Crystal Engineering of MOF-Derived Bimetallic Oxide Solid Solution Anchored with Au Nanoparticles for Photocatalytic CO2 Reduction to Syngas and C2 Hydrocarbons

Considering that CO2 reduction is mostly a multielectron reaction, it is necessary for the photocatalysts to integrate multiple catalytic sites and cooperate synergistically to achieve efficient photocatalytic CO2 reduction to various products, such as C2 hydrocarbons. Herein, through crystal engineering, we designed and constructed a metal-organic framework-derived Zr/Ti bimetallic oxide solid solution support, which was confirmed by X-ray diffraction, electron microscopy and X-ray absorption spectroscopy. After anchoring Au nanoparticles, the composite photocatalyst exhibited excellent performances toward photocatalytic CO2 reduction to syngas (H2 and CO production rates of 271.6 and 260.6 μmol g-1 h-1) and even C2 hydrocarbons (C2H4 and C2H6 production rates of 6.80 and 4.05 μmol g-1 h-1). According to the control experiments and theoretical calculations, the strong interaction between bimetallic oxide solid solution support and Au nanoparticles was found to be beneficial for binding intermediates and reducing CO2 reduction, highlighting the synergy effect of the catalytic system with multiple active sites.

Doping induced phase stabilization and electronic properties of alkaline earth metal doped zirconium (IV) oxide: A first principles study

The role of divalent dopant cations such as Ca and Mg in phase stabilization of ZrO2 has been demonstrated experimentally, with Mg emerging as a crucial dopant ion because of its ability to enhance the photocatalytic properties of ZrO2. To provide a theoretical basis for these experimental observations, the modifications of the crystal and electronic structure of the monoclinic phase of zirconia, m-ZrO2, upon doping with Mg have been studied at the atomic level using Density Functional Theory method. Additionally, the effect of dopant ionic radius on the electronic properties has been demonstrated by doping with Ca, which is isoelectronic with Mg. On 6.25 % doping, a structural distortion of the monoclinic crystal structure towards a tetragonal structure is observed. Additionally, the Density of States of doped m-ZrO2 exhibits the characteristics of t-ZrO2 in the Zr d orbitals in the unoccupied states and O unoccupied states emerge upon creation of an O vacancy in Mg/Ca doped m-ZrO2. The calculated band gap of m-ZrO2 is 3.6 eV. Upon doping there is a shift of the Fermi energy towards the valence band maximum.

Layered β-ZrNBr Nitro-Halide as Multifunctional Photocatalyst for Water Splitting and CO2 Reduction

Developing mixed-anion semiconductors for solar fuel production has inspired extensive interest, but the nitrohalide-based photocatalyst is still in shortage. Here we report a layered nitro-halide β-ZrNBr with a narrow band gap of ca. 2.3 eV and low defect density to exhibit multifunctionalities for photocatalytic water reduction, water oxidation and CO₂ reduction under visible-light irradiation. As confirmed by the results of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the formation of anion vacancies in the nitro-halide photocatalyst was inhibited due to its relatively high formation energy. Furthermore, performance of β-ZrNBr can be effectively promoted by a simple exfoliation into nanosheets to shorten the carrier transfer distance as well as to promote charge separation. Our work extends the territory of functional photocatalysts into the nitro-halide, which opens a new avenue for fabricating efficient artificial photosynthesis.

Plasmon-assisted photocatalytic CO2 reduction on Au decorated ZrO2 catalysts

ZrO₂ is one of the most stable metal oxides which is applicable to various chemical reactions in harsh environments. However, the photocatalytic performance of ZrO₂ is relatively poor due to the negligible use of the solar spectrum caused by the wide bandgap (E_g = 5.3 eV). Here, we report plasmon enhanced Au nanoparticles decorated onto ZrO₂ through a facile tannic acid-reduction method. The Au/ZrO₂ heterojunctions exhibited efficient and stable photocatalytic activity of reducing CO₂ into main CO and CH₄, at the rates of 25.6 μmol g^-1 h^-1 and 5.1 μmol g^-1 h^-1 at most, respectively, approximately 6-fold enhanced compared to the pristine ZrO₂, under simulated solar light. The reduction rates could also be improved over 10-fold under visible light when Au nanoparticles were loaded onto ZrO₂. UV-Vis diffuse reflectance spectra confirmed the enhanced visible-light absorption of Au/ZrO₂ caused by localized surface plasmon resonance (LSPR). Electrochemical impedance spectra (EIS) and photocurrent tests proved the more efficient charge transport and electron-hole separation of Au/ZrO₂ heterojunctions. This study demonstrates an effective strategy of LSPR effects to improve the photocatalytic performances of semiconductors.

