Elsevier

Materials Letters

Volume 264, 1 April 2020, 127354
Materials Letters

Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (0 1 0)-facet

https://doi.org/10.1016/j.matlet.2020.127354Get rights and content

Highlights

  • Tunable and controlled (0 1 0)-faceted anatase single crystals were grown by hydrothermal method.

  • F ions play a synergistic effect in the crystals phase from rutile to anatase with low-index facets.

  • At higher concentration of F ions dominate the (0 1 0)-faceted anatase TiO2.

  • Over 1.4-µm-size anatase TiO2 single crystal sheets were obtained.

Abstract

While most anatase TiO2 crystals reported were in the form of nanoparticle, herein the tuneable and controlled (0 1 0)-faceted anatase phase were grown directly onto a conductive substrate. F ions provided by addition of ammonium hexafluorotitanate (AHFT) play a synergistic effect in the crystals phase changed from rutile to anatase during crystal growth with low-index facets. The morphological transformation from vertically-aligned rutile-nanorods to highly crystalline anatase-nanosheets bounded with the most significant percentage of (0 0 1) facets with the effect of F ions. At higher concentration of AHFT dominate the (0 1 0)-faceted anatase TiO2 with suppression of (0 0 1) facets of nanosheets (side length of 1.4 µm), which can suppress the charge recombination for an electron transport layer in solar cell applications, and thus, efficient charge transportation can be achieved compared to nanoparticles.

Introduction

Among various metal oxide semiconductors (MOS), titanium dioxide (TiO2) has been widely used for many applications. TiO2 is the most commonly used for the electron transport layer (ETL) in perovskite solar cells, which is compared to other MOS, due to its chemical stability, abundance, low-cost, long-durability, quick electron transport, and long electron lifetime [1], [2]. Generally, (1 0 1) facets are thermodynamically stable for anatase TiO2 than other facets due to low surface energy (0.44 Jm−2). Although (0 0 1) facets have a higher surface energy (0.90 Jm−2), they usually diminish rapidly during the crystal growth process. Therefore, single crystals anatase TiO2 with exposed (0 0 1) facets were much needed for improving the perovskite absorber on the ETL surface. The (0 0 1) facet growth percentage has been increased by exploiting various reaction systems and capping agents [3]. The introduction of F ions as a capping agent stabilized the (0 0 1) facets of anatase TiO2, which increased 47% of (0 0 1) facets exposure [4]. Furthermore, similar methods were used to synthesize anatase TiO2 nanosheets with 89% exposure of (0 0 1) facets, and F and H+ ions have a significant role in changing the crystal structure of TiO2 [4], [5]. The introduction of F ion can be an effective strategy to control the growth to (0 0 1) facets of anatase TiO2 nanocrystals [6].

However, these facet-controlled syntheses are resulted in the form of nanoparticles and could be unfavourable for the fabrication of the photovoltaic devices. The nanoparticles with large grain boundaries hinder the charge transportation because of the high charge-recombination possibility which depreciate the device performance [7]. To overcome, we should develop interparticle boundaries with a high reactive facet that is highly beneficial. Many reports suggest that nanostructures have been directly grown on conducting subtracts. Among that rutile TiO2 nanorods grown on FTO substrate shows higher performance compared to other nanostructures [8]. However, in the 1D structures the attachment of dye molecules is less, and light trapping in the photoanode is also weaker which results in poor light harvesting [9]. Nevertheless, growing anatase TiO2 nanosheets directly on conducting substrate with facet controlled would be a benefit to the enhancement of interfacial materials improving the ETL/perovskite layers by the electron extraction [8]. In this work, a single-step hydrothermal method was used to grow on vertically aligned TiO2 nanostructures with tuneable directly and controlled (1 0 1), (0 0 1), and (0 1 0) facets on FTO substrate with uniform morphology and size. The sample with (0 1 0) and (0 0 1) facets dominated to the surface of particles can be used for the electron transport layer in solar cell applications.

Section snippets

Experimental details

30 mL of hydrochloric acid and 30 mL of deionized water (DI-water) were stirred for 5 min. To the above solution, 1 mL of titanium tetrachloride was added and stirred for 10 h. The mixture solution was transferred into Teflon-lined autoclave, and FTO was placed inside and maintained at 160 °C for 10 h. After the cooling process, the FTO substrate was taken out and rinsed with DI-water and dried at 80 °C for 30 min. For the synthesis of anatase TiO2 same procedure was repeated with the addition

Results and discussion

Fig. 1a shows the XRD patterns of as-synthesized samples. The XRD pattern of FTO with the diffraction peaks corresponds to the tetragonal phase of SnO2 (JCPDS 41-1445). For pure TiO2 (Fig. 1a (S1)) peaks at 36.1° and 62.8° were well-matched with rutile phase (JCPDS 21-1276). With the addition of AHFT (0.008 M) peaks at 39.1° and 68.7°, corresponds to (2 0 0) and (1 1 6) planes of rutile and anatase phase were observed (Fig. 1a (S2)). In further, the increase in concentration from 0.008 to

Conclusion

The controlled and tunable growth of single-crystalline anatase TiO2 with the exposed facet on the FTO substrate. F ions from AHFT transforms of phase as well as morphology during the crystal growth to expose the low-index facet. The XPS analysis confirmed Ti-F species on the surface of TiO2. The elemental analysis confirms the existence of F ions in the composition. At 0.032 M AHFT, (0 1 0) and (0 0 1) facets dominate the surface of particles with large surface nanosheets (side length of

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