Precise Regulation of Ga-Based Liquid Metal Oxidation

ConspectusLiquid metals, defined as metals or alloys with melting points below or near room temperature, can be regarded as an amorphous solid without any crystallinity in the molten state, exhibiting fundamentally different fluidities and metallicities from solid metals and other liquids. In the past decade, gallium as a typical representative liquid metal with a melting point of ∼29.8 °C, virtually nonexistent vapor pressure, and negligible toxicity has been proposed as a base material for the construction of gallium (Ga)-based liquid metals (LMs). This class of extraordinary materials with unique physicochemical properties, such as superb thermal and electrical conductivity, fluidity, shape transformability, self-healing capability and biocompatibility, biodegradability, catalytic properties, plasmonic effect, and facile functionalization accessibility, has attracted considerable attention in widespread applications. Generally, under the action of ambient oxygen and water, the ultrathin oxide layers will be formed at the LM–ambient environment interface, which may provide a physical, chemical, and electrical barrier to prevent the LMs from further oxidation. The introduction of excitations, such as electrical, chemical, electrochemical, mechanical, and ultrasonic, and the alteration of reaction conditions including ingredients, temperature, and time will promote oxide formation. However, the existence of oxides is a double-edged sword, sometimes considered as a nuisance because of the deterioration of performance and stability; for example, the oxides will adhere to the system, which brings problems for fluidic applications (such as heat-transfer media, pump media, and microfluidity). Conversely, in some cases, oxides are considered essential to improve functionality, such as shape transformation, substrate adhesion, intracellular uptake, etc. For this reason, the main aim of oxidation regulation is to alter the fundamental physicochemical properties or even endow distinct and fascinating properties for the LMs, thereby expanding the scope of applications. Although technological advances have shown dramatic progress and great potential of the LMs, their oxidation regulation remains in its infancy, thus deserving further attention. In this Account, we present a relatively elaborate summary of the oxidation regulation of LMs. First, the fundamental properties of LM oxides and their performance impact on LMs are reviewed. Then, the visions expanding to precise oxidation regulation in terms of vital structural statuses of LMs. After that, representative applications focusing on our own contributions to this field in recent years are described. Finally, brief perspectives and challenges are also presented here. Overall, this Account not only sheds light on the valuable balance between pristine LMs and oxides but also proposes prospective principles for the design and synthesis of advanced LM materials with tunable or even unprecedented properties.