Orientation Relationships and Interface Structuring in Au-Seeded TiO₂ Nanowires

Abstract

The Au–TiO₂ interface plays a critical role in heterogeneous catalysis and nanostructure synthesis relevant to renewable energy applications. Using Au-seeded TiO₂ nanowires as the model system, we observe that, in addition to the commonly reported orientation relationships (ORs) and atomically sharp interfaces, Au–TiO₂ interfaces can also exhibit ORs involving high-indexed planes, often accompanied by local disorder and atomic reconstructions involving multiple Ti-O monolayers. These interfacial rearrangements are promoted by high-temperature thermal treatment at 1000 °C during nanowire growth. The findings broaden our understanding of orientation relationships and interface structures in the Au–TiO₂ system, offering valuable insights into interface-driven synthesis of oxide nanostructures and guiding future strategies for interface engineering in catalytic and electronic applications.

1. Introduction

Nanoscale Au–TiO₂ exhibits remarkable catalytic performance in diverse chemical reactions, including the hydrogenation and oxidation of complex organic molecules such as methanol and carbon monoxide—processes central to numerous renewable energy applications [1,2,3,4,5,6]. Au–TiO₂ interfaces frequently operate as active sites for these reactions [7,8,9,10] and are crucial in enabling strong metal–support interactions (SMSIs) [11,12], which are known to induce the formation of a TiOₓ overlayer encapsulating Au nanoparticles. In parallel, Au nanoparticles frequently act as seeds for vapor–liquid–solid (VLS)-like growth of TiO₂ nanostructures, promoting TiO_x mass transport at elevated temperatures [13,14,15,16,17,18], and enabling their integration into solar energy and battery devices [19,20,21]. Despite their technological importance, the structural characteristics of Au–TiO₂ interfaces—including preferred orientation relationships (ORs) and atomic arrangements—remain insufficiently understood.

The crystallographic aspects of the Au–TiO₂ (rutile) interfaces are summarized by Cosandey [22], who reported four orientation relationships (ORs) that aligns the best lattice-matched direction pair, ${\langle 1\overline{1}0\rangle }_{\mathrm{A}\mathrm{u}}//{\langle 001\rangle }_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, along with low-indexed planes of Au parallel to TiO₂$\left\{110\right\},$ the rutile surface with the lowest reported energies. Amongst these, ORa, ${\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, is most detected across different preparation processes and thermal treatments, suggesting it represents a thermodynamically favored configuration. Other ORs (ORb–ORd) are less commonly observed and are generally associated with certain synthetic conditions. For instance, ORb, ${\left\{112\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, is reported during elevated-temperature deposition [23,24], whereas ORc, ${\left\{001\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, is frequently associated with precipitation utilizing HAuCl₄ precursors on TiO₂ powders following 200–400 °C treatment [25]. Gold clusters formed via ion implantation and thermal treatment can exhibit ORa–ORd (ORd: ${\left\{110\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$) [26]. Particularly at thermal treatment >800 °C, additional ORs, named as ORe–f which aligns high-indexed planes of Au with TiO₂$\left\{110\right\}$, are observed, with their fractions increasing as temperature rises [27]. ORa–f agree well with theoretical predictions based on the geometrical constraints of adjacent Au and TiO₂ lattices with the best-matched direction pair, ${\langle 1\overline{1}0\rangle }_{\mathrm{A}\mathrm{u}}//{\langle 001\rangle }_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ [27].

Focused on the most observed ORa interfaces, atomic-resolution electron microscopy reveals that the distance between the interfacial ${\left\{111\right\}}_{\mathrm{A}\mathrm{u}}$ and the first Ti–O plane is 0.33 nm [28], while the distance becomes 0.28 nm after thermal treatment at 500 °C in ultra-high vacuum (UHV) [22]. The equilibrium distance is sensitive to the TiO₂ surface terminations—whether stoichiometric, reduced, or reconstructed—according to the density functional theory (DFT) calculations for ORa and ORc [29]. In addition, ORb interfaces can contain several monolayers that undergo significant atomic arrangement after thermal treatment at 800 °C [30]. Interestingly, gold nanoparticles on TiO₂ substrates have been observed to rotate to 9.5$°$ during in situ impregnation and calcination, accompanied by interfacial step changes. This rotation is reversible and suggests a dynamic interplay between interfacial structure and processing conditions [10]. Given the possibility of diverse interfaces and their dependence on processing routes, a comprehensive study of the ORs and detailed structures of the interfaces, particularly following high-temperature thermal treatment and exposure to gaseous species, is essential.

