Analysis on growth mechanism of TiO2 nanorod structures on FTO glass in hydrothermal process

https://doi.org/10.1016/j.jiec.2021.08.045Get rights and content

Abstract

Understanding the growth mechanism of TiO2 nanorods (TNRs) is critical for producing high-performance materials with morphology and structure control. TNRs on FTO glass were prepared by hydrothermal method in acidic solution. The structural and morphological characteristics of thin films were investigated for different temperatures and reaction times. By the hydrolysis and protonation, monomers Ti(OH)n(OH2)6-n](4-n)+ can be formed at ambient conditions. TNRs were formed through the bonding between these monomers by olation and oxolation reactions during hydrothermal process. During the hydrothermal growth of TNRs on FTO glass, precursors of TNRs and nanoflowers were observed in the reactive solution and on top of TNR thin films. The preferential deposition of precursors and TiO2 nanostructures on top of primary TNRs from solution resulted in significant changes in their morphology, structure, and growth orientation. A new possible growth mechanism of TNRs is proposed based on these experimental observations. Our preliminary results show positive signs to apply the prepared TNRs as electron transfer layer of perovskite solar cells (PSCs). This study will become the basis for our further researches to apply the prepared TNR thin films with the most suitable structural and morphological properties to high-performance PSCs as well as other photovoltaic devices.

Graphical abstract

Illustration of the growth mechanism of TiO2 nanorods during hydrothermal process with a new approach from the crystallization and deposition of precursors in solution.

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Introduction

Nowadays, the applications of semiconductor materials are rapidly increasing especially in the field of electronics, photonics, and renewable energy [1], [2], [3]. TiO2 is a semiconducting oxide-based photocatalyst material which is most widely used on the industrial scale because of its high photocatalytic activity, excellent physicochemical stability, good mechanical rigidity, low cost, environmental friendliness, and non-toxic properties [4], [5]. It is also often used in various applications such as hydrogen generation under solar irradiation, batteries, electron transport layer in solar cells, and photovoltaics [6], [7], [8]. It is known that the morphology and crystal structure of TiO2 play an important role in determining the properties as well as the application of TiO2 nanomaterials [9]. For example, most of the commercial TiO2 nanomaterials with a nanoparticle shape (3D structure) show small specific surface area and rapid recombination rate of electron/hole pairs (e/h+), so the photocatalytic activities of them are relatively limited [10].

Among various nanostructures of TiO2, one-dimensional (1-D) structures show some outstanding properties which are well suited for photovoltaic applications because of direct electrical pathways for electron transport resulting in increasing the electron transport rate and also reducing the recombination rate of e/h + pairs and finally improving the performance of photovoltaic devices [11]. Therefore, many researchers have synthesized various 1-D TiO2 nanostructures such as nanorod [12], nanowire [13], and nanotube [14] and have applied them to different fields. TiO2 nanorods (TNRs) possess unique and better properties comparing to traditional TiO2 nanoparticles (TNPs) or other 1-D TiO2 nanostructures such as TiO2 nanotubes (TNTs) and TiO2 nanowires (TNWs), especially for photovoltaic applications. For instance, TNRs have a larger surface area, less grain boundary, enhanced light absorption, and faster electron transfer rate than 3-D TiO2 nanomaterials [15], [16], [17], [18]. TNRs are also more stable than TNWs or TNTs because of the ease in morphology control during synthesis and the difficulty in shape-breaking during applications. For example, morphology control of NWs is more difficult because NWs have their high aspect ratio and large inter-wire spacing and the significantly small and long architectures of NWs can cause them to collapse easily and hit each other [19], [20]. Moreover, rutile TNRs have lower inherent resistance for electron transport than TNTs and the electron lifetime in TNRs is longer than in TNTs [21].

There are several methods to synthesize TNRs on FTO glass such as chemical vapor deposition [22], template-assisted [23], and electrodeposition method [24], but the hydrothermal method is still applied widely for both research and industrial purposes owing to the simple implementation, low cost, and safety of process, and the high homogeneity, crystallinity, and performance of TNRs obtained [25]. The morphology and structure control of TNR thin film plays a key role in achieving the high-performance of final products [26]. Several studies have synthesized TNRs on FTO glass and applied them in different fields [27], [28], [29], [30], while some other studies have investigated the formation mechanism of TNRs on FTO glass during the hydrothermal process [31], [32]. In terms of growth mechanism, the surface hydroxide ions and concentration of hydrochloric acid were indicated as factors leading to the growth of 1-D rutile TNRs on FTO glass [32], [33], [34].

In this study, TiO2 nanorod thin films on FTO glass substrates were synthesized systematically by the hydrothermal method for different temperatures and reaction times in acidic solutions containing TiO2 precursors. By combining the experimental observations on the nucleation and deposition of TiO2 precursors on the FTO substrate during the hydrothermal process, we proposed a new mechanism of TiO2 nanostructure development which combines the phenomena of TNR formation and growth. These results will help extend our understanding about the growth mechanism of TNR thin film and will also contribute to better control of TiO2 nanostructures on FTO glass substrates, thereby, improving the performances of TiO2 thin film in various applications.

Section snippets

Sample preparation

The TNR thin films were synthesized by the hydrothermal method using FTO glasses (1.5 cm × 1.5 cm) as substrates. First, FTO-glass substrates were cleaned by an ultrasonic bath in commercial detergents, acetone and isopropanol, followed by rinsing with de-ionized (DI) water and drying in an oven. 20 mL of DI water was mixed with 20 mL of concentrated hydrochloric acid solution (36–38% by weight) to reach a total volume of 40 mL, and the mixture was stirred for 15 min. Afterward, 0.48 ml

Morphology and crystal phase

Fig. 1a, Fig. 1b show the top-view and cross-sectional SEM images of TiO2 nanostructures, respectively, grown on FTO glass via hydrothermal reaction at 150 °C for various reaction times from 2 to 24 h. It is observed that a tetragonal faceted-nanorod structure with four smooth lateral facets is the main structure of TiO2, which has grown on FTO glass substrate by the hydrothermal method. By extending the reaction time, the diameter and length of TNRs both increase. After a short reaction time

Conclusions

TiO2 nanorod thin films were systematically synthesized on FTO substrates for various temperatures and reaction times by hydrothermal process and a new mechanism of TNR formation and growth was proposed based on experimental results. In the early stage of growth, TNRs were formed and grew with various orientations on the rough surface of FTO substrate, and they grew more vertically with preferred growth in [001] direction when the reaction time was prolonged. As the hydrothermal temperature

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Mid-career Researcher Program through NRF funded by the MSIP (2019R1A2C1004716).

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