Twinning-mediated anomalous alignment of rutile films revealed by synchrotron X-ray nanodiffraction.

Nanotwin structures in materials engender fascinating exotic properties. However, twinning usually alter the crystal orientation, resulting in random orientation and limited performances. Here, we report a well-aligned rutile TiO2 nanotwin film with superior preferential orientation than its isostructural substrate. By means of the synchrotron X-ray Laue nanodiffraction technique, the crystal orientation, twin boundaries, and deviatoric stresses of the film were quantitatively imaged at unprecedented spatial resolution to unravel the underlying mechanism of this anomalous alignment. Massive {101}-type rutile nanotwins were observed, and a crystallographic relationship of the heteroepitaxy was proposed. The rapid twinning and twin-controlled heteroepitaxy are responsible for the texture improvement. This work would open up opportunities for rational design of better twin-based functional materials, and implies the powerful capabilities of X-ray nanodiffraction technique for multidisciplinary applications.

Introduction

Twinned crystals, especially the ordered and dense nanotwins, have attracted considerable interest due to their unique interface or defect structures, novel properties, and promising application prospects (Lu et al., 2009; Behrens et al., 2012; Nie et al., 2015; Zhu et al., 2018; Liu et al., 2018). Generally, a coherent twin boundary (TB) could be deemed as a stable two-dimensional (2D) high-pressure polymorph of the parent crystal due to its higher atomic density (e.g., rutile TiO2 {101} twin (Hwang et al., 2000)). Similar to high-angle grain boundaries (GBs), TBs can serve as barriers against dislocation motion, that is, the hardening effect (Lu et al., 2009; Zhang et al., 2004). The excess energy of coherent TBs is about one order of magnitude lower than that of the ordinary high-angle GBs, which together with the hardening effect make twinned materials possess higher hardness, higher stability, higher toughness, lower compressibility, and near zero thermal expansion (Tian et al., 2013; Huang et al., 2014; Lu et al., 2004, 2009; Zhu et al., 2018; Zhang et al., 2004). In addition, the electrical conductivity of coherent TBs is higher than that of high-angle GBs, and the enhanced ionic transportation along TBs was observed in compounds such as WO3 and SnO2 (Lu et al., 2004; Aird and Salje, 2000; Nie et al., 2015). Moreover, the (photo)catalytic activity of a (photo)catalyst can be enhanced by nanotwin domains via twin-induced active sites or homojunctions caused by staggered band alignment (Behrens et al., 2012; Liu et al., 2011; Huang et al., 2018; Lu et al., 2020a, 2020b).

As an important class of multifunctional materials, rutile TiO2 can be twinned on (101) and (301) planes (Hwang et al., 2000; Li et al., 1999; Lu et al., 2012a, 2020a; Jordan et al., 2018; Gao et al., 1992; Daneu et al., 2007, 2014). However, the synthetic TiO2 twins generally possess lower texture because twinning trends to alter the growth direction or crystal orientation, which is detrimental to practical applications and the discovery of novel properties, whereas the natural rutile twin minerals can directionally form on their substrates (Li et al., 1999; Lu et al., 2012a; Jordan et al., 2018; Gao et al., 1991, 1992; Daneu et al., 2007, 2014; Lee et al., 2006; Sosnowchik et al., 2010). Recently, we have successfully synthesized rutile TiO2 nanotwin films on fluorine-doped tin oxide (FTO) glass substrates with the rare b-axis preferred orientation (instead of the c-axis in common self-aligned rutile films) via a rapid nucleation/twinning strategy (Lu et al., 2020a). In this study, to uncover the underlying mechanism of the anomalous alignment, we applied the state-of-the-art scanning synchrotron X-ray Laue nanodiffraction technique (XND, see supplemental information Figure S1) and the developed data analyzing software to image the crystal orientation, twin boundaries, and stresses at ∼80 nm spatial resolution. A rutile {101} twin model on the substrate was proposed to illustrate the anomalous orientation. This study will facilitate the rational design and fabrication of better nanostructured materials.

