Joel B. Varley

Oxygen vacancies and donor impurities in β-Ga2O3

Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.

Understanding Trends in the Electrocatalytic Activity of Metals and Enzymes for CO2 Reduction to CO.

We develop a model based on density functional theory calculations to describe trends in catalytic activity for CO2 electroreduction to CO in terms of the adsorption energy of the reaction intermediates, CO and COOH. The model is applied to metal surfaces as well as the active site in the CODH enzymes and shows that the strong scaling between adsorbed CO and adsorbed COOH on metal surfaces is responsible for the persistent overpotential. The active site of the CODH enzyme is not subject to these scaling relations and optimizes the relative binding energies of these adsorbates, allowing for an essentially reversible process with a low overpotential.

The Mechanism of CO and CO2 Hydrogenation to Methanol over Cu-Based Catalysts

Methanol, an important chemical, fuel additive, and precursor for clean fuels, is produced by hydrogenation of carbon oxides over Cu‐based catalysts. Despite the technological maturity of this process, the understanding of this apparently simple reaction is still incomplete with regard to the reaction mechanism and the active sites. Regarding the latter, recent progress has shown that stepped and ZnOx‐decorated Cu surfaces are crucial for the performance of industrial catalysts. Herein, we integrate this insight with additional experiments into a full microkinetic description of methanol synthesis. In particular, we show how the presence or absence of the Zn promoter dramatically changes not only the activity, but unexpectedly the reaction mechanism itself. The Janus‐faced character of Cu with two different sites for methanol synthesis, Zn‐promoted and unpromoted, resolves the long‐standing controversy regarding the Cu/Zn synergy and adds methanol synthesis to the few major industrial catalytic processes that are described on an atomic level.

Mechanism of Visible‐Light Photocatalysis in Nitrogen‐Doped TiO2

IO N Semiconductor-based photocatalysis is growing at an unprecedented rate, with TiO 2 leading the way as an important material in applications ranging from the degradation of pollutants to water splitting. [ 1 , 2 ] The basic mechanism is the creation of an electron-hole pair by exciting an electron from the valence to the conduction band through light absorption. Since rutile and anatase have bandgaps of 3.1 eV and 3.2 eV, respectively, only a small fraction of the solar spectrum is absorbed and great efforts have been devoted to extending the TiO 2 photoabsorption to the visible region of the spectrum. Adding N impurities has been shown to enhance visible-light absorption, leading to enhanced photochemical activity. [ 3–7 ] The behavior of N in TiO 2 has been widely discussed, [ 3 , 8–10 ] but the fundamental mechanisms underlying the visible-light absorption remain unclear. It is generally accepted that the visible-light transitions involve electrons from N-related states in the gap to the conduction band. Nonetheless, whether the predominant active species are N atoms at interstitial sites or on substitutional O sites (N O ), and whether the behavior of N in rutile is different from that in anatase are still open issues. In this work we address the stabilities of the relevant N-related defects in both polymorphs, fi nding that impurity-band transitions from N O are the origin of the lower absorption threshold in N-doped titania. Early experimental work on N-doped TiO 2 focused on the correlation between visible-light absorption, photocatalytic activity, and X-ray photoelectron spectroscopic (XPS) measurements of the N 1 s state. [ 3 ] Two main peaks attributed to the N 1 s have been correlated with photocatalytic activity, [ 3 , 9 , 10 ] with one at ∼ 396 eV and the other at ∼ 400 eV. More recent reports on rutile and anatase fi lms grown by plasma-assisted molecular-beam epitaxy have indicated that N incorporates as N O with a characteristic peak at 396.6 eV seen by XPS. [ 11 ] We note that different groups have suggested different causes for each peak, with N O , interstitial N, and N–H complexes as proposed sources. [ 3 , 9–12 ]

Experimental electronic structure of In2O3 and Ga2O3

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019?cm?3 (In2O3) and ~1017?cm?3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.

