This review article summarizes the last few decades of research on nickel hydroxide, an important material in physics and chemistry, that has many applications in engineering including, significantly, batteries. First, the structures of the two known polymorphs, denoted as α-Ni(OH)2 and β-Ni(OH)2, are described. The various types of disorder, which are frequently present in nickel hydroxide materials, are discussed including hydration, stacking fault disorder, mechanical stresses and the incorporation of ionic impurities. Several related materials are discussed, including intercalated α-derivatives and basic nickel salts. Next, a number of methods to prepare, or synthesize, nickel hydroxides are summarized, including chemical precipitation, electrochemical precipitation, sol-gel synthesis, chemical ageing, hydrothermal and solvothermal synthesis, electrochemical oxidation, microwave-assisted synthesis, and sonochemical methods. Finally, the known physical properties of the nickel hydroxides are reviewed, including their magnetic, vibrational, optical, electrical and mechanical properties. The last section in this paper is intended to serve as a summary of both the potentially useful properties of these materials and the methods for the identification and characterization of 'unknown' nickel hydroxide-based samples.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.2014.0792

Nickel hydroxides and related materials: a review of their structures, synthesis and properties.

Inspired by biological neural systems, neuromorphic devices may open up new computing paradigms to explore cognition, learning and limits of parallel computation. Here we report the demonstration of a synaptic transistor with SmNiO₃, a correlated electron system with insulator-metal transition temperature at 130°C in bulk form. Non-volatile resistance and synaptic multilevel analogue states are demonstrated by control over composition in ionic liquid-gated devices on silicon platforms. The extent of the resistance modulation can be dramatically controlled by the film microstructure. By simulating the time difference between postneuron and preneuron spikes as the input parameter of a gate bias voltage pulse, synaptic spike-timing-dependent plasticity learning behaviour is realized. The extreme sensitivity of electrical properties to defects in correlated oxides may make them a particularly suitable class of materials to realize artificial biological circuits that can be operated at and above room temperature and seamlessly integrated into conventional electronic circuits.

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A correlated nickelate synaptic transistor.

Interfaces between insulating oxides have revealed exotic electronic and magnetic properties. It is now shown that a complex magnetic structure can emerge in an oxide superlattice, and that specific interfaces can unexpectedly exhibit exchange bias. The observations reveal the induction of antiferromagnetism in a material that is usually paramagnetic.

Exchange bias in LaNiO3-LaMnO3 superlattices.

Perovskite nickelates (RNiO3, where R is rare earth or a heavy metal such as Tl or Bi) display sharp metal–insulator transitions, unusual magnetic order, charge order and, perhaps, orbital order. Furthermore, there are strong reasons to believe that some of them may be magnetoelectric multiferroics. In this article the author reviews recent research perovskite nickelates, highlighting the important role that thin film research has contributed to our understanding of their properties. A special emphasis is placed on open questions and highly topical issues such as whether or not nickelates have orbital order, and whether or not (and why) some of them may be multiferroic.

Progress in perovskite nickelate research

A complex hydroxide/metal Ni(OH)2@Ni core–shell electrode was developed for a high-performance and flexible pseudocapacitor. Compared to the conventional Ni(OH)2 electrode, the as-prepared amorphous Ni(OH)2@ three-dimensional (3D) Ni core–shell electrode shows a large specific capacitance of 2868 F g−1 at a scan rate of 1 mV s−1 and a good cycling stability (3% degradation after 1000 cycles) at a scan rate of 100 mV s−1. Furthermore, the high rate capability with a specific capacitance of 2454 F g−1 can be achieved at a charge–discharge current density of 5 A g−1. An amorphous Ni(OH)2@3D Ni-AC based asymmetric supercapacitor could be cycled reversibly in the high-voltage region of 0–1.3 V, and the specific capacitance of 92.8 F g−1 at 1 A g−1. This research demonstrates that introduction of a metal core to conventional hydroxide supercapacitor electrodes could open up new opportunities for designing and developing high-performance supercapacitors.

Amorphous Ni(OH)2 @ three-dimensional Ni core–shell nanostructures for high capacitance pseudocapacitors and asymmetric supercapacitors

We present a detailed study of the magnetic properties of sol-gel prepared nickel oxide nanoparticles of different sizes. We report various measurements such as frequency, field, and temperature dependence of ac susceptibility, temperature and field dependence of dc magnetization, and time decay of thermoremanent magnetization. Our results and analysis show that the system behaves as a spin glass.

https://arxiv.org/pdf/cond-mat/0503433.pdf

K. P. Rajeev

Papers

Signatures of spin-glass freezing in NiO nanoparticles

Low-temperature electronic properties of a normal conducting perovskite oxide (LaNiO3)

Magnetic properties of NiO nanoparticles

Low-temperature electrical transport in La0.7A0.3MnO3, (A: Ca, Sr, Ba)

Effect of oxygen stoichiometry on the electrical resistivity behaviour of NdNiO3-δ