Overtone Spectroscopy of Hydrogen in MOF-5

Metal-Organic Frameworks, or MOFs, are an exciting class of nanoporous crystalline materials with applications that include hydrogen storage and hydrogen isotope separation. The dynamics of adsorbed molecular hydrogen in the prototypical material known as MOF-5 have previously been studied using infrared spectroscopy. However, the rovibrational spectrum of the isotopologues, HD, and D2 were obscured due to overlap with the MOF peaks. Overtone infrared spectroscopy in conjunction with a diffuse reflectance geometry is used to observe the spectrum of H2, HD and D2. The overtone spectrum is shown to facilitate the identification of hydrogen peaks. Further, the spectrum of trapped H2 near the crystallographic metal site is greatly enhanced relative to other sites and displays a greater intensity relative to the fundamental spectrum than is seen in gas phase hydrogen. The ability of the MOF to catalyze ortho to para conversion of trapped species is also discussed

Overtone Spectroscopy of Hydrogen in MOF-5

Metal-Organic Frameworks, or MOFs, are an exciting class of nanoporous crystalline materials with applications that include hydrogen storage and hydrogen isotope separation. The dynamics of adsorbed molecular hydrogen in the prototypical material known as MOF-5 have previously been studied using infrared spectroscopy. However, the rovibrational spectrum of the isotopologues, HD, and D2 were obscured due to overlap with the MOF peaks. Overtone infrared spectroscopy in conjunction with a diffuse reflectance geometry is used to observe the spectrum of H2, HD and D2. The overtone spectrum is shown to facilitate the identification of hydrogen peaks. Further, the spectrum of trapped H2 near the crystallographic metal site is greatly enhanced relative to other sites and displays a greater intensity relative to the fundamental spectrum than is seen in gas phase hydrogen. The ability of the MOF to catalyze ortho to para conversion of trapped species is also discussed

Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO3_3

The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO$_3$, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO$_3$ substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO$_3$, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings between the higher-order replicas, as well as other self-energy effects, allow us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of $\lambda = 0.19 \pm 0.02$.Comment: 5 pages, 4 figure

Infrared overtone spectroscopy of adsorbed hydrogen in MOF-5

Overtone spectroscopy is used to observe the rovibrational spectra of the hydrogen isotopologues H2, HD, and D2 adsorbed in the metal–organic framework known as MOF-5. It is shown that the overtone spectrum facilitates the identification of hydrogen modes which are obscured in the fundamental region by the presence of MOF-5 features. Further, the overtone spectrum of H2 at the primary adsorption site is greatly enhanced relative to other sites, and thus ambiguities about feature assignment can be avoided. The frequency (wavenumber) of the overtone modes are in good agreement with a Buckingham perturbative model while the relative intensity of the Q2Q2(0) pure vibrational mode is found to be anomalously large, most likely arising through mode coupling to the MOF-5 framework

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO2

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide IrO2. Using a combination of in situ angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile IrO2 have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of IrO2 in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure. © 2018 American Physical Societ

Influence of Surface Adsorption on the Oxygen Evolution Reaction on IrO2(110)

A catalyst functions by stabilizing reaction intermediates, usually through surface adsorption. In the oxygen evolution reaction (OER), surface oxygen adsorption plays an indispensable role in the electrocatalysis. The relationship between the adsorption energetics and OER kinetics, however, has not yet been experimentally measured. Herein we report an experimental relationship between the adsorption of surface oxygen and the kinetics of the OER on IrO2(110) epitaxially grown on a TiO2(110) single crystal. The high quality of the IrO2 film grown using molecular-beam epitaxy affords the ability to extract the surface oxygen adsorption and its impact on the OER. By examining a series of electrolytes, we find that the adsorption energy changes linearly with pH, which we attribute to the electrified interfacial water. We support this hypothesis by showing that an electrolyte salt modification can lead to an adsorption energy shift. The dependence of the adsorption energy on pH has implications for the OER kinetics, but it is not the only factor; the dependence of the OER electrocatalysis on pH stipulates two OER mechanisms, one operating in acidic solution and another operating in alkaline solution. Our work points to the subtle adsorption−kinetics relationship in the OER and highlights the importance of the interfacial electrified interaction in electrocatalyst design