In this study, we investigate the Au–TiO₂ (rutile) interfaces using Au-seeded vapor-phase growth of rutile nanowires as the model system. This system undergoes thermal treatment at 1000 °C and involves substantial mass transport of vaporized TiO₂ species for the growth of TiO₂ nanowires [14,31,32]. Transmission electron microscopy (TEM) investigations are conducted to examine the orientation relationships (ORs) and detailed structures of the Au–TiO₂ interfaces beneath Au seeds. Our results reveal a range of interfaces, highlighting their role in mediating the mass transport essential for nanowire growth, and deepening the understanding of these technologically important interfaces.

2. Materials and Methods

Au-seeded TiO₂ nanowires were obtained following our recently developed approach [14,31,32]. Rutile TiO₂$\left(110\right)$ single-crystal substrates (5 × 5 × 1 mm, MTI Corp., Richmond, VA, USA) were ultrasonically cleaned for 30 min in high-purity acetone (99.99%) and cleaning repeated six times using a fresh solvent each time. Polycrystalline gold films (approximately 10 nm thick) were deposited at room temperature using a Hitachi E-1045 ion sputter coater. The gold-deposited substrates were placed in quartz tubes, which were then filled with high-purity argon gas (99.99%) at a pressure of 100 mmHg and sealed. The sealed tubes were heat-treated at 1000 °C for 60 min in a GSL-1400X tube furnace (MTI Corp., Richmond, VA, USA) and subsequently cooled in the furnace.

Nanowires were scratched from the substrate using a sharp blade and transferred onto copper grids for transmission electron microscopy (TEM) analysis. To study the crystallographic orientation and detailed structure of the Au–TiO₂ interfaces, bright-field (BF) imaging, selected area electron diffraction (SAED), and high-resolution TEM imaging of the Au–TiO₂ interfaces were conducted using an FEI Talos 200X operating at 200 kV (Thermo Fisher Scientific Inc., Waltham, MA, USA). The beam current is in the nA range. The exposure times for imaging range from 0.5 s to 2 s at a binning of 1.

3. Results

As shown in the bright-field (BF) micrographs in Figure 1, Figure 2 and Figure 3, the grown single-crystal nanowires exhibit a bead-like morphology that resembles the reported Wulff shape of rutile, enclosed by two ${\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and eight ${\left\{101\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ sidewall facets. The gold seed particles are positioned atop the rutile nanowires, with the Au–TiO₂ interfaces parallel to the ${\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. This is consistent with nanowires synthesized using the same growth approach [14,31,32]. The orientation relationships (ORs) and associated Au–TiO₂ interfaces are examined with 21 individual nanowires. To facilitate analysis of the interfacial structures, nanowires are preferentially viewed along the best lattice-matched direction pair, i.e., ${\langle 1\overline{1}0\rangle }_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, with ${\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ edge-on, corresponding to an edge-on Au–TiO₂ interface. Due to the limited tilting range of the TEM, nanowires are also analyzed from other low-indexed direction pairs, such as ${\langle 11\overline{2}\rangle }_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}3⟩}_{{TiO}_2}$ and ${\langle 1\overline{1}0\rangle }_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. For nanowires where no common low-indexed direction pairs are accessible, the Au seeds or TiO₂ nanowires are viewed individually along their low-indexed directions by tilting the specimen along the Kikuchi lines associated with the best lattice-matched direction pair.

As summarized in

Table 1. The Au-seeded TiO₂ nanowire interfaces: the detected orientation relationships (ORs), the number of nanowires exhibiting each OR, their type, and measured deviations from the ideal ORs.

