Controlling the Spatial Direction of Hydrothermally Grown Rutile TiO₂ Nanocrystals by the Orientation of Seed Crystals

Abstract

Hydrothermally grown TiO₂ nanorods are a key material for several electronic applications. Due to its anisotropic crystal structure, the electronic properties of this semiconductor depend on the crystallographic direction. Consequently, it is important to control the crystal orientation to optimize charge carrier pathways. So far, the growth on common polycrystalline films such as fluorine tin oxide (FTO) results in randomly distributed growth directions. In this paper, we demonstrate the ability to control the growth direction of rutile TiO₂ nanocrystals via the orientation of the seed crystals. The control of the orientation of such nanocrystals is an important tool to adjust the electronic, mechanical, and chemical properties of nanocrystalline films. We show that each employed macroscopic seed crystal provides the growth of parallel nanofingers along the [001] direction under specific angles. The parallel growth of these nanofingers leads to mesocrystalline films whose thickness and surface structure depends on the crystal orientation of the seed crystal. In particular, the structure of the films is closely linked with the known inner structure of hydrothermally grown rutile TiO₂ nanorods on FTO. Additionally, comprehensive 1D structures on macroscopic single-crystals are generated by branching processes. These branched nanocrystals form expanded 2D defect planes, which provide the opportunity of defect doping-induced two-dimensional electronic systems (2DES).

1. Introduction

Rutile TiO₂ nanostructures have versatile capabilities in applications such as photocatalysis, [1,2,3,4] surface wetting, [5,6,7] disinfection, [8,9] gas and molecule sensing, [10,11] displays, [12] transparent electrodes, [12] photodetectors, [13,14] photovoltaics, [15,16,17] fuel cells, [18] data storage, [19] capacitors, [20] batteries, [21,22,23,24] and medical engineering [25]. Often, these nanostructures are created using an inexpensive, fast, and scalable hydrothermal method [26]. Orientated TiO₂ nanorod arrays can be grown hydrothermally on polycrystalline rutile fluorine tin oxide (FTO) films using the substrate as a seed layer [17]. These nanorods grow along the [001] direction with {110} facets on their side walls, {100} facets on their side edges, and {001} and {111} facets on their tips [27]. They exhibit a fine structure consisting of bundled nanofingers, which are aligned in parallel to the [001] direction. Hence, the nanofingers have the same shape and length as the nanorods, but they are significantly thinner. Previously, we found that the fine structure results from crystal defects in the early growth state and propagates throughout the growth [27]. The additional grain boundaries affect not only the chemical stability but also the electronic properties such as charge carrier mobility [28,29,30,31]. Within a nanorod, these nanofingers merge and form a single crystal for sufficiently high annealing temperatures [32].

Besides the fine structure, another important effect occurs, which offers great opportunities for electronic applications. The hydrothermal growth of rutile TiO₂ nanorods is accompanied by branching events in the early growth stage. In as-grown nanorods, the principal crystal and the branch are separated by a thin 2D defect plane. The existence of flat defect planes in crystals gives rise for specific electronic applications. Such a defect layer represents a local change of the stoichiometry and a shift of the conduction band. Density functional theory (DFT) calculations performed by Morgan and Watson indicate a low formation energy of oxygen vacancies on specific interfaces [33]. Dependent on the kind of defect levels induced by such vacancies, the defect plane has either an n-n⁻-n or n-p-n type character [33,34].

In this study, we demonstrate that the growth direction and orientation is controllable via the crystallographic orientation of the seed crystal. Specifically, we performed the nanorod growth on macroscopic rutile TiO₂ single crystals with defined crystal facets resulting in a similar fine structure as observed in the nanorods grown on polycrystalline seed films. Here, the orientation of the nanofingers along the [001] direction is used as an indicator for the crystallographic direction of the grown mesocrystalline films [21,27]. Besides the inner structure, nanorods and the presented mesocrystalline films share another feature. Branching, as typically observed for rutile TiO2 nanorods, [35] appear on the mesocrystalline films as well as provide expanded two-dimensional electron systems (2DES), [36] which have the potential for sensing and transistor applications [37,38].

2. Materials and Methods

The hydrothermal growth was performed on commercial rutile TiO₂ single crystals (Latech Scientific Supply Pte. Ltd., Singapore) with polished {100}, {001}, {110}, and {111} facets fabricated by a float zone crystal growth method. For all crystals, the rms-roughness is less than 1 nm and the purity is above 99.99%. The hydrothermal growth was performed by heating a 20 mL of hydrochloric acid (HCl, reagent grade, VWR Chemicals, 14.8wt% concentration in distilled water) and 350 μL titanium(IV) butoxide (C₁₆H₃₆O₄Ti, reagent grade, 97%, Sigma-Aldrich, now Merck KGaA, Darmstadt, Germany) solution in a Teflon lined autoclave at 180 °C. The growth process was stopped after 3 h by rapid quenching in water. After the growth, the samples were split mechanically and the edge was investigated with a scanning electron microscope (SEM). The field-effect scanning electron microscope (FE SEM) imaging was executed with a Zeiss CrossBeam 1540XB (Carl Zeiss Microscopy GmbH, Jena, Germany) using an acceleration voltage of 5 keV. Powdered X-ray diffraction (PXRD) data were acquired on a Bruker AXS D8 Discover (Bruker Corporation, Billerica, USA) with an IµS microfocus X-ray source (Cu-Kα radiation) equipped with a 2D-Detector Vantec500.

