Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle-support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa2Nb3O10, K4Nb6O17, and RbTaO3 were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96), and α-Zr(HPO4)2·H2O. High surface area SiO2 and γ-Al2O3 supports were also used for comparison in the ITC experiments. A Born-Haber cycle analysis of ITC data revealed an exothermic interaction between Rh(OH)3 nanoparticles and the layered niobate and tantalate supports, with ΔH values in the range -32 kJ·mol(-1) Rh to -37 kJ·mol(-1) Rh. In contrast, the interaction enthalpy was positive with SiO2 and γ-Al2O3 supports. The strong interfacial bonding in the former case led to "reverse" ripening of micrometer-size Rh(OH)3, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and α-Zr(HPO4)2·H2O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh(OH)3 to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh(OH)3, Rh2O3, and Rh nanoparticles on niobate and tantalate supports.

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize high surface area TiO2 electrodes functionalized with light absorbing sensitizers and water oxidation catalysts. Because water splitting requires vectorial electron transfer from the catalyst to the sensitizer to the TiO2 surface, attaching both sensitizer and catalyst to TiO2 in the correct sequence and stabilizing them under photoelectrochemical conditions has been a challenging problem. Rutile-phase IrO2 nanoparticles can be deposited directly on the TiO2 electrode by adsorbing citrate-capped amorphous IrO x and then sintering at 450 °C. Electrodes functionalized with these nanocrystalline particles show higher activity than those made from ligand-capped amorphous IrO x without sintering. In the WS-DSPEC, the Coulombic efficiency for oxygen evolution from the sintered nanoparticle photoelectrodes was near unity. The loading of colloidal IrO x and IrO2 particles onto the porous TiO2 electrodes was quantified by neutron activation analysis. Photovoltage measurements suggest that at high catalyst loading the dominant charge recombination pathway is from photoinjected electrons to IrO2.

The interfacial interactions between late transition metal/metal oxide nanoparticles and oxide supports impact catalysts’ activity and stability. Here, we report the use of isothermal titration calorimetry (ITC), electron microscopy and density functional theory (DFT) to explore periodic trends in the heats of nanoparticle-support interactions for late transition metal and metal oxide nanoparticles on layered niobate and silicate supports. Data for Co(OH)2, hydroxyiridate-capped IrOx.nH2O, Ni(OH)2, CuO, and Ag2O nanoparticles were added to previously reported data for Rh(OH)3 grown on nanosheets of TBA0.24H0.76Ca2Nb3O10 and a layered silicate. ITC measurements showed stronger bonding energies in the order Ag < Cu ≈ Ni ≈ Co < Rh < Ir on the niobate support, as expected from trends in M-O bond energies. Nanoparticles with exothermic heats of interaction were stabilized against sintering as revealed by temperature resolved images recorded using transmission electron microscopy. In contrast, ITC measurements showed endothermic interactions of Cu, Ni, and Rh oxide/hydroxide nanoparticles with the silicate and poor resistance to sintering. These trends in interfacial energies were corroborated by DFT calculations using single-atom and four-atom cluster models of surface-bound metal/metal oxide nanoparticles. Density of states and charge density difference calculations reveal that strongly bonded metals (Rh, Ir) transfer d-electron density from the adsorbed cluster to niobium atoms in the support; this mixing is absent in weakly binding metals, such as Ag and Au, and in all metals on the layered silicate support. The large differences between the behavior of nanoparticles on niobate and silicate supports highlight the importance of d-orbital interactions between the nanoparticle and support in controlling the nanoparticles’ stability.

Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrO(x)·nH2O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrO(x)·nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)5(H2O)](2-) and [Ir(OH)6](2-), respectively. These monomeric anions strongly adsorb onto IrO(x)·nH2O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrO(x)·nH2O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell. The growth of a dense TiO2 barrier layer prevents access of soluble monomeric anions to the interface between the oxide semiconductor and the electrode back contact (a fluorinated tin oxide transparent conductor) and leads to improved photoanode performance. Purified IrO(x)·nH2O colloids, which contain no adsorbed monomer, give improved performance at the same electrodes. These results explain earlier observations that IrO(x)·nH2O catalysts can dramatically degrade the performance of metal oxide photoanodes for the OER reaction.

The loss of centrosymmetry via oxygen octahedral rotations is demonstrated in the n = 2 Dion-Jacobson family of layered oxide perovskites, A′LaB 2 O 7 (A′ = Rb, Cs; B = Nb, Ta). Ab initio density functional theory calculations predict that all four materials should adopt polar space groups, in contrast to the results of previous experimental studies that have assigned these materials to the centrosymmetric P4/ mmm space group. Optical second harmonic generation experiments confi rm the presence of a noncentrosymmetric phase at ambient temperature. Piezoresponse force microscopy experiments also show that this phase is piezoelectric. To elucidate the symmetry-breaking and assign the appropriate space groups, the crystal structure of CsLaNb 2 O 7 is refi ned as a function of temperature from synchrotron X-ray diffraction data. Above 550 K, CsLaNb 2 O 7 adopts the previously determined centrosymmetric P4/ mmm space group. Between 550 and 350 K, the symmetry is lowered to the noncentrosymmetric space group A mm 2. Below 350 K, additional symmetry lowering is observed as peak splitting, but the space group cannot be unambiguously identifi ed.

