Potassium niobate nanoscrolls incorporating rhodium hydroxide nanoparticles for photocatalytic hydrogen evolution Renzhi Ma, ab Yoji Kobayashi, b W. Justin Youngblood b and Thomas E. Mallouk * b Received 14th July 2008, Accepted 18th September 2008 First published as an Advance Article on the web 5th November 2008 DOI: 10.1039/b812003j Well-dispersed rhodium trihydroxide nanoparticles (below 1 nm) were deposited into the interlayer galleries of scrolled K 4 Nb 6 O 17 nanosheets. The unmodified nanoscrolls are good catalysts for UV light- driven hydrogen evolution from aqueous methanol solutions, and their activity can be significantly improved by anchoring a small amount (0.1 wt% Rh) of Rh(OH) 3 or Rh 2 O 3 to the surface. The high hydrogen generation rate achieved in this system, 1480 mmol/h per 1g of catalyst, shows promise towards overall water splitting using these catalytic composites. Introduction Layered K 4 Nb 6 O 17 is composed of corrugated sheets of edge- sharing NbO 6 octahedra. In each layer, the top and bottom faces are different from each other, giving rise to two different interlayer environments for hydration and intercalation. 1 The asymmetry is regarded as the driving force leading to spontaneous scrolling when proton-exchanged H + /K 4 Nb 6 O 17 (or H x K 4x Nb 6 O 17 , x z 3) is exposed to aqueous tetra (n-butyl)ammonium hydroxide (TBA + OH  ) and exfoliated. 2 K 4 Nb 6 O 17 loaded with interlayer Ni or Pt clusters has shown high photocatalytic activity for the photolysis of water and alcohols under UV light. 3 Porous composites obtained by exfoliation of H + /K 4 Nb 6 O 17 and subsequent precipitation of MgO fine particles also showed high photocatalytic activity for H 2 evolution from various aqueous alcohol solutions. 4 The properties of surface-modified, platinized K 4 Nb 6 O 17 in the photolysis of HI with visible light have also been investigated. 5 However, expansion of K 4 Nb 6 O 17 interlayer gallery by silica pillaring was not successful though it could be accomplished for clays and KCa 2 Nb 3 O 10 . 6 In our previous report, the uniform dispersion of Rh(OH) 3 nanoparticles, which can be converted to Rh 2 O 3 by calcination, in the interlayer galleries of exfoliated–restacked KCa 2 Nb 3 O 10 and HCa 2 Nb 3 O 10 was achieved, which brought about a high rate of H 2 evolution under UV light. 7 We hypothesized that Rh(OH) 3 and Rh 2 O 3 nanoparticles were anchored to the calcium niobate sheets by covalent interactions, i.e., by Rh–O–Nb bonding. This motivated us to investigate another Rh–O–Nb system, specifically based on K 4 Nb 6 O 17 nanoscrolls, to look for the possibility of expanding the interlayer space with active Rh(OH) 3 nanoparticles and interesting photocatalysis functionalities. Experimental Materials preparation and characterization Synthetic K 4 Nb 6 O 17 crystals with a typical size of 1–10 mm were obtained by solid calcination of K 2 CO 3 and Nb 2 O 5 . 2 1.0 g K 4 Nb 6 O 17 was stirred in 300 ml of 1M HCl solution for 4 days to produce H x K 4x Nb 6 O 17 (x z 3); presumably the remaining K + ions reside mainly in the slowly exchanging interlayer galleries. The acid solution was renewed 3 times to promote a complete exchange. The proton-exchanged solid H x K 4x Nb 6 O 17 (0.2 g) was shaken in 50 mL of aqueous TBAOH solution (25 mM, pH 11) for 24 h to obtain a colloidal suspension of exfoliated (TBA) x K 4x Nb 6 O 17 . In order to incorporate rhodium hydroxide, the colloidal suspension was combined with different volumes of aqueous RhCl 3 (8.2 mM) solution and vigorously stirred for another day. The added amount of RhCl 3 corre- sponded to 0.1, 1.0 and 10.0 wt% Rh loading of H x K 4x Nb 6 O 17 , respectively. The yellowish suspension was then poured into 50 mL of 2 M KOH solution. As TBA + ions were replaced by K + , the exfoliated (TBA) x K 4x Nb 6 O 17 was instantly flocculated into a wool-like sediment. The precipitated product was centrifuged and rinsed with copious amounts of water to remove excess KOH. A portion of the Rh(OH) 3 /K 4 Nb 6 O 17 was calcined at 623 K in air for 1 h to convert deposited Rh(OH) 3 into Rh 2 O 3 . After calcination, a color change from yellow to grey (0.1, 1.0 wt%) or dark brown (10.0 wt%), indicating the formation of Rh 2 O 3 , was observed. Rh 2 O 3 /K 4 Nb 6 O 17 was further acid- exchanged with 1M HNO 3 to obtain Rh 2 O 3 /H x K 4x Nb 6 O 17 . X-Ray powder diffraction (XRD) patterns were obtained with a Philips X’Pert MPD diffractometer (monochromatized Cu Ka 0.15418 nm). Transmission electron microscope (TEM) images were obtained using a Philips 420 T microscope at an acceler- ating voltage of 120 kV and a JEOL JM-2010 microscope (operating at 200 kV). Samples for TEM observation were prepared by depositing a drop of ethanol-dispersed sample suspensions onto a carbon-coated copper grid and air-dried. Photocatalysis 5.0 mg catalyst was suspended in 2.0 mL of aqueous 10 vol% methanol solution in a quartz reaction cell (5.0 mL) sealed with a Nanoscale Materials Center, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan. E-mail: MA.Renzhi@nims.go.jp; Fax: +81 29 854 9061; Tel: +81 29 860 4124 b Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, 16802, USA. E-mail: tom@chem.psu.edu; Fax: +1 814 863 8403; Tel: +1 814 863 9637 5982 | J. Mater. Chem., 2008, 18, 5982–5985 This journal is ª The Royal Society of Chemistry 2008 PAPER www.rsc.org/materials | Journal of Materials Chemistry