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Electronic doping of transition-metal oxides (TMOs) is typically accomplished through the synthesis of nonstoichiometric oxide compositions and the subsequent ionization of intrinsic lattice defects. As a result, ambipolar doping of wide-band-gap TMOs is difficult to achieve because the formation energies and stabilities of vacancy and interstitial defects vary widely as a function of the oxide composition and crystal structure. The facile formation of lattice defects for one carrier type is frequently paired with the high-energy and unstable generation of defects required for the opposite carrier polarity. Previous work from our group showed that the brucite (β-phase) layered metal hydroxides of Co and Ni, intrinsically p-type materials in their anhydrous three-dimensional forms, could be n-doped using a strong chemical reductant. In this work, we extend the electron-doping study to the α polymorph of Co(OH)2 and elucidate the defects responsible for n-type doping in these two-dimensional materials. Through structural and electronic comparisons between the α, β, and rock-salt structures within the cobalt (hydr)oxide family of materials, we show that both layered structures exhibit facile formation of anion vacancies, the necessary defect for n-type doping, that are not accessible in the cubic CoO structure. However, the brucite polymorph is much more stable to reductive decomposition in the presence of doped electrons because of its tighter layer-to-layer stacking and octahedral coordination geometry, which results in a maximum conductivity of 10-4 S/cm, 2 orders of magnitude higher than the maximum value attainable on the α-Co(OH)2 structure.

Cobalt sulfide nanomaterials are among the most active and stable catalysts for the electrocatalytic oxygen reduction reaction in pH 7 electrolyte. However, due to the complexity and dynamism of the catalytic surfaces in cobalt sulfide bulk materials, it is challenging to identify and tune the active site structure in order to achieve low overpotential oxygen reduction reactivity. In this work, we synthesize isolated Co sites supported on colloidal WS2 nanosheets and develop a synthetic strategy to rationally control the first-shell coordination environment surrounding the adsorbed Co active sites. By studying Co–WS2 materials with a range of Co–S coordination numbers, we are able to identify the optimal active site for pH 7 oxygen reduction catalysis, which comprises cobalt atoms bound to the WS2 support with a Co–S coordination number of 3–4. The optimized Co–WS2 material exhibits an oxygen reduction onset potential of 0.798 V vs. RHE, which is comparable to the most active bulk phases of cobalt sulfide in neutral electrolyte conditions.

The interaction between catalyst and support is well known to influence the reactivity and stability of heterogeneous catalysts, and electrochemical hydrogen evolution catalysts based on amorphous ...

Ambipolar doping of metal oxides is critical toward broadening the functionality of semiconducting oxides in electronic devices. Most metal oxides, however, show a strong preference for a single doping polarity due to the intrinsic stability of particular defects in an oxide lattice. In this work, we demonstrate that layered metal hydroxide nanomaterials of Co and Ni, which are intrinsically p-doped in their anhydrous rock salt form, can be n-doped using n-BuLi as a strong electron donor. A combination of X-ray characterization techniques reveal that hydroxide vacancy formation, Li+ adsorption, and varying degrees of electron delocalization are responsible for the stability of injected electrons. The doped electrons induce conductivity increases of 4-6 orders of magnitude relative to the undoped M(OH)2. We anticipate that chemical electron doping of layered metal hydroxides may be a general strategy to increase carrier concentration and stability for n-doping of intrinsically p-type metal oxides.

Single-atom functionalization of transition-metal dichalcogenide (TMD) nanosheets is a powerful strategy to tune the optical, magnetic, and catalytic properties of two-dimensional materials. In thi...

Incorporation of a metal oxide or hydroxide species into Pt-based electrocatalysts has been shown to lower the overpotential required to oxidatively remove carbon monoxide from the catalyst surface, a reaction that is critical to preventing catalyst poisoning and deactivation in fuel cell devices. In this work, we report a general synthetic method toward Pt-metal oxide composite nanoparticles via the adsorption of metal halide ligands onto 1–2 nm colloidal Pt nanoparticles. Pt-metal oxide composite nanoparticles spanning across the first-row transition metals and post-transition metals are synthesized and characterized with transmission electron microscopy and energy dispersive X-ray scattering. CO stripping and steady-state CO oxidation experiments reveal that Mn, Fe, Co, Ni, and In oxides are capable of participating in the catalysis as a bifunctional partner and reduce the overpotential required for CO electrooxidation by ∼200 mV relative to pure Pt.