The Orientation Relationships (ORs)	No.	Rotation	Type
${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{\left\{111\right\}}_{\mathrm{A}\mathrm{u}}$$\mathrm{with}\rho$$\mathrm{off}{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	9	$\delta <$$6°$	ORa
${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}$$\mathrm{with}\rho$$\mathrm{off}{⟨1\overline{1}2⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{{\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	4	$\rho <$$8°$	ORa
${⟨11\overline{2}⟩}_{\mathrm{A}\mathrm{u}}{//⟨1\overline{1}3⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{{\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	3	$\rho ~$$0.93°$	ORa
${⟨10\overline{1}⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}1⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{{\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	1	$\rho ~$$5.5°$	ORa
${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{\left\{11\overline{1}\right\}}_{\mathrm{A}\mathrm{u}}$$\mathrm{with}2°$$\mathrm{off}{\left\{020\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	1	$\delta$$~2°$	ORe2
${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}~{\left\{117\right\}}_{\mathrm{A}\mathrm{u}}{//\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	1	$\eta ~$$15°$	~ORe2
${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}$$\mathrm{with}~2°$$\mathrm{off}{⟨011⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{{\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//\left\{100\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	1	$\delta$$~1.7°$	ORe2
${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}$$\mathrm{with}~15°$$\mathrm{off}{⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$$\mathrm{and}{{\left\{110\right\}}_{\mathrm{A}\mathrm{u}}//\left\{001\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$	1	$\delta$$~3.43°$	ORg2

Note: $\delta$: out-of-plane rotation about ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$; $\rho$: in-plane rotation; $\eta$: deviation from ideal ORs.

, approximately 80% nanowires exhibit ORa, most with minimal angular deviations ($\delta <6°$) from the ideal ORa. The measured rotation angle $\delta$ corresponds to out-of-plane rotation about ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ while $\rho$ denotes the in-plane rotation of ${⟨111⟩}_{\mathrm{A}\mathrm{u}}//{⟨110⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, which is normal to the ORa interface. Notably, $\delta <6°$ and $\rho <8°$ for most nanowires. The angular difference between ORa and ORf1 is $\delta =8.2°$, suggesting that some nanowires classified as ORa may lie close to ORf1. Approximately 15% nanowires adopt ORe2, also accompanied by modest angular deviations. $\delta ~2°$ also represents out-of-plane rotation about ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ while $\rho ~15°$ is seen for the in-plane rotation of ${~⟨117⟩}_{\mathrm{A}\mathrm{u}}//{⟨110⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. Only one nanowire (~5%) shows ORg2, with a notable out-of-plane rotation of ~3.43° from the ideal ORg2. Typical examples of each type are further illustrated in Figure 1, Figure 2 and Figure 3.

Figure 1 presents the BF micrographs and corresponding selected area electron diffraction (SAED) patterns of representative Au-seeded TiO₂ nanowires exhibiting ORa. All nanowires maintain distinct contrast along its length, resulted from their bead-like shape. TiO₂ reflections are labeled in white, as a white square in Figure 1b,d and a white hexagon in Figure 1f, while Au reflections are highlighted by yellow hexagons in all diffraction patterns. Figure 1a–d show nanowires viewed along ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, selected to illustrate possible out-of-plane rotations. Figure 1e,f show nanowires viewed along ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}1⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, with the diffraction pattern acquired separately from the Au seed and TiO₂ nanowires to highlight potential in-plane rotation.

Figure 1a,b depict the ideal ORa, defined by the epitaxial alignment ${\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. In contrast, the nanowire in Figure 1c has ${\left\{111\right\}}_{\mathrm{A}\mathrm{u}}$ reflections deviate from ${\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ reflections in Figure 1d, indicating a slight out-of-plane rotation of roughly 4° about the ${⟨110⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ common axis. Amongst the 9 listed ORa nanowires in Table 1, the out-of-plane rotations range from 0 to 6°. Figure 1e–g illustrate other nanowires with ${⟨10\overline{1}⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}1⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and ${{\left\{111\right\}}_{\mathrm{A}\mathrm{u}}//\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. The direction pair of ${⟨10\overline{1}⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}1⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ has an in-plane rotation of $~5.5°$ from ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ of the ideal ORa, with a rotation axis of ${⟨111⟩}_{\mathrm{A}\mathrm{u}}//{⟨110⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. While the in-plane rotation varies from nanowire to nanowire, it generally remains below 8°.

Figure 2 presents typical examples of Au-seeded TiO₂ nanowires exhibiting ORe2. ORe1–2 is defined by the alignment of ${\{114\mp 3\sqrt{2}\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, where ${\left\{111\right\}}_{\mathrm{A}\mathrm{u}}$ reflections overlap with ${\left\{020\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ reflections when viewed along ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. The nanowire shown in Figure 2a slightly deviates from the ideal ORe2, as evidenced by a ~2° offset between ${\left\{111\right\}}_{\mathrm{A}\mathrm{u}}$ and ${\left\{020\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ reflections in the corresponding SAED pattern in Figure 2b. This offset indicates a small out-of-plane deviation of ~2° around ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//⟨{001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. Figure 2c,d shows the high-resolution TEM micrograph and overlapped SAED pattern acquired with the electron beam aligned along ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. Figure 2d indicates a nearly parallel alignment of ${\left\{117\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, characteristic of the ORe2 (i.e., ${\{114+3\sqrt{2}\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$). ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ suggests a significant deviation of ~15° from the ideal direction pair, ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$.

