Synthesis and Characterization of (Sn,Zn)O Alloys

SnO exhibits electrical properties that render it promising for solar energy conversion applications, but it also has a strongly indirect band gap. Recent theoretical calculations predict that this disadvantage can be mitigated by isovalent alloying with other group II oxides, such as ZnO. Here, we have synthesized new metastable isovalent (Sn,Zn)­O alloy thin films by combinatorial reactive co-sputtering and characterized their structural, optical, and electrical properties. The alloying of ZnO into SnO leads to a change of the valence state of the tin from Sn⁰ via Sn²⁺ to Sn⁴⁺, which can be counteracted by reducing the oxygen partial pressure during the deposition. The optical characterization of the smooth <10 at. % Sn_1–<i>x</i>Zn_<i>x</i>O thin films showed an increase in the absorption coefficient in the range from 1 eV to 2 eV, which is consistent with the theoretical predictions for the isovalent alloying. However, the experimentally observed alloying effect may be convoluted with the effect of local variations of the Sn oxidation state. This effect would have to be minimized to improve the (Sn,Zn)O optical and electrical properties for their use as absorbers in solar energy conversion applications

Possible <i>n–</i>type carrier sources in In₂O₃(ZnO)_k

Homologous compounds with the formula In₂O₃(ZnO)_k, where k is an integer, have potential applications as transparent conducting oxides and high temperature thermoelectric materials. In this study, we focus on the defect properties. Using the <i>k</i> = 3 phase as a prototype, we calculate with the first-principles method the defect formation energies and transition levels of the most probable <i>n</i>-type carrier producers, which include oxygen vacancy (V_O), indium antisite on zinc (In_Zn), indium interstitial (In_i), and zinc interstitial (Zn_i). The site-preference of these defects has been explored by comparing the total energies of defects at different sites. Under the <i>n</i>-type environment, In_Zn has a low formation energy and meanwhile a transition energy level close to the conduction band minimum (CBM); V_O also has a lower formation energy, however a deep transition energy level in the band gap; the cation interstitials have high formation energies, although their defect transition energy levels are quite shallow. Besides, we find that V_O and In_Zn tend to form a defect complex when the two isolated defects take the nearest-neighboring atomic sites in the same <i>ab</i>-plane. We conclude that In_Zn and its related defect-complex are the possible <i>n</i>–type carrier sources in In₂O₃(ZnO)_k. Besides, we found that V_O has a significant site-preference, which can modify the site-preference of In_Zn by forming defect-complexes. This may lead to high anisotropy in relaxation time, and then the experimentally reported strong anisotropy in electrical conductivities in In₂O₃(ZnO)₅

Experimental Characterization of a Theoretically Designed Candidate p‑Type Transparent Conducting Oxide: Li-Doped Cr₂MnO₄

The development of a p-type transparent conducting oxide (p-TCO) requires the deliberate design of a wide band gap and high hole conductivity. Using high-throughput theoretical screening, Cr₂MnO₄ was earlier predicted to be a p-TCO when doped with lithium. This constitutes a new class of p-TCO, one based on a tetrahedrally coordinated d⁵ cation. In this study, we examine and experimentally validate a few central properties of this system. Combined neutron diffraction and anomalous X-ray diffraction experiments give site occupancy that supports the theoretical prediction that lithium occupies the tetrahedral (Mn) site. The lattice parameter of the spinel decreases with lithium content to a solubility limit of [Li]/([Li] + [Mn]) ∼ 9.5%. Diffuse reflectance spectroscopy measurements show that at higher doping levels the transparency is diminished, which is attributed to both the presence of octahedral Mn and the increased hole content. Room-temperature electrical measurements of doped samples reveal an increase in conductivity of several orders of magnitude as compared to that of undoped samples, and high-temperature measurements show that Cr₂MnO₄ is a band conductor, as predicted by theory. The overall agreement between theory and experiment illustrates the advantages of a theory-driven approach to materials design

Water Oxidation Catalyzed by Cobalt Oxide Supported on the Mattagamite Phase of CoTe₂

A chemical synthesis for the cobalt pertelluride mineral mattagamite is reported. Synthetic nanocrystalline mattagamite was investigated for electrochemical water oxidation and showed catalytic activity. Electrochemical water oxidation occurred at an overpotential of 380 mV at 10 mA/cm⁻² with a Tafel slope of 58 mV/decade and with a Faradaic efficiency of 96% and turnover frequency of 0.021 s^–1 at 0.5 V

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (<i>h</i> = 70 mmol of O₂/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase