Synthesis of Hierarchical SnO2 Nanowire–TiO2 Nanorod Brushes Anchored to Commercially Available FTO-coated Glass Substrates

Nano-Micro Lett. (2017)9:33 DOI 10.1007/s40820-017-0136-6 ARTICLE Synthesis of Hierarchical SnO2 Nanowire–TiO2 Nanorod Brushes Anchored to Commercially Available FTO-coated Glass Substrates Derek R. Miller1 . Sheikh A. Akbar1 . Pat A. Morris1 Received: 23 December 2016 / Accepted: 24 January 2017  The Author(s) 2017. This article is published with open access at Springerlink.com Highlights • • • 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 dyesensitized solar cells. Abstract Growth of single-crystal SnO2 nanowires using a fluorine-doped SnO2 (FTO) thin film as both the source and substrate is demonstrated for the first time at relatively low temperature (580 C) which preserves the integrity of the underlying glass support and improves scalability to devices. Furthermore, a microwave hydrothermal process is shown to grow TiO2 nanorods on these nanowires to create a hierarchical nanoheterostructure that will lead to efficient photogenerated charge carrier separation and rapid transport of electrons to the substrate. This process simplifies nanowire growth by using commercially available and widely used FTO substrates without the need for an additional upstream Sn source and can be used as a high surface area host structure to many other hierarchical structures. & Sheikh A. Akbar 1 Keywords SnO2
Nanowire
Fluorine-doped SnO2 (FTO)
Vapor–liquid–solid (VLS)
TiO2
Nanorod brush Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43212, USA 123 33 Page 2 of 8 1 Introduction Stannic oxide (SnO2) has been greatly studied in many fields for its electronic, catalytic, and optical properties [1–3]. 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) [4–6]. 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 [7]. 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 [12]. Carney et al. [13] 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 [16]. 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 [17–19]. This is a promising and simple process for creating nanoheterostructures on 123 Nano-Micro Lett. (2017)9:33 commercially available and widely used FTO substrates that may also lead to enhanced properties in photocatalysis, DSSCs, and gas sensing. 2 Experimental Section SnO2 nanowires were grown from a polycrystalline FTOcoated glass substrate by a high-temperature gold-catalyzed VLS mechanism. The FTO glass slides (TEC 7, Hartford Glass Company) were square shape with 2.25 cm sides and 2.2 mm thick, as shown in Fig. 1a. The polycrystalline FTO coating was measured to be 600 nm thick. The slides were cleaned by sonication for 5 min each in acetone, ethanol, and H2O and then dried in an oven at 140 C. The slides were sputter-coated with a *15-nm Au coating using a Pelco Model 3 Sputter Coater. A KaptonTM tape mask was placed over the surface during sputtering to restrict gold-catalyzed growth to a pre-defined region in the center. The substrate was loaded horizontally into a quartz tube with 2.54 cm inner diameter with no other precursors present. For optimal growth, the quartz tube was heated to 580 at 5 C min-1 under zero flow conditions. When the growth temperature was reached, 750 sccm 5% H2/N2 was flowed through a room-temperature upstream water bubbler to humidify the gas and was held for 8–12 h to facilitate growth of the nanowires. The gas flow was then shut off and the furnace cooled at 2 C min-1. The reported conditions were suitable to grow gold-tipped rutile SnO2 nanowires approximately 20–100 lm long and 40–80 nm in diameter as shown in Fig. 1b. The preparation of FTO precursor solution for a sacrificial coating followed that as described previously [4]. 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 syste (...truncated)