Facile Fabrication of WO 3 Nanoplates Thin Films with Dominant Crystal Facet of (002) for Water Splitting Jin You Zheng, † Guang Song, † Jisang Hong, # Thanh Khue Van, † Amol Uttam Pawar, † Do Yoon Kim, † Chang Woo Kim, † Zeeshan Haider, † and Young Soo Kang* ,† † Korea Center for Artificial Photosynthesis, Department of Chemistry, Sogang University, Seoul 121-742, South Korea # Department of Physics, Pukyong National University, Busan 608-737, South Korea * S Supporting Information ABSTRACT: Single crystalline orthorhombic phase tungsten trioxide monohydrate (O-WO 3 ·H 2 O, space group: Pmnb) nanoplates with a clear morphology and uniform size distribution have been synthesized by the hydrothermal method and fabricated on the surface of fluorine doped tin oxide (FTO) coated glass substrates with selective exposure of the crystal facet by the finger rubbing method. The rubbing method can easily arrange the O-WO 3 ·H 2 O nanoplates along the (020) facet on the FTO substrate. The O-WO 3 ·H 2 O nanoplate can be converted to monoclinic phase WO 3 (γ-WO 3 , space group: P21/n) with dominant crystal facet of (002) without destroying the plate structure. Crystal morphologies, structures, and components of the powders and films have been determined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, etc. The band gap energies of the O-WO 3 ·H 2 O and γ-WO 3 nanoplates were determined as ca. 2.26 and 2.49 eV, respectively. Photoelectrochemical properties of the films with (002) dominant crystal facet have also been checked for discussion of further application in water oxidation. The advantage of (002) facet dominant film was investigated by comparing to one spin-coated γ- WO 3 thin film with the same thickness via photoelectrochemical characterizations such as photocurrent, incident photon to current efficiency, and electrochemical impedance spectroscopy. 1. INTRODUCTION WO 3 has many potential applications in electrochromic devices, 1,2 gas sensors, 3 photocatalytic systems, 4 and photo- electrochemical (PEC) water splitting. 5 For PEC water splitting, mainly n-type semiconductors such as TiO 2 , 6 ZnO, 7 α-Fe 2 O 3 , 8 BiVO 4 , 9 and WO 3 5,10 are very popular. Among them, WO 3 is a very important 5d 0 transition metal oxide with a smaller band gap (∼2.8 eV) than that of other semiconductors such as TiO 2 (∼3.2 eV) and ZnO (∼3.2 eV). This results in the absorption of solar light in the visible range. WO 3 crystals show five phase transitions in the temperature range of −180 to 900 °C changing from tetragonal (α-WO 3 , > 740 °C) → orthorhombic (β-WO 3 , 330−740 °C) → monoclinic I (γ- WO 3 , 17−330 °C) → triclinic (δ-WO 3 , −43−17 °C) → monoclinic II (ε-WO 3 ,< −43 °C). 11,12 Among them, the γ- WO 3 is the most stable phase in bulk WO 3 at room temperature. Thus, the generally mentioned WO 3 refers in particular to γ-WO 3 . WO 3 possesses good hole mobility (10 cm 2 V −1 s −1 ) and long diffusion length (150 nm), much better than those of α-Fe 2 O 3 (10 −2 −10 −1 cm 2 V −1 s −1 and 2−20 nm). 13,14 WO 3 has attracted a lot of interest due to its photosensitivity, good electron transport properties, and stability against photocorrosion. 15 However, the conduction band minimum of bulk WO 3 is about 0.4 V (vs NHE at pH = 0) below the hydrogen redox potential; 16,17 thus, WO 3 photoanode can only drive half of the water splitting reaction for O 2 ; another p-type photocathode (such as p-Cu 2 O and p- Si) or external bias is required for water reduction to obtain H 2 . 18,19 The photocatalytic reactivity of a semiconductor photocatalyst is affected by its surface environment such as surface electronic and atomic structures, which critically depend on the different crystal facets. 20 The surface atomic structure tunable by crystal facet engineering can easily adjust the properties of the semiconductor, such as electronic band structure, surface energy and surface active sites, the adsorption of reactant, and desorption of reaction production. 21 Guo et al. 22 have reported that the preferential orientation of the (002) planes was possibly more favorable in adsorption and redox reaction of pollutants than preferential orientation of the (020) planes. Valdé s and Kroes 23 have investigated that photo- oxidation of water on the γ-WO 3 surfaces requires 1.04 V overpotential for (200), 1.10 V for (020), and 1.05 V for (002) by using density functional theory (DFT) calculations. Most recently, Xie et al. 24 have reported a quasi-cubic-like monoclinic WO 3 crystal with {002}, {200}, and {020} facets, which show a much higher photocatalytic O 2 evolution; a {002}-dominant sheet-like WO 3 can reduce CO 2 to CH 4 under light illumination. Up to now, the active sites at different facets and the underlying reaction mechanisms in photocatalytic Received: August 16, 2014 Revised: September 29, 2014 Published: October 3, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 6057 dx.doi.org/10.1021/cg5012154 | Cryst. Growth Des. 2014, 14, 6057−6066