Article pubs.acs.org/JPCC
Preparation of RuO2/TiO2 Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations Md. Tamez Uddin,† Yohann Nicolas,† Céline Olivier,† Thierry Toupance,*,† Mathis M. Müller,‡ Hans-Joachim Kleebe,‡ Karsten Rachut,‡ Jürgen Ziegler,‡ Andreas Klein,‡ and Wolfram Jaegermann‡ †
Institut des Sciences Moléculaires, Univ. Bordeaux, UMR 5255 CNRS, 351 Cours de la Libération, F-33405 Talence Cédex, France Institute of Material Science, Technische Universität Darmstadt, Petersenstrasse 23, D-64287 Darmstadt, Germany
‡
S Supporting Information *
ABSTRACT: Nanoporous RuO2/TiO2 heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO2 nanoparticles in a ruthenium(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO2 amount (0.5−5 wt %), the asprepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO2 nanocrystallites modified with RuO2 according to N2 sorption, TEM, and XRD analyses. Furthermore, a careful attention has been paid to determine the energy band alignment diagram by XPS and UPS in order to rationalize charge separation at the interface of RuO2/TiO2 heterojunction. At first, a model experiment involving stepwise deposition of RuO2 on the TiO2 film and an in situ XPS measurement showed a shift of Ti 2p3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO2/TiO2 heterojunction. The band bending for the heterostructure RuO2/TiO2 nanocomposites was then found to be 0.2 ± 0.05 eV. Photocatalytic decomposition of methylene blue (MB) in solution under UV light irradiation revealed that the 1 wt % RuO2/TiO2 nanocatalyst led to twice higher activities than pure anatase TiO2 and reference commercial TiO2 P25 nanoparticles. This higher photocatalytic activity for the decomposition of organic dyes was related to the higher charge separation resulting from built-in potential developed at the interface of RuO2/TiO2 heterojunction. Finally, these mesoporous RuO2−TiO2 heterojunction nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant that opens new avenues toward potential industrial applications.
1. INTRODUCTION Since the pioneer works of Fujishima and Honda in 1972 who succeeded in splitting water into hydrogen and oxygen under light irradiation employing titanium dioxide (TiO2) as anode material,1 numerous research activities have been carried out over the past two decades to develop semiconductor photocatalysts with high activities for purifying various environmental pollutants in water and air.2−4 Among the different metal oxide semiconductors, TiO2 has been widely used as a photocatalyst because of its relatively high photocatalytic activity, biological and chemical stability, low cost, nontoxicity, and long-term stability against photocorrosion and chemical corrosion.2,3 However, the high recombination rate of photoinduced electron−hole pairs produced during photocatalytic processes limits the application of TiO2. As a consequence, careful attention has been paid to tune TiO2-based nanomaterials in order to improve their photocatalytic activity. Among the different approaches, surface modification by coating noble metal on TiO2 surface is one of the effective methods to reduce electron−hole recombination in the photocatalytic process.5−11 © 2013 American Chemical Society
Depending on alignment of the energy levels, the deposits act as electron sinks or hole accumulators, favoring separation of the photogenerated charges and enhancing the oxidation and reduction reactions required for organic degradation. This is due to the possible induced space charge layer when a metal comes into contact with a semiconductor. A Schottky barrier is thus created that facilitates the transfer of electrons or holes from the semiconductor to the metal depending upon the work function of the latter.12 Along noble metals like gold or platinum, other metallic compounds can also be associated with TiO2 to enhance its photocatalytic performances. As an example, ruthenium(IV) oxide (RuO2), which belongs to the family of transition-metal oxides with rutile-like structure, shows an interesting variety of properties. Because of its high chemical stability, electrical (metallic) conductivity, and excellent diffusion barrier properReceived: July 29, 2013 Revised: September 22, 2013 Published: September 24, 2013 22098
dx.doi.org/10.1021/jp407539c | J. Phys. Chem. C 2013, 117, 22098−22110
The Journal of Physical Chemistry C
Article
obtained powder was dried at 300 °C for 30 min and then bleached under UV irradiation for 3 h to yield 5.3 g of TiO2 as a white powder that is named pure TiO 2 . RuO 2 /TiO 2 heterostructure were then prepared in the second step using the impregnation method. In a typical procedure, a suitable amount of ruthenium(III) pentan-2,4-dionate (Alfa Aesar) was dissolved in 50 mL of tetrahydrofuran, and TiO2 powder (0.3 g) was then dispersed into the solution. After vigorous stirring for 4 h at room temperature, the solvent was evaporated. Subsequently, the solids were dried in an oven at 70 °C overnight and finally calcined at 400 °C in air for 6 h to achieve the desired RuO2/TiO2 catalyst. The content of RuO2 in RuO2/TiO2 samples was controlled by changing the concentration of the ruthenium(III) pentan-2,4-dionate solution. The samples containing 0.5, 1, 2.5, and 5 wt % of RuO2 were obtained by adding 0.0045, 0.0091, 0.023, and 0.047 g of ruthenium(III) pentan-2,4-dionate, respectively, to THF (50 mL). The resulting samples are hereafter named 0.5 wt % RuO2/TiO2, 1 wt % RuO2/TiO2, 2.5 wt % RuO2/TiO2, and 5 wt % RuO2/TiO2 nanocomposites. 2.2. Characterization Methods. Fourier transform infrared (FTIR) spectra (KBr pellets) were recorded with a PerkinElmer Spectrum 100 FTIR spectrophotometer. Raman studies were carried out in the solid state on a Labram 1B spectrometer using a red laser beam (632 nm). X-ray diffraction (XRD) studies were carried out with a Bruker AXS diffractometer (D2 PHASER A26-X1-A2B0D3A) using a Cu anode (Kα radiation). A continuous scan mode was employed to collect 2θ data from 10° to 80° with a 0.1° sampling pitch and a 2° min−1 scan rate. The average crystallite size of the nanomaterials prepared was deduced from fwhm (full width at half-maximum) according to Scherrer’s formula (applied to the {101} reflection).31 The textural properties (specific surface area, pore size distribution) were evaluated from nitrogen adsorption−desorption isotherms using a ASAP2010 Micromeritics equipment. Samples were degassed at 120 °C under vacuum for a time interval long enough to reach a constant pressure (
