Synthesis of Hierarchical SnO2 Nanowire–TiO2 Nanorod Brushes Anchored to Commercially Available FTO-coated Glass Substrates
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KeywordsSnO2 Nanowire Fluorine-doped SnO2 (FTO) Vapor–liquid–solid (VLS) TiO2 Nanorod brush
Single-crystal SnO2 nanowires can be grown from polycrystalline thin films of fluorine-doped SnO2 (FTO) on glass substrates at a reduced temperature of 580 °C without the need for an upstream source powder.
A novel hierarchical nanobrush-like structure was designed to maximize the surface area of the photoactive material while aiding more efficient photogenerated charge carrier separation and extraction through the nanowire core for photocatalysis and dye-sensitized solar cells.
The SnO2 nanowires were used as an anchored 3D host to create a novel hierarchical nanobrush-like structure, and this immobilized structure was designed to maximize the surface area of the photoactive material while aiding more efficient photogenerated charge carrier separation and extraction through the nanowire core for photocatalysis and dye-sensitized solar cells.
Stannic oxide (SnO2) has been greatly studied in many fields for its electronic, catalytic, and optical properties [1, 2, 3]. It is often employed as a transparent conducting oxide (TCO) by doping with indium (ITO) or fluorine. These transparent substrates are heavily used in optoelectronic applications as transparent conductors such as thin film LEDs and dye-sensitized solar cells (DSSC) [4, 5, 6]. In DSSC applications, it is imperative that nanomaterials can effectively separate photogenerated electron–hole pairs and that the electrons can be harvested into the external circuit from the nanomaterials before recombination or trapping in defect states . To this end, high-quality single-crystal nanowires or nanorods are often employed as the active material [8, 9]. However, nanowires deposited as a slurry are randomly oriented and do not necessarily lead to the TCO layer or the external circuit. TiO2 nanorods have been grown, deposited, or attached to FTO substrates in a number of clever ways including double-sided brush-like flakes via hydrothermal reaction [10, 11] and around carbon fibers via microwave hydrothermal reaction . Carney et al.  originally showed that Au-tipped SnO2 nanowires could be grown directly from pressed and sintered 95% SnO2–5% CoO pellets with a sputtered Au thin film through a vapor–liquid–solid (VLS) mechanism at 700–800 °C. This method greatly simplifies nanowire growth compared to many other techniques that utilize an upstream source and downstream substrate [14, 15], often needing independently controlled temperature zones to induce condensation . The present work shows that a modification of this method can grow single-crystal SnO2 nanowires from polycrystalline thin films of fluorine-doped SnO2 (FTO) on glass substrates at a reduced temperature of 580 °C. These nanowires are then used as an anchored 3D host to create a novel hierarchical nanobrush-like structure using hydrothermally grown TiO2 nanorods. This immobilized structure is designed to maximize the surface area of the photoactive material while aiding more efficient photogenerated charge carrier separation and extraction for photocatalysis and dye-sensitized solar cells. Photogenerated electrons should move into the single-crystal SnO2 nanowire core due to a larger work function, leaving the separated hole to react with the solution or dye at the surface. The high-quality core nanowire can then help shuttle the electrons directly to the thin film on which it is anchored and grown, where it can then be collected to pass through an external circuit. Similar morphologies of nanoheterostructures have been synthesized with different combinations of materials previously and led to unique or enhanced properties [17, 18, 19]. This is a promising and simple process for creating nanoheterostructures on commercially available and widely used FTO substrates that may also lead to enhanced properties in photocatalysis, DSSCs, and gas sensing.
3 Experimental Section
The preparation of FTO precursor solution for a sacrificial coating followed that as described previously . SnCl4 ·5H2O (99.99%), NH4F (99.99%), and ethanol (99.5%) were purchased from Sigma–Aldrich. Microwave hydrothermal growth of TiO2 nanorods was carried out using a CEM Discover SP microwave digestion system. TEM samples were prepared by sonicating the nanowires or powders into a dispersion in methanol and dropping the dispersion onto 3-mm copper lacey carbon grids, followed by drying at 140 °C. TEM electron diffraction was performed by an FEI/Phillips CM200T. X-ray diffraction (XRD) was performed using a Rigaku Smart Lab with Cu-Kα radiation (λ = 1.5408 Å).
4 Results and Discussion
The growth of the nanowires is attributed to the well-known VLS mechanism as evidenced by the Au-tipped nanowires. The main difference from previous studies is that the source of the Sn and O is from the substrate itself, rather than from an upstream source powder. The source/substrate FTO is a relatively stable oxide, so the 5% H2/N2 is needed to reduce the oxide to induce vaporization as SnO or Sn metal, while the humidified flow provides the necessary oxygen to re-oxidize the material pushed out of the Au nanoparticles into nanowires. This atmosphere is likely the key to allowing a lower growth temperature than other studies.
The FTO thin film was measured at roughly 600 nm thick by tracking the ratio of Sn (FTO) to Si (glass) in an energy-dispersive X-ray spectroscopy (EDX) line scan of the substrate cross section and also directly imaged using the electron backscatter detector. EDX analysis in Fig. 1d of the as-received FTO thin film detected only a small bump in signal where a fluorine elemental peak is expected, but not significant enough to be assigned by the program. X-ray photoelectron spectroscopy (XPS) shown in Fig. 1e also detected only a small increase in signal at 687 eV. This is compared to that of the nanowires, which shows no increase, inset in Fig. 1e. These methods are barely sensitive enough to detect any presence of fluorine in the as-received FTO, so it is unknown if any of the original fluorine doping was incorporated into the nanowires.
To reduce the degradation of the FTO thin film, an additional solution-based FTO layer was spin-coated over the original layer to act as a sacrificial source for growth. Sacrificial coatings were added using approximately 60 µL of solution via pipette onto the substrate while on a spin coater at 500 rpm. Although the catalyst layer was still necessary, this additional layer assisted growth of longer nanowires and allowed adequate growth at slightly lower temperatures of 560–570 °C, as shown in Fig. 2d. However, even if the sacrificial layer protected the underlying thin film, the resistance of the top layer was quite high at 3 kΩ (560 °C) and 20 kΩ (570 °C), likely due to the poor quality of the spin-coating that leaves many microcracks after drying and annealing. Careful modification of the thickness of the deposited coating and growth conditions may eventually achieve growth solely from the sacrificial layer without affecting the high-quality transparent thin film. This coating was also applied to a bare Si wafer, and after gold sputtering, nanowire growth was achieved at the same conditions as on the FTO slide. This demonstrates that a simple solution-based coating can be used to grow SnO2 nanowires on nearly any substrate that is stable above 580 °C under reducing conditions without the need for an upstream precursor or a reaction chamber with multiple independent temperature-controlled zones.
TiO2 nanorods were precipitated onto the SnO2 nanowires using a microwave hydrothermal method similar to that described previously . However, the previous study employed traditional hydrothermal growth in a PTFE autoclave over 1–24 h and grew nanorods directly on a blank FTO slide. This was first replicated in the microwave hydrothermal chamber, and then a slide with pre-grown SnO2 nanowires was used as the growth substrate. The SnO2 nanowire-covered FTO substrates were placed upright into a quartz vial reaction chamber containing a solution of 5 mL H2O, 5 mL HCl, and 0.5 mL titanium (IV) butoxide. The inner diameter of the quartz tube was 2.54 cm, which allowed two FTO slides to stand upright back-to-back with no additional support. This prevented TiO2 deposition on the back of the slides, allowed a small magnetic stir bar to freely spin in the tapered bottom of the vial, and prevented settling of large TiO2 particles on the surface of the slides. A typical growth experiment used 150 W of intermittent microwave power to hold the reaction chamber temperature at 150 °C for 5 min with the resulting pressure holding between 150 and 250 psi. Reaction times greater than 5 min did not increase the coverage of nanorods on the nanowires, suggesting the nucleation and growth is complete after that period. Nanorods did not sufficiently grow at temperatures below 150 °C or pressure below 150 psi, but higher pressure and temperature did not change the morphology. The previously cited study  conducted extensive analysis of varying reaction parameters and growth mechanisms of the TiO2 nanorods, so the most promising formula and reaction conditions were chosen for this study, but achieved instead via microwave heating. The reader is referred to this paper  for more details on the nucleation and growth process of the nanorods.
This study has demonstrated a simple process for growing high-quality SnO2 nanowires anchored directly to a FTO thin film. Additional solution-deposited FTO coatings offer a versatile method to grow SnO2 nanowires from any substrate that can withstand the processing conditions. The novel hierarchical nanobrushes consisting of SnO2 nanowires and hydrothermally grown TiO2 nanorods are promising for future applications of photocatalysis, water splitting, and dye-sensitized solar cells (DSSCs). The remaining challenge is to fabricate such a device without affecting the quality or properties of the thin film or substrate. This could be overcome by fabricating a thicker FTO film that is not as susceptible to degradation with nanowire growth. The underlying glass substrate could also be substituted with sapphire or quartz to allow growth at higher temperatures without softening of the slide. Future studies will explore the properties of this nanostructure and its implications in fields such as catalysis, photocatalysis, and gas sensing.
This work was funded by a NASA Space Technology Research Fellowship and a Facilities Grant from the Institute for Materials Research (IMR) at The Ohio State University.
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