A signifi cant amount of research in the piezoelectric and ferroelectric communities is presently being dedicated to fi nding new electronic materials for applications in sensors, actuators, and transducers. This fi eld of research has long been dominated by AB O 3 perovskite oxides in which the inversion centers in the crystal structures are removed by atomic displacements due to the second-order Jahn-Teller (SOJT) effect. [1][2][3] There are only a few SOJT-active cations such as Ti 4+ and Zr 4+ with empty n d ( n = 3, 4) orbitals, and Pb 2+ and Bi 3+ with n s ( n = 6) lone-pair electrons, which limit the choices of chemical elements that can be used to design noncentrosymmetric materials. On the other hand, the most ubiquitous distortion in the perovskite oxides is the rotation or tilting of B O 6 oxygen octahedra enclosing the B-site cations. [ 4 ] These rotations do not break the inversion symmetry in simple AB O 3 perovskites because of the nature of octahedral connectivity. Further, the oxygen octahedral rotations (OORs) encompassing A-site cation displacements suppress the instability of polar distortions in most perovskites that are available in nature. [4][5][6] Though OORs cannot break inversion symmetry in simple perovskites, they can break inversion symmetry in layered oxide structures with different topologies (arising from different connectivities), such as Ruddlesden-, and perovskite superlattices (PS) [ABO 3 ] m /[A′B′O 3 ] n . [7][8][9][10][11][12][13][14][15] In addition to topology , layered oxides provide three additional tuning knobs for realizing new properties: dimensionality (subscript integers m and n above), chemical ordering (cations A, A′, B, and B′), and strain such as when grown as an epitaxial fi lm on a single crystal substrate.Benedek et al. [ 8,12 ] and Rondinelli et al. [ 13 ] have proposed that the most abundant OOR, represented by a − a − c + in the Glazer notation, [ 16 ] can remove inversion centers in n = 2 RP phase and AB O 3 / A ′ B O 3 PS, respectively. This idea of breaking inversion symmetry by OORs has been developed in other layered perovskites such as RP phases with and without A-site cation An improper mechanism for breaking inversion symmetry is revealed and thus inducing piezoelectricity in the family of layered perovskites, Li R TiO 4 ( R = rare earths), which are previously reported as centrosymmetric. Noncentrosymmetry in this family of compounds arises from TiO 6 octahedral rotation represented by a − b o c o / b o a − c o in the perovskite blocks between R O rock salt and LiO antifl uorite layers. X-ray diffraction and optical second harmonic generation complemented by density functional theory predictions are crucial in determining the new structures. High transition temperature ( T ac ) of up to 1200 K from noncentrosymmetric to centrosymmetric phase is observed. Piezoelectric coeffi cients ( d 36 ) of up to −15 pC/N are predicted, and piezoelectric force microscopy experiments confi rm a piezoelectric response. The demonstrated improper mechanism in this...

We report the observation of noncentrosymmetricity in the family of HRTiO4 (R = Eu, Gd, Dy) layered oxides possessing a Ruddlesden–Popper derivative structure, by second harmonic generation and synchrotron X-ray diffraction with the support of density functional theory calculations. These oxides were previously thought to possess inversion symmetry. Here, inversion symmetry is lifted by rotations of the oxygen-coordinated octahedra, a mechanism that is not active in simple perovskites. We observe a competition between rotations of the oxygen octahedra and sliding of a combined unit of perovskite–rocksalt–perovskite blocks at the proton layers. For the smaller rare earth ions, R = Eu, Gd, and Dy, which favor the octahedral rotations, noncentrosymmetricity is present but the sliding is absent. For the larger rare earth ions, R = Nd and Sm, the octahedral rotations are absent, but the sliding at the proton layers is present to optimize the length and direction of hydrogen bonding in the crystal structure. The study reveals a new mechanism for inducing noncentrosymmetricity in layered oxides, and chemical–structural effects related to rare earth ion size and hydrogen bonding that can turn this mechanism on and off. We construct a phase diagram of temperature versus rare earth ionic radius for the HRTiO4 family.

Through the systematic investigation of zinc alkylbisphosphonates, four new structural families have been obtained. These families are named zinc alkyl-tunnel, -gate, -cation, and -sheet (ZAT, ZAG, ZAC, and ZAS) for convenience and have been synthesized and further extended through isoreticular design utilizing alkylbis(phosphonic acid) ligands of the formula H2O3P­C n H 2n ­PO3H2 (n = 3–6) (H 4 L n ). Both even- and odd-length chains were utilized to help determine the effect of chain conformation on structure formation. The investigation lead to two known compounds (ZAG-4, and ZAS-3) and nine new compounds, two of which contain large 1-D channels. The crystal structures of all compounds were determined by single-crystal X-ray diffraction. Of the nine new compounds, only seven of them fall into the new families. In three of the four families, the structure is controlled by alkyl-chain length and conformation (i.e., odd vs even), and in the fourth, a conformational distortion allows both odd and even lengths to form the given structure. Isoreticular species using n = 3 and 5 were obtained in both the ZAT and the ZAS families; using n = 4 and 6 were obtained in the ZAG family; and n = 4–6, in the ZAC family.