Liquid metal/metal oxide frameworks with incorporated Ga2O3 for photocatalysis

Advancing Nanoelectronics Applications: Progress in Non-van der Waals 2D Materials

Extending the inventory of two-dimensional (2D) materials remains highly desirable, given their excellent properties and wide applications. Current studies on 2D materials mainly focus on the van der Waals (vdW) materials since the discovery of graphene, where properties of atomically thin layers have been found to be distinct from their bulk counterparts. Beyond vdW materials, there are abundant non-vdW materials that can also be thinned down to 2D forms, which are still in their early stage of exploration. In this review, we focus on the downscaling of non-vdW materials into 2D forms to enrich the 2D materials family. This underexplored group of 2D materials could show potential promise in many areas such as electronics, optics, and magnetics, as has happened in the vdW 2D materials. Hereby, we will focus our discussion on their electronic properties and applications of them. We aim to motivate and inspire fellow researchers in the 2D materials community to contribute to the development of 2D materials beyond the widely studied vdW layered materials for electronic device applications. We also give our insights into the challenges and opportunities to guide researchers who are desirous of working in this promising research area.

Galvanic Replacement Synthesis of VOx@EGaIn-PEG Core-Shell Nanohybrids for Peroxidase Mimics

The development of high-performance biosensors is a key focus in the nanozyme field, but the current limitations in biocompatibility and recyclability hinder their broader applications. Herein, we address these challenges by constructing core-shell nanohybrids with biocompatible poly(ethylene glycol) (PEG) modification using a galvanic replacement reaction between orthovanadate ions and liquid metal (LM) (VOx@EGaIn-PEG). By leveraging the excellent charge transfer properties and the low band gap of the LM surface oxide, the VOx@EGaIn-PEG heterojunction can effectively convert hydrogen peroxide into hydroxyl radicals, demonstrating excellent peroxidase-like activity and stability (Km = 490 μM, vmax = 1.206 μM/s). The unique self-healing characteristics of LM further enable the recovery and regeneration of VOx@EGaIn-PEG nanozymes, thereby significantly reducing the cost of biological detection. Building upon this, we developed a nanozyme colorimetric sensor suitable for biological systems and integrated it with a smartphone to create an efficient quantitative detection platform. This platform allows for the convenient and sensitive detection of glucose in serum samples, exhibiting a good linear relationship in the range of 10-500 μM and a detection limit of 2.35 μM. The remarkable catalytic potential of LM, combined with its biocompatibility and regenerative properties, offers valuable insights for applications in catalysis and biomedical fields.

Mechanism behind the Controlled Generation of Liquid Metal Nanoparticles by Mechanical Agitation

The size-controlled synthesis of liquid metal nanoparticles is necessary in a variety of applications. Sonication is a common method for breaking down bulk liquid metals into small particles, yet the influence of critical factors such as liquid metal composition has remained elusive. Our study employs high-speed imaging to unravel the mechanism of liquid metal particle formation during mechanical agitation. Gallium-based liquid metals, with and without secondary metals of bismuth, indium, and tin, are analyzed to observe the effect of cavitation and surface eruption during sonication and particle release. The impact of the secondary metal inclusion is investigated on liquid metals' surface tension, solution turbidity, and size distribution of the generated particles. Our work evidences that there is an inverse relationship between the surface tension and the ability of liquid metals to be broken down by sonication. We show that even for 0.22 at. % of bismuth in gallium, the surface tension is significantly decreased from 558 to 417 mN/m (measured in Milli-Q water), resulting in an enhanced particle generation rate: 3.6 times increase in turbidity and ∼43% reduction in the size of particles for bismuth in gallium liquid alloy compared to liquid gallium for the same sonication duration. The effect of particles' size on the photocatalysis of the annealed particles is also presented to show the applicability of the process in a proof-of-concept demonstration. This work contributes to a broader understanding of the synthesis of nanoparticles, with controlled size and characteristics, via mechanical agitation of liquid metals for diverse applications.

Constructing liquid metal/metal-organic framework nanohybrids with strong sonochemical energy storage performance for enhanced pollutants removal

With endogenous redox systems and multiple enzymes, the storage and utilization of external energy is general in living cells, especially through photo/ultrasonic synthesis/catalysis due to in-situ generation of abundant reactive oxygen species (ROS). However, in artificial systems, because of extreme cavitation surroundings, ultrashort lifetime and increased diffusion distance, sonochemical energy is rapidly dissipated via electron-hole pairs recombination and ROS termination. Here, we integrate zeolitic imidazolate framework-90 (ZIF-90) and liquid metal (LM) with opposite charges by convenient sonosynthesis, and the resultant nanohybrid (LMND@ZIF-90) can efficiently capture sonogenerated holes and electrons, and thus suppress electron-hole pairs recombination. Unexpectedly, LMND@ZIF-90 can store the ultrasonic energy for over ten days and exhibit acid-responsive release to trigger persistent generation of various ROS including superoxide (O₂•^-), hydroxyl radicals (•OH), and singlet oxygen (¹O₂), presenting significantly faster dye degradation rate (short to seconds) than previously reported sonocatalysts. Moreover, unique properties of gallium could additionally facilitate heavy metals removal through galvanic replacement and alloying. In summary, the LM/MOF nanohybrid constructed here demonstrates strong capacity for storing sonochemical energy as long-lived ROS, enabling enhanced water decontamination without energy input.

Liquid Metal as Bioinspired and Unusual Modulator in Bioorthogonal Catalysis for Tumor Inhibition Therapy

Bioorthogonal catalysis mediated by Pd-based transition metal catalysts has sparked increasing interest in combating diseases. However, the catalytic and therapeutic efficiency of current Pd⁰ catalysts is unsatisfactory. Herein, inspired by the concept that ligands around metal sites could enable enzymes to catalyze astonishing reactions by changing their electronic environment, a LM-Pd catalyst with liquid metal (LM) as an unusual modulator has been designed to realize efficient bioorthogonal catalysis for tumor inhibition. The LM matrix can serve as a "ligand" to afford an electron-rich environment to stabilize the active Pd⁰ and promote nucleophilic turnover of the π-allylpalladium species to accelerate the uncaging process. Besides, the photothermal properties of LM can lead to the enhanced removal of tumor cells by photo-enhanced catalysis and photothermal effect. We believe that our work will broaden the application of LM and motivate the design of bioinspired bioorthogonal catalysts.

Synthesis and Application of Liquid Metal Based-2D Nanomaterials: A Perspective View for Sustainable Energy

With the continuous exploration of low-dimensional nanomaterials, two dimensional metal oxides (2DMOs) has been received great interest. However, their further development is limited by the high cost in the preparation process and the unstable states caused by the polarization of surface chemical bonds. Recently, obtaining mental oxides via liquid metals have been considered a surprising method for obtaining 2DMOs. Therefore, how to scientifically choose different preparation methods to obtain 2DMOs applying in different application scenarios is an ongoing process worth discussing. This review will provide some new opportunities for the rational design of 2DMOs based on liquid metals. Firstly, the surface oxidation process and in situ electrical replacement reaction process of liquid metals are introduced in detail, which provides theoretical basis for realizing functional 2DMOs. Secondly, by simple sticking method, gas injection method and ultrasonic method, 2DMOs can be obtained from liquid metal, the characteristics of each method are introduced in detail. Then, this review provides some prospective new ideas for 2DMOs in other energy-related applications such as photodegradation, CO₂ reduction and battery applications. Finally, the present challenges and future development prospects of 2DMOs applied in liquid metals are presented.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Ultrasoft and Ultrastretchable Wearable Strain Sensors with Anisotropic Conductivity Enabled by Liquid Metal Fillers

Herein, ultrasoft and ultrastretchable wearable strain sensors enabled by liquid metal fillers in an elastic polymer are described. The wearable strain sensors that can change the effective resistance upon strains are prepared by mixing silicone elastomer with liquid metal (EGaIn, Eutectic gallium-indium alloy) fillers. While the silicone is mixed with the liquid metal by shear mixing, the liquid metal is rendered into small droplets stabilized by an oxide, resulting in a non-conductive liquid metal elastomer. To attain electrical conductivity, localized mechanical pressure is applied using a stylus onto the thermally cured elastomer, resulting in the formation of a handwritten conductive trace by rupturing the oxide layer of the liquid metal droplets and subsequent percolation. Although this approach has been introduced previously, the liquid metal dispersed elastomers developed here are compelling because of their ultra-stretchable (elongation at break of 4000%) and ultrasoft (Young’s modulus of <0.1 MPa) mechanical properties. The handwritten conductive trace in the elastomers can maintain metallic conductivity when strained; however, remarkably, we observed that the electrical conductivity is anisotropic upon parallel and perpendicular strains to the conductive trace. This anisotropic conductivity of the liquid metal elastomer film can manipulate the locomotion of a robot by routing the power signals between the battery and the driving motor of a robot upon parallel and perpendicular strains to the hand-written circuit. In addition, the liquid metal dispersed elastomers have a high degree of deformation and adhesion; thus, they are suitable for use as a wearable sensor for monitoring various body motions.

A Review on Gallium Oxide Materials from Solution Processes

Gallium oxide (Ga₂O₃) materials can be fabricated via various methods or processes. It is often mentioned that it possesses different polymorphs (α-, β-, γ-, δ- and ε-Ga₂O₃) and excellent physical and chemical properties. The basic properties, crystalline structure, band gap, density of states, and other properties of Ga₂O₃ will be discussed in this article. This article extensively discusses synthesis of pure Ga₂O₃, co-doped Ga₂O₃ and Ga₂O₃-metal oxide composite and Ga₂O₃/metal oxide heterostructure nanomaterials via solution-based methods mainly sol-gel, hydrothermal, chemical bath methods, solvothermal, forced hydrolysis, reflux condensation, and electrochemical deposition methods. The influence of the type of precursor solution and the synthesis conditions on the morphology, size, and properties of final products is thoroughly described. Furthermore, the applications of Ga₂O₃ will be introduced and discussed from these solution processes, such as deep ultraviolet photodetector, gas sensors, pH sensors, photocatalytic and photodegradation, and other applications. In addition, research progress and future outlook are identified.

Liquid-Metal-Coated Magnetic Particles toward Writable, Nonwettable, Stretchable Circuit Boards, and Directly Assembled Liquid Metal-Elastomer Conductors

Liquid metal is a promising conductor material for producing soft and stretchable circuit "boards" that can enable next-generation electronics by electrically connecting and mechanically supporting electronic components. While liquid metal in general can be used to fabricate soft and stretchable circuits, magnetic liquid metal is appealing because it can be used for self-healing electronics and actuators by external magnetic fields. Liquid metal can be rendered into particles that can then be used for sensors and catalysts through sonication. We used this feature to produce "novel" conductive and magnetic particles. Mixing ferromagnetic iron particles into the liquid metal (gallium) produces conductive ferrofluids that can be rendered into gallium-coated iron particles by sonication. The gallium shell of the particles is extremely soft, while the rigid iron core can induce high friction in response to mechanical pressure; thus, hand-sintering of the particles can be used to directly write the conductive traces when the particles are cast as a film on elastic substrates. The surface topography of the particles can be manipulated by forming GaOOH crystals through sonication in DI water, thus resulting in nonwettable circuit boards. These gallium-coated iron particles dispersed in uncured elastomer can be assembled to form conductive microwires with the application of magnetic fields.

Self-Standing, Photothermal-Actuating, and Motion-Monitoring Janus Films One-Pot Synthesized by Green Carboxymethyl Glucomannan/Liquid Metal Nanoinks

Downsizing bulk liquid metals (LM) at the nanometer scale with biocompatibility and multifunction is a key process for electronic or medical applications. Here, we report a stable and green LM aqueous colloidal ink by wrapping eutectic gallium-indium alloys (EGaIn) with carboxymethyl glucomannan (CGM) derived from radiata pine chip, which is capable of being prepared into a free-standing, photothermal-actuating, and motion-monitoring Janus film. With the assistance of CGM, the bulk EGaIn was ultrasonicated into stable nanodroplets (∼500 nm) with a typical "core-shell" structure, in which the colloidal inks can be stored for more than 1 week under room temperature. The stable CGM/EGaIn inks can be patterned on different substrates to form coating layers or self-assembled into free-standing Janus films with high mechanical strength and modulus (∼94 MPa and ∼3.8 GPa) by density deposition. Such a Janus film with anisotropic thermal conductivity made it a potential photothermal actuator. In addition, the biocompatible film demonstrated both high conductivity and large resistance variation in response to strain change (gauge factor >500), allowing for human motion monitoring. This work provides a new prospect for the development of biocompatible and high-performance nano-LM materials.

Non-Noble Plasmonic Metal-Based Photocatalysts

Solar-to-chemical energy conversion via heterogeneous photocatalysis is one of the sustainable approaches to tackle the growing environmental and energy challenges. Among various promising photocatalytic materials, plasmonic-driven photocatalysts feature prominent solar-driven surface plasmon resonance (SPR). Non-noble plasmonic metals (NNPMs)-based photocatalysts have been identified as a unique alternative to noble metal-based ones due to their advantages like earth-abundance, cost-effectiveness, and large-scale application capability. This review comprehensively summarizes the most recent advances in the synthesis, characterization, and properties of NNPMs-based photocatalysts. After introducing the fundamental principles of SPR, the attributes and functionalities of NNPMs in governing surface/interfacial photocatalytic processes are presented. Next, the utilization of NNPMs-based photocatalytic materials for the removal of pollutants, water splitting, CO₂ reduction, and organic transformations is discussed. The review concludes with current challenges and perspectives in advancing the NNPMs-based photocatalysts, which are timely and important to plasmon-based photocatalysis, a truly interdisciplinary field across materials science, chemistry, and physics.

Controlled Transformation of Liquid Metal Microspheres in Aqueous Solution Triggered by Growth of GaOOH

Liquid metals (LMs) are playing an increasingly important role in the fields of flexible devices, electronics, and thermal management due to their low melting point and excellent thermal and electrical conductivity, and the transformation of LMs in deionized water has recently received much attention. In this paper, we investigate the transformation process of EGaIn microspheres in deionized water and propose a two-step process of microspherical transformation, whereby the microspheres are first deformed into a spindle shape and then into lamellar nanorods. It is also shown that the growth of GaOOH crystals drives the transformation. Based on this result, EGaIn microspheres with controllable transformation could be prepared, such as spindle or lamellar rod shapes, extending the application area of LMs.

Piezoelectric effect enhanced photocatalysis in environmental remediation: State-of-the-art techniques and future scenarios

Photocatalysis has been widely used as an advanced oxidation process to control pollutants effectively. However, environmental photocatalysis' decontamination efficiency is restricted to the photogenerated electron-hole pairs' rapid recombination. Recently, emerging investigations have been directed to generate internal electric field by piezoelectric effect to enhance the separation efficiency of photogenerated charge carriers for better photocatalytic performance; however, there are still huge knowledge gaps on the rational application of piezo-photocatalysis in environmental remediation and disinfection. Thus, we have conducted a comprehensive review to better understand the physicochemical properties of piezoelectric materials (non-centrosymmetric crystal structures, piezoelectric coefficient, Young's modulus, and etc.) and current study states. We also elucidated the strategy of piezo-photo catalysis system constructions (mono-component, core-shell structure, and etc.) and underlying mechanisms of enhanced remediation performance. Addressing the current challenges and future scenarios (degradation of organic pollutants, disinfection, and etc.), the present review would shed light on the advanced wastewater treatment development towards sustainable control of emerging containments.

Applications of liquid metals in nanotechnology

Post-transition liquid metals (LMs) offer new opportunities for accessing exciting dynamics for nanomaterials. As entities with free electrons and ions as well as fluidity, LM-based nanomaterials are fundamentally different from their solid counterparts. The low melting points of most post-transition metals (less than 330 °C) allow for the formation of nanodroplets from bulk metal melts under mild mechanical and chemical conditions. At the nanoscale, these liquid state nanodroplets simultaneously offer high electrical and thermal conductivities, tunable reactivities and useful physicochemical properties. They also offer specific alloying and dealloying conditions for the formation of multi-elemental liquid based nanoalloys or the synthesis of engineered solid nanomaterials. To date, while only a few nanosized LM materials have been investigated, extraordinary properties have been observed for such systems. Multi-elemental nanoalloys have shown controllable homogeneous or heterogeneous core and surface compositions with interfacial ordering at the nanoscale. The interactions and synergies of nanosized LMs with polymeric, inorganic and bio-materials have also resulted in new compounds. This review highlights recent progress and future directions for the synthesis and applications of post-transition LMs and their alloys. The review presents the unique properties of these LM nanodroplets for developing functional materials for electronics, sensors, catalysts, energy systems, and nanomedicine and biomedical applications, as well as other functional systems engineered at the nanoscale.

Recent advances in the fabrication of 2D metal oxides

Atomically thin two-dimensional (2D) metal oxides exhibit unique optical, electrical, magnetic, and chemical properties, rendering them a bright application prospect in high-performance smart devices. Given the large variety of both layered and non-layered 2D metal oxides, the controllable synthesis is the critical prerequisite for enabling the exploration of their great potentials. In this review, recent progress in the synthesis of 2D metal oxides is summarized and categorized. Particularly, a brief overview of categories and crystal structures of 2D metal oxides is firstly introduced, followed by a critical discussion of various synthesis methods regarding the growth mechanisms, advantages, and limitations. Finally, the existing challenges are presented to provide possible future research directions regarding the synthesis of 2D metal oxides. This work can provide useful guidance on developing innovative approaches for producing both 2D layered and non-layered nanostructures and assist with the acceleration of the research of 2D metal oxides.

Supported Cu/W/Mo/Ni—Liquid Metal Catalyst with Core-Shell Structure for Photocatalytic Degradation

Room-temperature liquid metal is a very ideal material for the design of catalytic materials. At low temperatures, the liquid metal enters the liquid state. It provides an opportunity to utilize the liquid phase in the catalysis, which is far superior to the traditional solid-phase catalyst. Aiming at the low performance and narrow application scope of the existing single-phase liquid metal catalyst, this paper proposed a type of liquid metal/metal oxide core-shell composite multi-metal catalyst. The Ga2O3 core-shell heterostructure was formed by chemical modification of liquid metals with different nano metals Cu/W/Mo/Ni, and it was applied to photocatalytic degrading organic contaminated raw liquor. The effects of different metal species on the rate of catalytic degradation were explored. The selectivity and stability of the LM/MO core-shell composite catalytic material were clarified, and it was found that the Ni-LM catalyst could degrade methylene blue and Congo red by 92% and 79%, respectively. The catalytic mechanism and charge transfer mechanism were revealed by combining the optical band gap value. Finally, we provided a theoretical basis for the further development of liquid metal photocatalytic materials in the field of new energy environments.