3. Results and Discussion

The surface of these hydrothermally grown structures, as drawn in Figure 1, consists of a tip with {001} and {111} facets, side walls composed of {110} facets, and the edges directed along the [100] direction. This is attributed to the preferred [001] growth direction. Consequently, it appears possible to control the spatial growth direction by controlling the crystallographic orientation of the seed crystal. Hence, the hydrothermal growth was performed on macroscopic rutile TiO₂ single crystals with {001}, {111}, {110}, and {100} facets. The top view and cross-section images are shown in Figure 1. On each facet, a densely packed nanocrystalline film consisting of 10 to 15 nm thick parallel nanofingers was formed. Similar nanofingers have been observed in hydrothermally grown rutile TiO₂ nanorods on FTO substrates [27]. The X-ray diffraction pattern (Figure S1, supporting information) indicates that the grown films consist mainly of rutile TiO₂ as expected. A comparison of the fine structure in the represented nanocrystalline films and typical nanorods is shown in the supplementary information (Figure S2). The nanofingers are aligned with the [001] direction of rutile TiO₂ as determined from the angles between the nanofingers and the crystallographic direction of the substrate. For the substrates with {100} and {110} facets, the main growth direction is parallel to the substrate surface. Consequently, the hydrothermally grown layers remain thin as the growth rate perpendicular to the substrate surface is low. In contrast, the substrates with {001} and {111} facets provide a component of the main growth direction perpendicular to the substrate surface and accordingly, the hydrothermally grown films become thick. This reflects the direction-dependent growth rates that are responsible for the rod-like shape of nanocrystals on common polycrystalline TiO₂ or FTO seed layers.

The hydrothermal growth on the {001} facet is shown in Figure 1A,B. On these {001} facets, the nanofingers align normal to the substrate surface, which is parallel to both the main growth direction and the [001] direction of rutile TiO₂. As the nanofinger growth proceeds, the diameter decreases by approximately 5%. This is attributed to changing process parameters such as temperature and precursor concentration during the growth process [35,39]. The decrease in diameter causes cracks in the film as shown in the inset of Figure 1A. Alternatively, the nanofingers include an angle of 45° on the {111} facets, which corresponds to the angle between the [111] and [001] direction (Figure 1C,D). As demonstrated in the inset of Figure 1C, cracks are in line with the tilted growth direction since breaking up the mesocrystalline film at grain boundaries costs less energy than breaking the nanofingers directly.

The growth on {110} (Figure 1E,F) and {100} (Figure 1G,H) facets results in thin films. For the {110} facet, nanofingers are observed, which are aligned in parallel to the substrate surface. Here, the spatial growth direction along the [001] direction is perpendicular to the [110] direction. This outcome is reasonable since {110} facets form the side walls of nanorods. The thin film thickness results from a slow growth perpendicular to the main growth direction (Figure 1F). The growth on {100} facets results in a thin film with a ribbed surface. The {100} facets are found at the vertical edges of the nanorods. The ribbed surface is attributed to the growth along the face of the substrate, resulting in the edges of the nanofingers being exposed. The high density of parallel gables leads to the distinctive rough structure. Summarizing the presented observations on all four different crystal facets, we can say that the growth direction of rutile TiO₂ nanocrystals is strictly correlated with the crystal orientation of the subjacent rutile TiO₂ seed.

In addition to the seed crystal-dependent growth, the nanorods also exhibit branching, which results in further, uncontrolled growth directions as demonstrated in Figure 2A for {100} facets. Since the preferred growth direction on {100} facets is parallel to the surface, this phenomenon is strongly related to the branching of single rutile nanorods [35]. A similar branching event observed on a single rutile nanorod, which was created in solution without any expanded seed layer, is presented in the inset of Figure 2A. The inclination angle of 65° corresponds to the presence of a {101} twin plane at the interface between the row of nanorods and the surface of the single crystal [35]. We suppose that the branching is correlated to ruptures on {100} facets as presented in Figure 2B. If these expanded crystal defects form an intersection line with the crystal surface, a straight 1D surface defect is created. A rupture on a crystal surface provides additional exposed crystal facets with different crystallographic orientations. Such facets support branching and hence, the creation of the nanorod wall shown in Figure 2A. Yang et al. and Zhou et al. reported about similar wall-like structures grown at the side wall of a primary single freestanding TiO₂ nanorod [40,41]. If surface defects on the substrates are responsible for the emergence of the observed branches, they can also be generated selectively and one can profit from the local properties of such extended nanostructures. These structures provide a bunch of important features. Twin planes between rutile nanocrystals have a stoichiometric composition that differs from defect-free TiO₂. This affects the band structure at these interfaces resulting in more n- or p-type two-dimensional layers as required for thin film transistors. Beside local electronic applications, a nanocrystalline surface supports the attachment of different kinds of particles. Specific body cells are known to form a strong contact with TiO₂ nanostructures and hence, these structures are suitable for applications in biological research and lab-on-a-chip devices [25]. The versatility of the applications can be significantly extended by applying a topcoat. For example, the V-shaped row of nanorods as shown in Figure 2A covered with a metallic coating serves as a waveguide for electromagnetic waves similar to the channel plasmon–polariton (CPP) geometry [42].

4. Conclusions

We investigated the hydrothermal growth of rutile TiO₂ nanocrystals on macroscopic rutile TiO₂ {100}, {110}, {111}, and {001} facets. Macroscopic facets provide the growth of dense mesocrystalline rutile TiO₂ films consisting of thin nanofingers. These nanofingers are aligned in parallel with the [001] direction and indicate the spatial growth direction. If the seed crystal does not exhibit a {001} facet, the spatial growth direction is tilted towards the seed surface with a well-defined angle. Thus, it is shown that the spatial growth direction is well controlled with the orientation of the employed seed crystal. Surface defects on seed crystals might support the appearance of additional spatial growth directions. Hence, an efficient control of the spatial growth direction is based on defect-free seed crystals. Nevertheless, a controlled creation of branches could be used to fabricate 2D layers with technically important electronic properties.