Figure 3 illustrates a TiO₂ nanowire exhibiting ORg2 with its Au seed. ORg2 is defined by the alignment of ${\left\{113\sqrt{2}\right\}}_{\mathrm{A}\mathrm{u}}//{\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, where ${\left\{002\right\}}_{\mathrm{A}\mathrm{u}}$ reflections overlap with ${\left\{2\overline{1}0\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ reflections when viewed along ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. The bead-like morphology of the nanowires is clearly viewed in the BF micrograph in Figure 3a acquired along ${⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. Within the accessible tilting range of the TEM, the low-indexed directions, ${⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and ${⟨110⟩}_{\mathrm{A}\mathrm{u}}$, are related by a 15° rotation about the best lattice-matched direction pair of ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. Viewed along ${⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, Figure 3b–d show the SAED pattern of TiO₂, the convergent-beam electron diffraction (CBED) pattern of TiO₂, and the CBED pattern of Au, respectively. The Kikuchi lines, corresponding to ${\left\{001\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ and ${\left\{1\overline{1}0\right\}}_{\mathrm{A}\mathrm{u}}$, labeled by the white lines in Figure 3c,d, are parallel, confirming the alignment of their normal, the best lattice-matched direction pair, ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. Similarly, when viewed along ${⟨110⟩}_{\mathrm{A}\mathrm{u}}$, Figure 3e–g present the SAED of Au, the CBED of TiO₂, and the CBED of Au. The above two viewing directions are reached by tilting the specimen approximately 0.5° in $\alpha$ and 15° in $\mathsf{\beta }$, where $\alpha$ and $\mathsf{\beta }$ are the tilting axes of the TEM holder. After rotating back by 15° from ${\left\{002\right\}}_{\mathrm{A}\mathrm{u}}$ around the ${⟨110⟩}_{\mathrm{A}\mathrm{u}}$ axis, the Au seed is identified as having ${\left\{115.3\right\}}_{\mathrm{A}\mathrm{u}}$, indicating that ${\left\{001\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ is nearly parallel to ${\left\{115.3\right\}}_{\mathrm{A}\mathrm{u}}$. Therefore, this nanowire is consistent with ORg2 having ~3.43$°$ out-of-plane rotation.

Figure 4 shows the high-resolution transmission electron microscopy (HRTEM) characterization of ORa (in Figure 4a) and ORe2 Au–TiO₂ interfaces (in Figure 4b), respectively. The Au lattice fringes are barely visible, likely due to the thickness of the seed exceeding ~100 nm and the large atomic number of Au. Consistent with previous reports [14], TiO_x layers, with discernible lattice fringes, are observed on the Au surface, as indicated by the black arrows. Away from the seed–nanowire–vapor triple line, Au–TiO₂ interfaces appear flat and are parallel to ${\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, though they lack atomic sharpness. The measured spacing of ${\left\{110\right\}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ is ~0.31 nm, closely matching the theoretical value of 0.32 nm. It is worth noting that the ORe2 interface has a reduced spacing of ~0.2 nm between the topmost Ti-O monolayer and the second layer, and ~0.27 nm between the second and third layers (see Figure 4b). Near the seed–nanowire–vapor triple line, the interface gradually transitions from a sharp to a diffuse interface over several atomic layers. In this region, the atomic configuration becomes locally disordered and misoriented; the interatomic spacing of ~0.32 nm is slightly larger than the 0.31 nm measured within the nanowire.

4. Discussion

The Au–TiO₂ interfaces between the seed and the grown nanowires predominantly exhibit ORa, identified in nearly 80% of the nanowires. In addition, several other ORs, including ORe2 (~15%) and ORg2 (~5%), are also identified. In contrast to Cosandey’s reports [22], the current results reveal ORs involving high-indexed planes, which are first predicted in our earlier work using dewetted Au nanoparticles on TiO₂(110) substrates [27]. However, the relative fractions of these ORs are not statistically reliable due to the limited number of nanowires analyzed and the inherent challenges of identifying the viewing direction pairs needed to resolve these ORs involving high-indexed planes. Future research utilizing a more extensive dataset would allow for a more robust validation of the frequency of different ORs.

