., 2014). Therefore, the octahedra are linked by sharing edges and corners to form a 3D framework. The chain-like complexes elongate and consequently tend to aggregate into bundles. These bundles form anisotropic rod shapes owing to the interaction between the Ti and O atoms, compared to the van der Waals interactions between the conjugated chains. According to the symmetry and screw axis, rutile has 4 2 screw axes along the crystallographic c-axis. The structure promotes crystal F I G U R E 6 SEM images of the samples prepared at 100 C (a, b), 130 C (c, d), and 160 C (e, f) for 12 h. growth along the {001} direction, resulting in a crystal morphology dominated by the (110) faces. Therefore, the rutile-phase nanoparticles are typically rod-like. Upon autoclaving at 100 C, further growth proceeds by an oriented coalescence, yielding rutile nanorods. Upon a prolonged autoclaving at 130 C, aggregation of nanorods occurs owing to the "oriented attachment growth." At a growth temperature of 160 C, a complete nanoflower composed of nanorods is formed by the selfassembly method.
3.2 | Effects of different growth temperatures with a growth period of 12 h Figure 6 shows the low-and high-magnification SEM images of the samples synthesized at 100 C (Figure 6a,b), 130 C (Figure 6c,d), and 160 C (Figure 6e,f) for a growth period of 12 h. TiO 2 nanorods were obtained at 100 C with a length of 2.5 μm. With increasing temperature (130 C), 3D TiO 2 nanostructures were obtained with fine nanorod building units. 3D nanostructures were obtained by Ostwald ripening.
When the temperature increased to 160 C, the length of the nanorods increased to 5 μm. Figure 7 shows the low-and high-magnification SEM images of the samples synthesized at 100 C (a, b), 130 C (c, d), and 160 C (e, f) for a growth period of 18 h. Figure 7a shows that the obtained products were nanofibers with a length of 7 μm and aggregated triangle shapes.

| Effects of different growth temperatures with a growth period of 18 h
The starting point of the nanofibers is shown in Figure 7b. When the temperature increased to 130 C, the aggregated nanostrips changed to a sphere-shaped morphology with a cracked surface on top ( Figure 7c). Figure 7d shows the nanoflower obtained layer by layer; the core of the nanoflower is also visible. Figure 8e,f shows the sample prepared at 160 C with a nanorod morphology. The nanoflower F I G U R E 7 SEM images of the samples prepared at 100 C (a, b), 130 C (c, d), and 160 C (e, f) for 18 h. morphology fully collapsed and separated as nanorods with a length of 1 μm. The synthesis of these monodisperse 3D TiO 2 spheres exhibited good reproducibility.
3.4 | Effect of growth period on the growth rate of the nanostructures Figure 8 shows the variation in the growth rate at different growth temperatures for growth periods of 6, 12, and 18 h. Table 1 shows that, according to the growth period, different lengths of nanowires were obtained.

| CURRENT-VOLTAGE CHARACTERIZATION
The TiO 2 photoanode was deposited by spray technique and the thickness of the deposited film was about $15-16 μm. Figure 9a shows a schematic of a DSSC comprised of the prepared nanoflower.

CONFLICT OF INTEREST STATEMENT
The authors report there are no competing interests to declare.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.