Results and discussion

Characterization of [010]-oriented rutile twin films

Several methods can be adopted to determine the crystal orientation and grain boundaries in materials, including electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and electron tomography (Hsiao et al., 2012; Schwartz et al., 2009; Langille et al., 2012; Fultz and Howe, 2008; Midgley and Dunin-Borkowski, 2009). Although EBSD can image the orientation and twin boundaries of the crystals on the surface, the surface of the sample is required to be flat and the angular resolution is limited to 1–0.1° (Schwartz et al., 2009; Hsiao et al., 2012). TEM and electron tomography can characterize the crystal structure and twin boundaries or flat interfaces in an atomic resolution (Fultz and Howe, 2008; Midgley and Dunin-Borkowski, 2009; Langille et al., 2012), but the specimen preparation is complicated or the sample should be small/thin (generally, size/thickness <100 nm) due to the low penetration depth of electron beam. X-ray diffraction (XRD) in laboratory provides the ensemble-average crystal structure of multiple grains (size >1 μm). Alternatively, the high-brilliance X-ray nanobeam with strong penetration capacity enables the quantitative and nondestructive mapping of crystal orientation and buried grain boundaries of bulk/thick sample with larger strain and surface roughness by X-ray nanodiffraction (Chen et al., 2016), which makes it suitable for studying nanotwinned materials and in situ probing the processes of the growth and transformation of materials at extreme conditions, e.g., high/low temperature and high pressure/stress.

The TiO2 films (Figure 1A) were synthesized on the FTO glass substrates by a rapid reaction method (see transparent methods) (Lu et al., 2020a). The XRD patterns (Figures 1B and S2) show that the TiO2 film and FTO substrate are of rutile structure and a preferred orientation of TiO2 film along the [010] direction, with a degree about 16 times stronger than that of the isostructural substrate. The scanning electron mircroscopy (SEM) image (Figure 1C) shows the ship-like rutile TiO2 crystals inverted on the substrate, with predominant exposed {111} and {110} facets (Lu et al., 2020a). Notably, there is a handful of crystals with reentrant shape, symbolizing twinned crystals. The high-resolution TEM image (Figure 1D) demonstrates that the sample possesses twin structure with a composition plane of (101). The thickness of the rutile twin film is about 2 μm (Figure S3).

Figure 1

Texture-improved rutile TiO2 twin film on F-doped SnO2 substrate

(A) Schematic illustration of the rutile TiO2 twin film on FTO (or F:SnO2) substrate (side view).

(B) XRD patterns of the rutile TiO2 twin film and the FTO conductive glass substrate.

(C) SEM image of the rutile TiO2 twin crystals. The red dashed circles indicate the reentrant facets.

(D) TEM image of the twinned TiO2. Inset is the corresponding fast Fourier transformation (FFT) pattern.

Laue nanoimaging of crystal orientation and twin boundaries

The dedicated XND setup of Taiwan Photon Source (TPS, beamline 21A) (Figure S1) can achieve an angular resolution down to 0.01°, with a nanoscale spatial resolution (∼80 nm in this work), and the deviatoric strain tensors resolution of 10−4 (Chung and Ice, 1999). Figure 2A shows the SEM image of the rutile twin film surrounding the XND-scanned region. A representative XND image (No. 14993) of the film was indexed as rutile phase (see Figure 2B). The (0, K, 0) spot of TiO2 located near the center of the Laue image indicates that the a- or b-axis preferred orientation of the grain as inferred from the geometric relationship of the X-ray, sample, and detector (Figure S1). In the scanned region, there are 40,401 images recorded and auto-indexed. As shown in Figure 2C, the crystal orientation map (overlapped with grain boundaries) of the film along the normal direction (z axis, other directions can be find in Figures S4A and S4B) is predominantly occupied by green area with only little blue and red area, revealing that the TiO2 film is preferentially oriented with the (100) or (010) crystal planes parallel to the glass substrate.

Figure 2

Nanoimaging of morphology, crystal orientation, and twin boundaries of rutile film

(A) SEM image of the TiO2 film around the scanning region (~20 μm × 20 μm).

(B) Typical indexed XND image of the film (No. 14993).

(C) Crystal orientation map for TiO2 grains along the normal direction of film (z axis) overlapped with grain boundaries (GBs). The white or transparent area represents the patterns failed to be indexed as rutile structure due to the complex or weak diffraction patterns from the joint area of grains.

(D) Spatial distribution of ordinary grain boundaries (OGBs, gray), {101} twin boundaries (red), and {301} twin boundaries (blue). Scale bar (bottom left) for Figures 2C and 2D: 2 μm. Sample coordinate system (bottom) for Figures 2C and 2D: O-XYZ.