Afterglow Properties and Trap-Depth Control in ZrO2:Ti, M (M = Ca2+, Y3+, Nb5+, W6+)

Ti-doped ZrO₂ is a chemically stable and persistent luminescence material. Doping and co-doping is an effective approach for improving the afterglow properties of phosphors, but few studies have investigated the co-doping of ZrO₂:Ti systems. This study aimed to synthesize ZrO₂:Ti, M (M = Ca²⁺, Y³⁺, Ti single-doped, Nb⁵⁺, W⁶⁺) and evaluate the luminescent properties of the resulting materials, with a specific focus on the relationship between trap depth and the valence state of the co-doped cation. The ratio of the luminescent center to co-doped ion was optimized using the combinatorial approach, where 0.09 mol % Ti led to the best afterglow duration. The emission decay curves of each co-doped sample differed significantly, where a change in curvature was observed in the Ti single-doped and W⁶⁺ co-doped samples due to the presence of multiple traps. From the thermoluminescence glow curves, the trap originating in an oxygen vacancy with a peak at around 270 K was observed. The trap depth was dependent on electrostatic interactions between the trapped electrons and their surrounding cations, and thus related to the valence of the co-dopant. Overall, co-doping with high-valent cations led to improved afterglow duration.

Preparation and Characterization of Ni/ZrTiAlOx Catalyst via Sol-Gel and Impregnation Methods for Low Temperature Dry Reforming of Methane

Recently, the dry reforming of methane (DRM) has received much attention as a conversion technology of greenhouse gases. Ni-based catalysts supported on ternary metal oxide composite (ZrTiAlOx) were prepared to improve the coke resistance properties in the DRM (CH4:CO2 = 1) at low temperature. The ZrTiAlOx supports with different ratios of Zr/Ti were prepared through the modified Pechini sol-gel method, and then the Ni was impregnated on the synthesized support via the incipient wetness impregnation method. Considering the Zr/Ti ratios, different catalytic activity and durability in the DRM were identified. The Ni/ZrTiAlOx catalyst with Zr/Ti of 2 exhibited enhanced coke inhibition property compared to the others at low temperature DRM for 50 h. The catalysts with a high Zr/Ti ratio under the same condition were rapidly deactivated, while the catalyst with a low Zr/Ti ratio showed deficient activity. It was found from temperature-programmed surface reactions (TPSR) and DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) analysis that the addition of Ti has led in to higher catalytic stability at Zr/Ti = 2, which could be as a result of oxygen vacancies generated by the ternary metal oxides. Ni/ZrTiAlOx catalyst with ratio of Zr/Ti = 2 showed high stability and good catalytic activity towards DRM for the production of syngas.

Influence of Stabilizing Ion Content on the Structure, Photoluminescence and Biological Properties of Zr1–xEuxO2–0.5x Nanoparticles

Quasi-spherical nanoparticles of ZrO2 containing EuO1.5 from 2 to 15 mol.% were synthesized from the chlorides of the corresponding metals under hydrothermal conditions. The structural changes of Zr1–xEuxO2–0.5x (x = 0.02 ÷ 0.15) nanoparticles depending on the content of europium (III) ions were studied using the complementary methods (X-ray diffraction, electron microdiffraction, Raman and photoluminescence spectroscopy). It was shown that increasing the Eu3+ concentration in the Zr1–xEuxO2–0.5x nanoparticles leads to a transition from the equilibrium monoclinic zirconia phase to metastable tetragonal and cubic polymorphic modifications. In this case, the size of the nanoparticles decreases from 11.5 nm to 9 nm; the specific surface area grows from 80.2 to 111.3 m2/g, and the electrokinetic potential increases monotonously from −8.7 to 16.3 mV. The evolution of the phase composition of Zr1–xEuxO2-0.5x nanoparticles from monoclinic to tetragonal/cubic allomorphs with an increase in the molar fraction of stabilizer ions was correlated with changes in the sublevel structure of 5D0 → 7F2 and 5D0 → 7F4 optical transitions for Eu3+ in the luminescence spectra. Besides, for the nanoparticles obtained by hydrothermal synthesis from chlorides, the quantum efficiency does not exceed 3%. According to the M.T.T. assay, as a result of three-day human fibroblast cultivation in the aqueous dispersion of Zr1–xEuxO2–0.5x (x = 0.02 ÷ 0.15) nanoparticles, the proliferation activity of the cells is maintained, indicating that they do not have cytotoxic properties. Such nanoparticles can be used in organic–inorganic composites for medical applications in order to strengthen the polymer scaffolds and visualize changes in their structure within time.