CO and CO2 Hydrogenation to Methanol Calculated Using the BEEF-vdW Functional

Hydrogenation of CO and CO2 to methanol on a stepped copper surface has been calculated using the BEEF-vdW functional and is compared to values derived with RPBE. It is found that the inclusion of vdW forces in the BEEF-vdW functional yields a better description of CO2 hydrogenation as compared to RPBE. These differences are significant for a qualitative description of the overall methanol synthesis kinetics and it is suggested that the selectivity with respect to CO and CO2 is only described correctly with BEEF-vdW.Graphical Abstract

Hydrogenated cation vacancies in semiconducting oxides

Using first-principles calculations we have studied the electronic and structural properties of cation vacancies and their complexes with hydrogen impurities in SnO(2), In(2)O(3) and β-Ga(2)O(3). We find that cation vacancies have high formation energies in SnO(2) and In(2)O(3) even in the most favorable conditions. Their formation energies are significantly lower in β-Ga(2)O(3). Cation vacancies, which are compensating acceptors, strongly interact with H impurities resulting in complexes with low formation energies and large binding energies, stable up to temperatures over 730 °C. Our results indicate that hydrogen has beneficial effects on the conductivity of transparent conducting oxides: it increases the carrier concentration by acting as a donor in the form of isolated interstitials, and by passivating compensating acceptors such as cation vacancies; in addition, it potentially enhances carrier mobility by reducing the charge of negatively charged scattering centers. We have also computed vibrational frequencies associated with the isolated and complexed hydrogen, to aid in the microscopic identification of centers observed by vibrational spectroscopy.

Dual behavior of excess electrons in rutile TiO2

The behavior of electrons in the conduction band of TiO2 and other transition-metal oxides is key to the many applications of these materials. Experiments seem to produce conflicting results: optical and spin-resonance techniques reveal strongly localized small polarons, while electrical measurements show high mobilities that can only be explained by delocalized free electrons. By means of hybrid functional calculations we resolve this apparent contradiction and show that small polarons can actually coexist with delocalized electrons in the conduction band of TiO2, the former being energetically only slightly more favorable. We also find that small polarons can form complexes with oxygen vacancies and ionized shallow-donor impurities, explaining the rich spectrum of Ti3+ species observed in electron spin resonance experiments. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electronic structure of β-Ga2O3

β-Ga2O3 has the widest energy gap of the transparent conducting oxides. The interest in its electronic properties has recently increased because of its applications in various optoelectronic devices, semiconductor lasers, and ultrasensitive gas detecting systems. In contrast, information on the electronic structure of β-Ga2O3 is very scarce. Here, we present the experimental valence-band structure of β-Ga2O3 single crystals determined by high-resolution angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. We find good matching of the experimental band structure with the advanced density functional theory calculations employing hybrid functionals and projector augmented wave potentials.

Lithium and oxygen vacancies and their role in Li2O2 charge transport in Li–O2 batteries

It is now well accepted that charge transport through Li2O2 is a serious limitation in Li–O2 batteries. We report formation energies for the different charge states of Li, O and O2 vacancies in Li2O2 that could have important implications for charge transport through Li2O2. Charge transition levels are given as a function of the location of the Fermi level in Li2O2 relative to the valence band maximum (VBM). We argue that in Li–O2 discharge/charge electrochemistry, the Fermi level is pinned by LiO2 interface states at ∼0.35 eV above the VBM and this causes the vacancies to be in positively charged states (weakly violating local charge neutrality). We show by non-equilibrium Greens function calculations of charge transport through a Au|Li2O2 + VLiq |Au metal–insulator–metal structure (with VLiq a Li vacancy of charge state q), that the +1 and 0 charge state induces considerable scattering for tunneling holes. This tunneling is the previously proposed dominant mechanism of charge transport in Li–O2 batteries at practical current densities at room temperature, although we also proposed a contribution from hole polaron migration at very low currents and higher temperatures. We suggest that scattering of the tunneling holes by the positively charged vacancies (and possibly hole polarons) is the origin of the resistive loss observed in Li–O2 discharges in bulk electrolysis cells (where other forms of resistance are negligible). Thus, we argue that charged vacancies hinder charge transport through Li2O2 in Li–O2 electrochemical discharges.