Previous studies showed that in the synthetic rutile phase TiO2 powder or natural rutile minerals, the {101} and {301} twins are usually present together, with around six times many former than the latter (Li et al., 1999; Lu et al., 2012a; Jordan et al., 2018; Gao et al., 1992; Daneu et al., 2007, 2014). In our case, the possible twin boundaries were identified from the ordinary grain boundaries by checking the rotation angle of adjacent gains and the parallelity of their rotation axis (Li et al., 2015). As shown in Figure 2D, the red dots or lines signify that the grain boundaries fit well with the {101} twin boundaries, judging from the fact that the rotation angle of the adjacent grains along the [010] axis is 114.4° for the rutile {101}/[010] twin. The blue dots represent that the GBs match well with the {301} TBs (the mutual rotation angle of the rutile {301}/[010] twin is 54.7° (Lee et al., 1993)). Obviously, massive {101}-type rutile twins were detected, whereas only a few blue dots are observed in the map, which suggests the ratio and weight of the rutile {301} twins in the film are much lower than that reported elsewhere. We note that there are lots of nanotwins with thickness less than 80 nm in the film, which have been demonstrated by TEM observation (Lu et al., 2020a).

Taking the XND image of No. 1998 as an example, the image can be indexed as two sets of Laue diffraction spots of rutile TiO2 and one set of Laue diffraction spots of F-doped SnO2 (Figure S5 and Data S1). The orientation of both crystals plotted as pole figures shown in Figure 3A reveal that two {010} poles and two {101} poles overlap, respectively, i.e., {101} as composition planes and [010] as a twin axis. The misorientation between both twinned components is 65.66°/[0.0025 1.0000 0.0005], agreeing well with the theoretical 65.57°/[0 1 0]. Moreover, the XND image of No. 14993 was indexed as one set of Laue diffraction spots of rutile TiO2 and two sets of Laue diffraction spots of F-doped SnO2 (Figures 2B and S6). The corresponding pole figures (Figure 3B) indicate that the preferential [010] orientation of F:SnO2 substrate is weaker than that of TiO2 overlayer, which is consistent with the XRD spectra, crystal orientation maps, and pole figures (Figures 1B, S4, and S7).

Figure 3

Pole figures derived from typical XND images

(A) XND image of No. 1998.

(B) XND image of No. 14993.

Formation mechanism of well-aligned nanotwin film

During the process of rapid hydrothermal synthesis of our samples, there are roughly three mechanisms at play: (1) nucleation of twins in reactive solution, (2) attachment of twinned crystals or nuclei to the substrate, and (3) selective growth controlled by twin anisotropy. In growth twin, twinning is usually related to the stress partially generated by the presence of defects and impurities or the accidental attachment of crystalline grains during the initial stages of crystal growth (Penn and Banfield, 1998, 1999; Lebensohn and Tomé, 1993; Guermazi et al., 1985; Bursill et al., 1969). To reveal the mechanisms of twinning and attachment, crystallographic relationships and the stress distribution in the rutile nanotwin film were further analyzed. As shown in Figures 4A and S8, the uneven distribution of deviatoric stresses in rutile twin film suggests that the formation or attachment of twinned rutile crystals on the substrate is associated with the stress. In addition, twinned particles were discovered in the reaction solution, and their shape is related to that of twins grown on the substrate (Figure S9) (Lu et al., 2020a). During the dynamic dissolution of reaction precursor (TiN) at room temperature or rapidly elevated temperature, the generation of oxygen vacancies (Ti3+, see the electron paramagnetic resonance data in our previous work (Lu et al., 2020a)) or N species would induce twin nuclei to reduce the system energy at the initial reaction stage. Then, the rutile twin seeds attach and continuously grow on the F:SnO2 substrate. Hence, the F:SnO2 substrate is not the prerequisite for twinning but profoundly impact the heteroepitaxial growth and the preferred orientation of the TiO2 overlayer.

Figure 4

Strain distribution and twin-substrate model

(A) Spatial distributions of the normalized deviatoric stresses in the typical twinned rutile grains. The position of No. 1998 is denoted by dashed circles.

(B) Lattice mismatch between rutile TiO2 (blue line) and F:SnO2 (black dashed line). The angular mismatch (α) is around 7.5°.

(C) Scheme of the [010]-oriented rutile TiO2 {101} twin on the SnO2(010) viewed along the [010] direction.