Modelling the bulk properties of ambient pressure polymorphs of zirconia

We report a detailed survey of the calculated bulk properties of zirconia using GGA and meta-GGA functionals (PBE, PBEsol, RPBE, and TPSS), dispersion (Grimme's D2 and D3 approach), and on-site Coulomb repulsion correction (U = 2-8 eV). Structural, elastic, mechanical, and dielectric properties, as well as energetics, electronic structure, and phonon dispersion curves were computed and compared to previous investigations to identify the best DFT approach for a consistent in silico description of zirconia polymorphs. In general, inclusion of dispersion corrections led to only small changes in the calculated properties, whereas DFT+U (U = 2 or 4 eV) reduced the deviations of calculated properties from the experimental results, although deterioration of the structure and relative stabilities may be observed in some cases. Standard PBEsol, RPBE+U, and PBE+U were the best methodologies for a simultaneous description of the three polymorphs of ZrO2. RPBE+U, however, was the only functional to conserve the distinct structures and stabilities of c-, t-, and m-ZrO2 when U = 4 eV was used, resulting in the best in silico replication of the band gaps of ZrO2, whilst outperforming the other methodologies in the description of elastic, mechanical, and dielectric properties of this material. Overall, these results provide insight into the most appropriate DFT methodology for in silico investigations of ZrO2, and show that simultaneous description of all three ambient pressure zirconia polymorphs by DFT techniques with acceptable levels of accuracy can be achieved only when the correct choice of methodology is applied.

Sunlight activated anodic freestanding ZrO2 nanotube arrays for Cr(VI) photoreduction

Visible-light-active freestanding zirconia (ZrO₂) nanotube (FSZNT) arrays were fabricated by a facile electrochemical anodization method in fluoride containing ethylene glycol electrolyte added to it was 1 vol% of potassium carbonate (K₂CO₃) at 60 V for 1 h. Poor adhesion at the metal∣oxide interface was induced by K₂CO₃ leading to the formation of FSZNT flakes. The effect of the crystal structures of the FSZNTs e.g., amorphous, amorphous/tetragonal, and tetragonal/monoclinic was investigated towards the photocatalytic reduction of 10 ppm hexavalent chromium, Cr(VI) at pH 2 under sunlight. The results demonstrate the amorphous FSZNTs exhibited the highest Cr(VI) removal efficiency than the crystalline FSZNTs (95% versus 33% after 5 h). The high photocatalytic activity of the amorphous FSZNTs can be attributed to enhanced Cr(VI) adsorption, high visible light absorption, and better charge carrier separation. The low photocatalytic activity of the crystalline FSZNTs annealed at 500 °C was mainly attributed to poor Cr(VI) adsorption, low visible light absorption, and less photoactive monoclinic-ZrO₂.

The thickness-dependent band gap and defect features of ultrathin ZrO2 films studied by spectroscopic ellipsometry

The band gap and defect features of ultrathin ZrO2 films with varying thicknesses have been investigated by spectroscopic ellipsometry through the point-by-point data inversion method. The ε2-sprectra in the 3-6 eV range are extracted based on an optical model consisting of a Si substrate/effective ZrO2 film/air ambient structure where the effective ZrO2 film is a combination of interfacial layers and ZrO2. Evident widening of the band gap with a reducing size is observed when the effective ZrO2 films are below a critical thickness, somewhere between 8.80 nm and 17.13 nm. This is due to quantum-confinement and amorphous effects. Moreover, the sub-band-gap defects at interfacial layers and in bulk ZrO2 are identified and present strong thickness dependence as well. The interfacial defects at 3.26, 4.13, 4.43, and 4.77 eV mainly exist below the critical thickness and exhibit a significant suppression with increasing film thickness. The bulk defects at 4.15 eV and 4.46 eV dominate in ZrO2 films once they are over the critical thickness. The evolution of the band gap and defects is closely related to variance in the electronic structure of amorphous ZrO2. Our results may be helpful in understanding controversial problems concerning the size effect on ultrathin high-k oxide films and exploring the further miniaturization of electronic devices based on them.

Double-hole codoped huge-gap semiconductor ZrO2 for visible-light photocatalysis

Anionic codoping pairs make the huge-gap semiconductor ZrO₂ a promising catalyst for solar-driven water splitting.

Revealing the properties of the cubic ZrO2 (111) surface by periodic DFT calculations: reducibility and stabilization through doping with aliovalent Y2O3

The reducibility of the clean cubic ZrO₂ (111) surface, as well as its stabilization through doping, have been investigated by hybrid DFT calculations within a periodic approach and localized basis sets.

Tunable green to red ZrO2:Er nanophosphors

Pulsed laser ablation in water was validated as an effective method to produced erbium-doped ZrO₂ nanoparticles with intense up-conversion luminescence.