These findings are generally consistent with the increasing occurrence of these ORs, particularly ORe, at elevated temperatures, as evidenced by the X-ray diffraction (XRD) pole figures obtained from dewetted Au nanoparticles on TiO₂(110) substrates after thermal treatment at 800 °C and 1000 °C in air [27]. Although short TiO₂ bases [33] and ridges [27] are noted beneath the dewetted Au nanoparticles, no nanowire growth is observed. This suggests the formation of these ORs, involving high-indexed planes, does not result from the growth of TiO₂ nanowires underneath Au seeds.

Angular deviations, typically on the order of a few degrees, are observed around well-defined crystallographic directions, including out-of-plane rotation about the best lattice-matched direction pair, ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, as well as in-plane rotation of the interfaces, particularly for ORa. These out-of-plane and in-plane rotations are also consistent with the XRD pole figures [27], where the corresponding ORa poles appear in arc- and star-like shapes. It is worthy of note that the ORa poles in an arc-like shape, indicating that large in-plane rotations of up to 20° are also detected for dewetted Au nanoparticles on TiO₂ substrates after being thermally treated at ~500 °C [22]. Such misalignment is largely attributed to the insufficient alignment between Au and TiO₂ lattices at lower temperatures. In contrast, high-temperature thermal treatment likely facilitates the alignment of adjacent Au and TiO₂ lattices, contributing to the detected angular deviations of ORa.

It is interesting to note that the OR in Figure 2c,d aligns with an alternative low-indexed direction pair, ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨1\overline{1}0⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, which exhibits a significant deviation (~15°) from the best lattice-matched direction pairs, ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$. This direction pair is also characteristic of ORc, which is frequently seen in systems involving Au precipitation on TiO₂ powders [25] and Au precipitation within TiO₂ bulk [26]. Although the lattice mismatch for this pair is considerably larger than ${⟨1\overline{1}0⟩}_{\mathrm{A}\mathrm{u}}//{⟨001⟩}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$, its repeated occurrence suggests it may represent another energetically favorable alignment.

Unlike previously reported Au–TiO₂ interfaces with ORa [22], ORb [22], and ORe-f [27], where the Au lattice bridge onto the TiO₂$\left(110\right)$ with various surface terminations [22,29], the observed interfaces lack atomic sharpness and, in the case of the ORe type, comprises two monolayers. Toward the seed–nanowire–vapor triple line, the interface extends into multiple layers. These Ti-O monolayers become increasingly disordered or loosely bonded near the Au seed, possibly associated with the mass transport necessary for growing TiO₂ nanowires. Lattice defects, such as lattice distortions or partially amorphous patches, may produce supplementary diffusion paths with fewer migration barriers, thus facilitating local mass transport. Such local disorder may be associated with the local saturation of the vapor growth species [34,35] and influence their adsorption and desorption kinetics, ultimately affecting the grown morphology [36,37]. This region is recognized as a preferred nucleation site in vapor–liquid–solid (VLS)-like nanowire growth [32,38], further supported by the observed atomic reconstruction near the interface. It should be noted that one ORb interface also consists of multiple Ti-O layers, associated with the growth of short TiO₂ bases following thermal treatment at 800 °C [33].

The orientation relationships (ORs) and corresponding interfacial reconstructions identified in this study may influence the catalytic efficacy of Au–TiO₂ nano-catalysts. Various ORs can modify the density and distributions of active sites at the metal–oxide interfaces, influencing charge transfer efficiency and the adsorption energetics of reactants [6,39].

5. Conclusions

In this study, Au–TiO₂ interfaces, after treatment at 1000°, are systematically investigated using Au-seeded TiO₂ nanowires as a model system. The interfaces exhibit distinct orientation relationships, mainly ORa and ORs such as ORe2 and ORg2 which pertain to high-index planes. The observed interfaces often consist of multiple Ti-O monolayers, particularly near the seed–nanowire–vapor triple line. The formation of these ORs and interface reconstructions involving multiple monolayers are facilitated by high-temperature heat treatment. These findings deepen the understanding of ORs and detailed structures of Au–TiO₂ interfaces, providing valuable insight into the interface-driven synthesis of oxide nanostructures and providing guidance for interface engineering for energy-related applications.