Nb-substituted TiO 2 Nanosheet Exfoliated from Layered Titanate for photocatalysis
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Nb-substituted TiO2 Nanosheet Exfoliated from Layered Titanate for Photocatalysis Haiyan Song , Anja O. Sjåstad, Helmer Fjellvåg Department of Chemistry/SMN, University of Oslo, N-0315 Oslo, Norway ABSTRACT Photoelectrochemical behavior and photocatalytic decomposition of Methylene Blue (MB) were studied on (Nb,Ti)O2 nanosheet electrodes and (Nb,Ti)O2 particles produced from nanosheets. We observe that 1% Nb-substitution can drastically increase the photocurrent and photocatalytic activity of Ti1-yNbyO2 due to the formation of new defects and electron traps that can promote the separation of photo induced holes and electrons. However, high concentration of electron traps produced by significant Nb-substitution appear to serve as efficient recombination centers that cause loss of photocatalytic activity of the samples. INTRODUCTION Titania (TiO2) has many application areas, including photocatalysis [1], pigments and sensors [2]. Recently, TiO2 has also offered potentials as a mother compound of electronic materials, such as resistive memories [3], diluted magnetic semiconductors [4], and transparent conductors [5]. In particular, Nb- or Ta-substituted anatase TiO2 have attracted much attention as promising candidates for practical transparent electrode materials [6-10], which are interesting in e. g., photoelectrochemical systems. Mattsson and co-workers [11] reported photocatalytic decomposition of acetone on TiO2 and Nb-substituted TiO2 thin films, and they found that heavy Nb-substitution (t 10%) cannot improve the photocatalytic activity of TiO2, since charge transfer to the conduction band, defect and vacancy site formation must be tuned appropriately. Recently we reported a novel sol-gel assisted solid-state reaction (SASSR) route for production of phase-pure layered Nb-substituted titanates Cs0.70Ti1.8.25-xNbxO4 [12], in which the TiIV is partially substituted by NbV in the host layers. The SASSR route enables synthesis of Nbsubstituted titanates for subsequent exfoliation and opens hence up for design of Nb-substituted TiO2 films and particles. This soft chemistry route provides products with significant differences with respect to morphology, substitution level and homogeneity, as well extending the thermal stability window of the metastable anatase. Through a systematic study we are presently able to delaminate, reconstruct, and subsequently heat treating (Nb,Ti)O2 powders and films to tune both Nb-substitution as well as TiO2 polytype [13]. In the present study, we explore the potential of using Nb-substituted TiO2 nanosheets exfoliated from layered Nb-substituted titanates and transformed into anatase powders and films as the active photocatalytic material through photoelectrochemical behavior of a (Nb,Ti)O2 electrode and photocatalytic decomposition of Methylene Blue (MB) on (Nb,Ti)O2 particles. EXPERIMENT AND CHARACTERIZATION (Nb,Ti)O2 nanosheets were prepared as described in [12]. The stable suspensions of exfoliated Nb-titanates were restacked by freeze-drying; yielding a voluminous white solid with a
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