Reactivity of metal oxide nanocluster modified rutile and anatase TiO2: Oxygen vacancy formation and CO2 interaction

The reduction of CO2 to fuels is an active research topic with much interest in using solar radiation and photocatalysts to transform CO2 into higher value chemicals. However, to date there are no photocatalysts known that can use solar radiation to efficiently reduce CO2. One particularly difficult problem is activating CO2 due to its high stability. In this paper we use density functional theory simulations to study novel surface modified TiO2 composites, based on modifying rutile and anatase TiO2 with molecular-sized metal oxide nanoclusters of SnO, ZrO2 and CeO2 and the interaction between CO2 and nanocluster-modified TiO2. We show that reduction of the supported nanocluster is favourable which then provides reduced cations and sites for CO2 adsorption. The atomic structures and energies of different adsorption configurations of CO2 on the reduced modified TiO2 composites are studied. Generally on reduced SnO and CeO2 nanoclusters, the interaction of CO2 is weak producing adsorbed carbonates. On reduced ZrO2, we find a stronger interaction with CO2 and carbonate formation. The role of the energies of oxygen vacancy formation in CO2 adsorption is important because if reduction is too favourable, the interaction with CO2 is not so favourable. We do find an adsorption configuration of CO2 at reduced CeO2 where a CO bond breaks, releasing CO and filling the oxygen vacancy site in the supported ceria nanocluster. These initial results for the interaction of CO2 at surface modified TiO2 provide important insights for future work on CO2 reduction using novel materials

Metal oxide nanocluster-modified TiO2 as solar activated photocatalyst materials

In this review we describe our work on new TiO2 based photocatalysts. The key concept in our work is to form new composite structures by the modification of rutile and anatase TiO2 with nanoclusters of metal oxides and our density functional theory (DFT) level simulations are validated by experimental work synthesizing and characterizing surface-modified TiO2. We use DFT to show that nanoclusters of different metal oxides, TiO2, SnO/SnO2, PbO/PbO2, NiO and CuO can be adsorbed at rutile and anatase surfaces and can induce red shifts in the absorption edge to enable visible light absorption which is the first key requirement for a practical photocatalyst. We furthermore determine the origin of the red shift and discuss the factors influencing this shift and the fate of excited electrons and holes. For p-block metal oxides we show how the oxidation state of Sn and Pb can be used to tune both the magnitude of the red shift and also its mechanism. Finally, aiming to make our models more realistic, we present some new results on the stability of water at rutile and anatase surfaces and the effect of water on oxygen vacancy formation and on nanocluster modification. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with the suitable choice of nanocluster modifier can be applied to CO2 reduction

Design of novel visible light active photocatalyst materials: Surface modified TiO2

Work on the design of new TiO2 based photocatalysts is described. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metal oxides. Density functional theory (DFT) level simulations are compared with experimental work synthesizing and characterizing surface modified TiO2. DFT calculations are used to show that nanoclusters of metal oxides such as TiO2, SnO/SnO2, PbO/PbO2, ZnO and CuO are stable when adsorbed at rutile and anatase surfaces, and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. The origin of the red shift and the fate of excited electrons and holes are determined. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. Comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations are described. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions

Application of two-color LIF thermometry to nucleate boiling

International audienceThe laser-induced fluorescence (LIF) thermometry is applied to measure the temperature field surrounding a single vapor bubble growing at an artificial nucleation site. In order to correct measurement errors due to the non-uniformity of the incident laser intensity, the two-color LIF thermometry technique is used in this nucleate boiling experiment. This technique is based on the use of two fluorescent dyes: the temperature sensitive dye Rhodamine B and the temperature insensitive dye Sulforhodamine-101. The concentration of the dyes is optimized by analyzing the behavior of fluorescence intensities. The mapping between the two images is determined through a geometrical calibration procedure. This technique presents a success in correcting the non uniformities due to the reflection of the light at the bubble surface and to the temperature gradient. The obtained temperature fields show that the two-color LIF is a promising technique in the investigation of nucleate boiling

Length-dependent resistance model for a single-wall Carbon nanotube

The non-linear length-dependent resistance, $\mathcal{R}(l)$ observed in single-wall Carbon nanotubes (SNTs) is explained through the recently proposed ionization energy ($E_I$) based Fermi-Dirac statistics ($i$FDS). The length here corresponds to the Carbon atoms number ($\mathcal{N}$) along the SNT. It is also shown that $\mathcal{R}_y(l_y)$ $<$ $\mathcal{R}_x(l_x)$ is associated with $E_I^y$ $<$ $E_I^x$, which can be attributed to different semiconducting properties in their respective $y$ and $x$ directions.Comment: Publishe

First-principles study of surface properties of PuO2: Effects of thickness and O-vacancy on surface stability and chemical activity

The (111), (110), and (001) surfaces properties of PuO2 are studied by using density-functional theory+U method. The total-energy static calculations determine the relative order of stability for low-index PuO2 surfaces, namely, O-terminated (111) > (110) > defective (001) > polar (001). The effect of thickness is shown to modestly modulate the surface stability and chemical activity of the (110) surface. The high work function of 6.19 eV indicates the chemical inertia of the most stable (111) surface, and the surface O-vacancy with concentration C_V=25% can efficiently lower the work function to 4.35 eV, which is a crucial indicator of the difference in the surface chemical activities between PuO2 and \alpha-Pu2O3. For the polar (001) surface, 50% on-surface O-vacancy can effectively quench the dipole moment and stabilize the surface structure, where the residual surface oxygen atoms are arranged in a zigzag manner along the direction. We also investigate the relative stability of PuO2 surfaces in an oxygen environment. Under oxygen-rich conditions, the stoichiometric O-terminated (111) is found to be the most stable surface. Whereas under O-reducing conditions, the on-surface O-vacancy of C_V = 1/9 is stable, and for high reducing conditions, the (111) surface with nearly one monolayer subsurface oxygen removed (C_V = 8/9) becomes most stable.Comment: 9 JNM pages, 7 figure

Activation of CO2 at chromia-nanocluster-modified rutile and anatase TiO2

Converting CO₂ to fuels is required to enable the production of sustainable fuels and to contribute to alleviating CO₂ emissions. In considering conversion of CO₂, the initial step of adsorption and activation by the catalyst is crucial. In addressing this difficult problem, we have examined how nanoclusters of reducible metal oxides supported on TiO₂ can promote CO₂ activation. In this paper we present density functional theory (DFT) simulations of CO₂ activation on heterostructures composed of extended rutile and anatase TiO₂ surfaces modified with chromia nanoclusters. The heterostructures show non-bulk Cr and O sites in the nanoclusters and an upshifted valence band edge that is dominated by Cr 3d- O 2p interactions. We show that the supported chromia nanoclusters can adsorb and activate CO₂and that activation of CO₂ is promoted whether the TiO₂ support is oxidised or hydroxylated. Reduced heterostructures, formed by removal of oxygen from the chromia nanocluster, also promote CO₂ activation. In the strong CO₂ adsorption modes, the molecule bends giving O-C-O angles of 127 - 132^o and elongation of C-O distances up to 1.30 Å; no carbonates are formed. The electronic properties show a strong CO₂-Cr-O interaction that drives the interaction of CO₂ with the nanocluster and induces the structural distortions. These results highlight that a metal oxide support modified with reducible metal oxide nanoclusters can activate CO₂, thus helping to overcome difficulties associated with the difficult first step in CO₂ conversion