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New nanostructures often reflect new and exciting properties. Here, we present an two-dimensional, hitherto unreported PdO square network with lateral dimensions up to hundreds of nanometers growing on reduced graphene oxide (rGO), forming a hybrid nanofilm. An intermediate state of dissolved Pd(0) in the bacterium S. oneidensis MR-1 is pivotal in the biosynthesis and inspires an abiotic synthesis. The PdO network shows a lattice spacing of 0.5 nm and a thickness of 1.8 nm on both sides of an rGO layer and is proposed to be cubic or tetragonal crystal, as confirmed by structural simulations. A 2D silver oxide analog with a similar structure is also obtained using an analogous abiotic synthesis. Our study thus opens a simple route to a whole new class of 2D metal oxides on rGO as promising candidates for graphene superlattices with unexplored properties and potential applications for example in electronics, sensing, and energy conversion.

Following a renaissance from the 1980s, electrochemistry has developed into sophisticated interdisciplinary science integrating solid-state and surface science and further, biological sciences. Most remarkably, even the single-molecule can now be addressed. Single-molecule electrochemistry covers transition metal complexes, organic redox molecules, fragile biomolecules, and molecular-scale hybrids between metallic nanoparticles and complex molecules, supported by new theoretical frames. We overview here selected areas of molecular scale electrochemistry. After a theoretical minimum, we address complex molecules in electron transfer, enzyme catalysis, and nanoparticle catalysis. Our focus is on issues not so much previously highlighted, such as competition between superexchange and sequential conduction, and resonance features in the transition between the two limits. Another aspect is coherent multi-electron transfer for large bias voltages often needed to drive enough current through solute molecular junctions. We note finally some single-molecule perspectives relating to DNA-based molecules and to spin transitions via chiral molecules.

Blue copper enzymes often show no voltammetry themselves, whereas substrate binding triggers strong electrocatalytic signals. Similarly, electrochemical STM only gives strong contrasts when substrate (O2, NO2-) is present. AFM shows that CuNIR on Au(111)-electrodes modified by self-assembled cysteamine monolayers (SAMs) maintains constant height throughout the electrocatalytic range, while NO2- triggers substantial enzyme ‘swelling’. ‘Swelling’ does not accord with the crystalline state, which, however, is not the relevant catalytic environment.With a view on understanding these patterns, we present ab initio quantum chemical studies of CuNIR/OH2 and CuNIR/NO2- 740-atom fragments including the type I and type II Cu-centres. Replacing water at the type II centre by nitrite triggers 2-Å Cu–Cu distance increase, according with enzyme ‘swelling’. 2 Å Cu–Cu increase would close intramolecular ET entirely, but is compensated by efficient superexchange alignment of closely interacting LUMOs and HOMOs. In the water-bound enzyme these orbitals are separated by unfavourable through-space tunneling regions.

Single-molecule electrochemical science has advanced over the past decades and now extends well beyond molecular imaging, to molecular electronics functions such as rectification and amplification. Rectification is conceptually the simplest but has involved mostly challenging chemical synthesis of asymmetric molecular structures or asymmetric materials and geometry of the two enclosing electrodes. Here we propose an experimental and theoretical strategy for building and tuning in situ (in operando) rectification in two symmetric molecular structures in electrochemical environment. The molecules were designed to conduct electronically via either their lowest unoccupied molecular orbital (LUMO; electron transfer) or highest occupied molecular orbital (HOMO; "hole transfer"). We used a bipotentiostat to control separately the electrochemical potential of the tip and substrate electrodes of an electrochemical scanning tunneling microscope (EC-STM), which leads to independent energy alignment of the STM tip, the molecule, and the STM substrate. By creating an asymmetric energy alignment, we observed single-molecule rectification of each molecule within a voltage range of ±0.5 V. By varying both the dominating charge transporting LUMO or HOMO energy and the electrolyte concentration, we achieved tuning of the polarity as well as the amplitude of the rectification. We have extended an earlier proposed theory that predicts electrolyte-controlled rectification to rationalize all the observed in situ rectification features and found excellent agreement between theory and experiments. Our study thus offers a way toward building controllable single-molecule rectifying devices without involving asymmetric molecular structures.

Featuring high electrical conductivity and electrochemical activity, transition metal sulfides are promising materials for the positive electrode of hybrid supercapacitors. However, considering their poor cycling life and rate performance, critical challenges remain. In this contribution, we describe hollow NiS2/CoS2@C composites obtained by stepwise etching, annealing, and sulfuration of ZIF-67. Specifically, NiS2/CoS2 particles were dispersed on N/S co-doped carbon nanocages and found to exhibit outstanding stability and rate performance. The unique nanocage structure of N/S co-doped carbon and heterogeneous interfaces of NiS2 and CoS2 can promote both ion transport and electron transfer, enriching the active site for enhanced reactivity. The resulting NiS2/CoS2@C nanocage composites in fact showed considerable electrochemical performance, with a specific capacity of 1373 C g-1 at 1 A g-1. When combined with reduced graphene oxide (RGO)-based negative electrode, the assembled hybrid supercapacitor registered an energy density of 63.3 Wh kg-1 at a power density of 800 W kg-1, retaining exceeding 98% of the initial capacity even after 13,000 cycles.