An atomic model of rutile {101} twin on SnO2 was proposed, as shown in Figure 4C, based on the pole figures such as No. 1998, strain distributions, and previous studies (Lu et al., 2012b; Lee et al., 1993). Table 1 and Figure 4B show the lattice mismatch between rutile twin and substrate. It was found that the lattice mismatch between the isostructural F:SnO2 and one twin component (Grain A and twin boundary) is lower than 8%, which could facilitate its heteroepitaxy on F:SnO2 grains and the formation of the other twin component (Grain B) with [010]-preferred orientation regardless of the large (tensile or compressive) stress in the Grain B region (Steidl et al., 2017). Similar effect can be obtained in the case of the rutile {301} twin formation (Figure S10). Rutile twin can be grown on some types of hexagonal substrates, such as sapphire (Al2O3) (Gao et al., 1992; Lee et al., 2006) and hematite (α-Fe2O3) (Rečnik et al., 2015) or formed from other hexagonal precursors, e.g., ilmenite (FeTiO3) (Janssen et al., 2010; Stanković et al., 2015; Daneu et al., 2014). Interestingly, the (010)-oriented SnO2 substrate possesses an approximatively hexagonal atomic arrangement, showing the importance of the lattice-matching attachment in the twinning and heteroepitaxial growth. The attachment of twinned rutile nanocrystals onto SnO2 substrate may be controlled by some mode of van der Waals-force-driven self-assembly processes, as described in references (Penn and Banfield, 1998; Jordan et al., 2018). In this case, the orientation relationship between SnO2 and TiO2 is expected to be imperfect, as shown by XND data (Figure S4).

Table 1

Lattice mismatch between the rutile TiO2 {101} twin and F:SnO2 substrate

	Grain A region		Twin boundary		Grain B region
	a (Å)	c (Å)	d (Å)	h (Å)	a or h (Å)	c or d (Å)
TiO2	4.5940	2.9590	2.7322	4.9753	4.5940	2.9590
F:SnO2	4.7687	3.2036	2.8724	5.3644	5.3644	2.8724
Mismatch	3.73%	7.94%	5.00%	7.53%	15.47%	−2.97%

In previous studies, the untwinned rutile TiO2 films of nanorods or nanowires that were grown on FTO substrates commonly possess the [001]-preferred orientation, although the preferential orientation of FTO substrates is [010] direction (Lu et al., 2019; Liu and Aydil, 2009; Feng et al., 2008). The erected growth of untwinned prismatic rutile on substrates could suppress the prostrate growth due to the fact that the fastest growth rate is along [001] direction, which can account for the self-aligned behavior of one-dimensional rutile arrays on [010]-oriented substrates. From the view point of surface free energy, the tips of rutile nanorods or nanowires are commonly exposed with reactive high-energy facets ({111}, {001}, or {101}), which favors the fastest growth along [001] direction and the formation of lateral low-energy {110} facets (Lu et al., 2019, 2020a). However, in our case, after twinned TiO2 is formed, the [001] direction ceases to be the fastest growth direction. Instead, the twin plane becomes the fastest growth direction, pulling the rest of rutile crystal along its way. This is supported by the fact that the ship-like rutile twins mainly are predominated with {111} facets (some capping species may be formed in the rapid reaction environments). In effect, this type of geometric control actually governs crystal orientation in the TiO2 film, whereas the substrate only plays a minor role. Any crystals that have unfavorable orientations (with twin plane not vertical to the substrate) will be blocked by surrounding seeds, and only those that have twin plane oriented vertically to the substrate can grow freely. It is likely that they would grow in a similar manner on any substrate other than FTO, as described for ZnO films by Podlogar et al. (Podlogar et al., 2012). Therefore, the discovered texture improvement could be ascribed to the rapid formation of twinned seeds on the preferred orientation of FTO substrate in the special reaction environments.

Conclusions

In conclusion, the twin law and crystallographic texture in the anomalously aligned TiO2 nanotwin film were systematically analyzed by the synchrotron X-ray Laue nanodiffraction. The anomalous alignment was mainly ascribed to the twin-mediated heteroepitaxy with low lattice mismatch. This study also presents a state-of-the-art tool to investigate the twin structure, crystal orientation, and internal strain of functional materials at sub-100 nm resolution and would stimulate the widespread application of X-ray Laue nanodiffraction in materials science, geoscience, solar cells, and electronic and optoelectronic devices. With the rapid developments of X-ray focusing optics and advanced synchrotron light sources, we expect that the X-ray Laue nanodiffraction system will further enable the quantitative 2D/3D imaging of materials at several nanometers resolution.

Limitations of the study

It is not clear whether rutile {301} twins are present or not in the film. The orientation maps of FTO may be distorted because the grain size is too small and the software (XMAS, 2018) cannot automatically remove the indexed points from TiO2 during the auto-indexing.

Resource availability

Lead contact

yanh@idsse.ac.cn.

yang.lu@hpstar.ac.cn.

Data and code availability

yang.lu@hpstar.ac.cn; chiang.cy@nsrrc.org.tw.

Methods

All methods can be found in the accompanying transparent methods supplemental file .