Preparation of RuO₂/TiO₂Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations

Abstract

Nanoporous RuO 2 /TiO 2 heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO 2 nanoparticles in a ruthenium-(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO 2 amount (0.5−5 wt %), the asprepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO 2 nanocrystallites modified with RuO 2 according to N 2 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 RuO 2 /TiO 2 heterojunction. At first, a model experiment involving stepwise deposition of RuO 2 on the TiO 2 film and an in situ XPS measurement showed a shift of Ti 2p 3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO 2 /TiO 2 heterojunction. The band bending for the heterostructure RuO 2 /TiO 2 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 % RuO 2 /TiO 2 nanocatalyst led to twice higher activities than pure anatase TiO 2 and reference commercial TiO 2 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 RuO 2 /TiO 2 heterojunction. Finally, these mesoporous RuO 2 −TiO 2 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.

Influence of surface disorder, oxygen defects and bandgap in TiO2 nanostructures on the photovoltaic properties of dye sensitized solar cells

“…Recently Prouzet's [59] group has shown that the indirect bandgap model gives better agreement on the E g values of anatase nanoparticles. Further, Uddin et al's [60] report on the bandgap of TiO 2 show that the indirect bandgap model gives lower E g , close to the reported bulk values (3.2 eV), than the estimates obtained using direct bandgap model. We calculated both direct and indirect bandgap values as obtained from (Fhν) 2 vs. hν and (Fhν) 1/2 vs. hν plots, respectively, where 'F' is the Kubelka-Munk (K-M) function, 'h' is Planck constant and 'ν' is the frequency of the photon (Figs.…”

Visible Light-Driven H2 Production over Highly Dispersed Ruthenia on Rutile TiO2 Nanorods

“…19 However, different conclusions have been reported by several groups, where 1 ~ 5 wt% of RuO2 has been suggested to provide the best performance. 21,35 It can be concluded that optimizing the spatial distribution of RuO2 loading on rutile TiO2 support strongly depends on the synthesis method, support nature and experimental conditions. It is critical to develop a synthesis method that allows the isolation of small amounts of RuO2 on TiO2.…”

“…The broad and weak spectrum in the inset indicates the formation of mixtures of Ru 0 and oxidized Ru n+ species (n = 2 ~ 4). 8,10,11,16,21,35 Due to the intrinsic submetallic property of RuO2, [8][9][10][11]21,31,[33][34][35] these highly dispersed ruthenium species behave as quasi metallic contact materials to enhance both the conductivity and transfer of photoinduced holes from TiO2 valence band, further facilitating the charge separation, so that the electrons freely migrate from the conduction band of TiO2, reducing the protons and/or water to generate gaseous H2. The local geometry of the prepared materials was further studied by electron microscopy.…”

“…The formation of a heterojunction at the RuO2-TiO2 interface, band gap narrowing, and upward band bending at the interface effectively promote the separation of photoinduced electrons and holes, leading to an increase in photocatalytic H2 evolution. RuO2 via an intimate contact due to higher work function of RuO2, 19,21,35 subsequently reacting with protons, water and methanol. The photoinduced holes from VB of TiO2 NRs are irreversibly scavenged by methanol to produce various oxidation intermediates and products such as  OH radical,  CH2OH, HCHO, HCOOH and CO2.…”

“…The photoinduced holes from VB of TiO2 NRs are irreversibly scavenged by methanol to produce various oxidation intermediates and products such as  OH radical,  CH2OH, HCHO, HCOOH and CO2. 31,35,50 Meanwhile, the photogenerated electrons simultaneously reduce the protons and water to generate H2 gas.…”