Core-shell nanoparticles of Au@Pd with precise submonolayer, monolayer, or multilayer structure were synthesized using ligand-exchange reactions of palladate ions onto colloidal Au nanocrystals. Decoupling the palladate adsorption step from the subsequent reduction enables excellent precision, uniformity, and tunability in the Pd shell thickness. The redox properties of the surface Pd are directly correlated to the thickness of the Pd shell with a+200 mV shift in the PdO reduction potential for submonolayer Au@Pd nanoparticles compared to pure Pd. Using these precisely controlled core-shell materials, the oxygen reduction catalytic activity can be directly correlated to PdO reduction potential and Pd surface coverage on Au. When the Pd oxide reduction peak is shifted by +240 mV compared to pure Pd, a 50 mV reduction in overpotential and a 4-fold increase in kinetic current density for oxygen reduction are observed. Colloidal ligand-exchange synthesis may be particularly useful for noble metal core-shell catalysts as a strategy to subtly tune the electronic properties of surface atoms in order to lower overpotential and increase catalytic turnover.

The electrochemical conversion of CO and H2O into liquid fuel is made feasible at modest potentials with the use of oxide-derived nanocystalline Cu as the catalyst. Renewable electricity is often produced when it is not needed. If the surplus could be harnessed to drive the conversion of CO2 and water into liquid fuel, the energy would not go to waste and a use would be found for CO2 produced by carbon capture. All this requires efficient electrocatalysts that reduce CO2 not only to CO, but also further into fuel chemicals. Copper does this but with low efficiency and selectivity. Christina Li et al. now show that the intrinsic catalytic properties of copper can be improved by producing it from its oxide as interconnected nanocrystallites. Their enhanced catalyst generates primarily ethanol, demonstrating that a two-step conversion of CO2 to liquid fuel powered by renewable electricity might be possible. The electrochemical conversion of CO2 and H2O into liquid fuel is ideal for high-density renewable energy storage and could provide an incentive for CO2 capture. However, efficient electrocatalysts for reducing CO2 and its derivatives into a desirable fuel1,2,3 are not available at present. Although many catalysts4,5,6,7,8,9,10,11 can reduce CO2 to carbon monoxide (CO), liquid fuel synthesis requires that CO is reduced further, using H2O as a H+ source. Copper (Cu) is the only known material with an appreciable CO electroreduction activity, but in bulk form its efficiency and selectivity for liquid fuel are far too low for practical use. In particular, H2O reduction to H2 outcompetes CO reduction on Cu electrodes unless extreme overpotentials are applied, at which point gaseous hydrocarbons are the major CO reduction products12,13. Here we show that nanocrystalline Cu prepared from Cu2O (‘oxide-derived Cu’) produces multi-carbon oxygenates (ethanol, acetate and n-propanol) with up to 57% Faraday efficiency at modest potentials (–0.25 volts to –0.5 volts versus the reversible hydrogen electrode) in CO-saturated alkaline H2O. By comparison, when prepared by traditional vapour condensation, Cu nanoparticles with an average crystallite size similar to that of oxide-derived copper produce nearly exclusive H2 (96% Faraday efficiency) under identical conditions. Our results demonstrate the ability to change the intrinsic catalytic properties of Cu for this notoriously difficult reaction by growing interconnected nanocrystallites from the constrained environment of an oxide lattice. The selectivity for oxygenates, with ethanol as the major product, demonstrates the feasibility of a two-step conversion of CO2 to liquid fuel that could be powered by renewable electricity.

Carbon dioxide reduction is an essential component of many prospective technologies for the renewable synthesis of carbon-containing fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. We show that the reduction of thick Au oxide films results in the formation of Au nanoparticles ("oxide-derived Au") that exhibit highly selective CO(2) reduction to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycrystalline Au electrodes and several other nanostructured Au electrodes prepared via alternative methods require at least 200 mV of additional overpotential to attain comparable CO(2) reduction activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO(2)(•-) intermediate on the surfaces of the oxide-derived Au electrodes.