Thermal Evaporation Synthesis of Vertically Aligned Zn2SnO4/ZnO Radial Heterostructured Nanowire Arrays

Gill Sang Han, Min Je Kang, 2,3,a Yoo Jae Jeong, Sangwook Lee,* In Sun Cho* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, South Korea Department of Materials Science and Engineering, Ajou University, Suwon 16509, South Korea Department of Energy Systems Research, Ajou University, Suwon 16509, South Korea School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, South Korea


Introduction
Semiconductor metal oxide nanowires composed of earth-abundant elements are technologically essential materials for energy conversion/storage devices, optoelectronics, and sensors [1][2][3][4][5][6]. Nanowires often outperform the commonly utilized isotropic polycrystalline or particulate films in applications that require more complex and multifunctional materials [7][8][9]. This is because nanowires have two different length scales (small diameter and significant length) that can be independently tailored to match the characteristic lengths of disparate physical processes. In addition, nanowires can also serve as building blocks for constructing heterostructured nanowires with designed materials that manipulate the surface, interface, and charge-transport/transfer properties, enabling multi-functionality [10,11]. For example, core/shell nanowires with type-II band alignment (staggered band edge alignment) spatially improve charge separation, leading to increased charge carrier lifetime, advantages in photocatalytic and photovoltaic performances [12,13].
However, most previous studies have reported randomly aligned nanowires with often less controllability and uniformity in the nanowire or heterostructure morphology.
In this study, we report a thermal evaporation method to synthesize vertically aligned Zn2SnO4/ZnO heterostructured nanowire arrays (HNA), demonstrating a highly aligned and uniform morphology. Single-crystalline ZnO nanowire arrays were first grown on the fluorine-doped tin oxide (FTO) substrate. Subsequently, the Zn2SnO4 shell layer was formed by the thermal evaporation of the Zn and Sn metal mixture, followed by post-annealing at 550 °C. The thermal evaporation of the elemental metal mixture allows the control of the amount of Zn and Sn independently and ensures a high vapor pressure at a low temperature (700 °C). Notably, a highly crystalline Zn2SnO4 shell layer with an average thickness of ~15 nm was successfully formed on the ZnO nanowire array. The resulting ZSO/ZnO HNA exhibited a higher surface roughness, intimate interface, and superior charge-transport properties than the pristine ZnO nanowire array.
Subsequently, the samples were annealed at 350 °C for 1 h in air to form the ZnO nanoparticle seed layer and remove organic residues.

Hydrothermal growth of ZnO nanowires array
ZnO nanowire arrays were grown on ZnO nanoparticle-seeded FTO substrates through a hydrothermal route. The growth solution was prepared by dissolving zinc nitrate hexahydrate (1.487 g, > 99%, Sigma-Aldrich Chemicals) and hexamethylenetetramine (0.701 g, > 99%, Sigma-Aldrich Chemicals) in deionized water (100 mL). After stirring for 10 min, polyethyleneimine (1.0 g, PEI, branched, Mw ~25,000, Sigma-Aldrich Chemicals) and ammonia (3.0 cc, 25-30%, Ducksan Chemicals) were added and stirred for an additional 10 min. The growth solution was poured into a glass bottle (Schott bottle, 125 mL capacity). Then, the ZnO-seeded FTO substrates were vertically suspended in the solution. Finally, the growth solution was heated to 100 °C in an oven and held for 2-6 h. The obtained samples were washed with deionized water, followed by absolute ethanol, and dried with N2 in air.

Synthesis of Zn2SnO4/ZnO heterostructure nanowires array
The Zn2SnO4 shell layer was synthesized by the thermal evaporation of an elemental Zn and Sn powder mixture in a tube furnace, followed by post-annealing in a muffle furnace. First, a Zn-Sn-O amorphous shell layer was deposited on the ZnO nanowire array (sample size: 2 cm × 2 cm) by the thermal evaporation of Zn/Sn (molar ratio of Zn/Sn = 2, 2 g) in a vacuum (1 mTorr, 700 °C for 0.5-2 h). The substrate was positioned 10 cm away from the precursor crucible on the downstream side.
Next, the samples were annealed at 550 °C for 1 h in an air atmosphere to form a crystalline Zn2SnO4 shell layer on the surface of the ZnO nanowire array.

Characterization and measurement of materials
The crystal structures of the synthesized materials were determined using X-ray diffraction The amplitude of the sinusoidal voltage was 10 mV, and the examined frequency range was 7 MHz to 1 Hz. Mott-Schottky plots were measured using a three-electrode system (a Pt wire counter electrode and saturated calomel reference electrode) in the frequency range of 300-3000 Hz. Finally, they were annealed at 550 °C in air to form a crystalline ZSO shell layer on the ZnO NW.

Results and Discussion
Notably, the thickness and morphology of the ZSO shell layer can be controlled by adjusting the thermal evaporation time. Figure 2 shows the SEM images of the synthesized ZnO NW and ZSO/ZnO HNA. The growth conditions of the hydrothermal process (e.g., NH4OH amount, growth time, and cycle) were optimized to obtain dense and vertical ZnO NW on the FTO substrate (see Figures S1-S3). The resulting ZnO NW exhibited a high-density and vertically aligned nanowire morphology, with an average length of approximately 3.5 μm (Figure 2a). In addition, the nanowires exhibit intimate contact with the FTO substrate without voids. Notably, the nanowire had a smooth surface and tapered morphology near the tip (Figure 2b). As shown in Figure 2c, the ZSO/ZnO HNA also exhibits a comparable length of 3.6 μm. However, the ZSO/ZnO HNA exhibits a slightly larger nanowire diameter than the ZnO NW. Interestingly, their surface was much rougher than the ZnO NW because of the formation of nanoparticles at the surface (Figure 2d).
XRD and Raman spectroscopy were performed to confirm the formation of the ZSO shell layer on the ZnO NW (Figure 3). Figure 3a shows the XRD patterns of the ZnO NW and ZSO/ZnO HNA. The ZnO NW exhibits a strong (002) peak intensity, indicating a preferred growth orientation along the [00l] direction. The ZSO/ZnO HNA also exhibits a high (002) peak intensity, retaining the [00l] preferred orientation of the ZnO NW. In addition, three additional weak peaks are observed at 17.6, 29.2, and 34.3°, which are indexed to the (111), (220), and (311) planes of the cubic Zn2SnO4 phase, respectively [32]. Figure 3b shows the Raman spectra of both ZnO NW and ZSO/ZnO HNA.
The ZnO NW exhibits a broad peak centered at 443 cm -1 , which corresponds to the E2 mode for ZnO [34]. After deposition of the ZSO shell layer, that is, for the ZSO/ZnO HNA, two peaks, at 443 and 673 cm -1 , are observed, corresponding to the E2 mode for the ZnO and A1g modes (stretching vibration mode of SnO6 octahedra) of spinel-type Zn2SnO4 [35]. Consequently, a Zn2SnO4 shell layer with nanoparticle morphology was successfully formed on the ZnO NWs via thermal evaporation, followed by post-annealing.
TEM analyses were conducted to investigate the microstructure and interface of both ZnO NW and ZSO/ZnO HNA (Figure 4). Figures 4a and 4b show the TEM and high-resolution TEM images of the ZnO NW, respectively. The ZnO NW exhibits tapered tips and a smooth surface. The Accordingly, the ZnO and ZSO heterostructures have a staggered band edge; that is, they form a type-II heterojunction, which improves the spatial charge separation [32,36].
The charge-transport properties of both electrodes (ZnO NW and ZSO/ZnO HNA) were evaluated by EIS measurements. As shown in Figure 5c, the ZSO/ZnO HNA exhibited a smaller semicircle than the ZnO NW, indicating reduced charge transport and transfer resistance values. In addition, the relative surface area was estimated using a dye-adsorption method (Figure S6), suggesting that the ZSO/ZnO HNA has a 130% larger surface area than the ZnO NW. Therefore, the construction of the ZSO/ZnO HNA improved charge separation, transport, and transfer (injection) properties (Figure 5d), which is attributed to the formation of type-II heterojunctions, intimate interfaces, and superior surface roughness.

Conclusion
We