Nanotip Formation from Liquid Metals for Soft Electronic Junctions

Liquid metals and alloys with high-aspect-ratio nanodimensional features are highly sought-after for emerging electronic applications. However, high surface tension, water-like fluidity, and the existence of self-limiting oxides confer specific peculiarities to their characteristics. Here, we introduce a high accuracy nanometric three-dimensional pulling and stretching method to fabricate liquid-metal-based nanotips from room- or near-room-temperature gallium-based alloys. The pulling rate and step size were controlled with a resolution of up to 10 nm and yielded different nanotip morphologies and lengths as a function of the base liquid metal alloy composition and the pulling parameters. The obtained nanotips presented high aspect ratios over lengths of a few microns and apexes between 10 and 100 nm. The liquid metal alloys were found confined within nanotips with about 10 nm apexes when vertically pulled at 100 nm/s. An amorphous gallium oxide skin was shown to cover the surface of the nanotips, while the liquid core was composed of the initial liquid metal alloys. The electrical contact established at the nanotips was characterized under dynamic conditions. The liquid metal nanotips showed an Ohmic resistance when a continuous liquid metal channel was formed, and a controllable semiconductor state corresponding to a heterojunction formed at the junction between the liquid metal phase and the gallium oxide semiconductor skin. The variable threshold voltages of the heterojunction were controlled via stretching of the nanotips with a 10 nm step resolution. The liquid metal nanotips were also used for establishing soft electronic junctions. This novel method of liquid metal nanotip fabrication with Ohmic and semiconducting behaviors will lead to exciting avenues for developing electronic and sensing devices.

Synthesis of vertical WO3 nanoarrays with different morphologies using the same protocol for enhanced photocatalytic and photoelectrocatalytic performances

Tungsten trioxide (WO3) nanoarrays with different morphologies were successfully synthesized by a hydrothermal method on an FTO substrate. Various nanostructures of WO3 including nanoflakes, nanoplates, nanoflowers and nanorods were obtained by adjusting only the acidity of the precursor solution. XRD patterns confirmed that the as-prepared orthorhombic WO3·0.33H2O transformed to the monoclinic WO3 phase under annealing at 500 °C. UV-Vis absorbance spectroscopy indicated that the absorption edge of WO3 nanoflowers exhibited a slight red-shift compared to other morphologies of WO3. The obtained WO3 nanoflower arrays exhibit the highest photocurrent density and photocatalytic degradation activity towards methylene blue. Finally, the mechanism of the photocatalytic degradation of methylene blue by WO3 is discussed.

Structural and mechanical properties of nanocrystalline Ga2O3 films made by pulsed laser deposition onto transparent quartz substrates

This work reports on the correlation between structure, surface/interface morphology and mechanical properties of pulsed laser deposited (PLD) β-Ga2O3 films on transparent quartz substrates. By varying the deposition temperature in the range of 25 °C–700 °C, ∼200 nm thick Ga2O3 films with variable microstructure and amorphous-to-nanocrystalline nature were produced onto quartz substrates by PLD. The Ga2O3 films deposited at room temperature were amorphous; nanocrystalline Ga2O3 films were realized at 700 °C. The interface microstructure is characterized with a typical nano-columnar morphology while the surface exhibits the uniform granular morphology. Corroborating with structure and surface/interface morphology, and with increasing deposition temperature, tunable mechanical properties were seen in PLD Ga2O3 films. At 700 °C, for nanocrystalline Ga2O3 films, the dense grain packing reduces the elastic modulus Er while improving the hardness. The improved crystallinity at elevated temperatures coupled with nanocrystallinity, the β-phase stabilization is accounted for the observed enhancement in the mechanical properties of PLD Ga2O3 films. The structure-morphology-mechanical property correlation in nanocrystalline PLD β-Ga2O3 films deposited on quartz substrates is discussed in detail.

Mini/Micro/Nano Scale Liquid Metal Motors

Swimming motors navigating in complex fluidic environments have received tremendous attention over the last decade. In particular, liquid metal (LM) as a new emerging material has shown considerable potential in furthering the development of swimming motors, due to their unique features such as fluidity, softness, reconfigurability, stimuli responsiveness, and good biocompatibility. LM motors can not only achieve directional motion but also deformation due to their liquid nature, thus providing new and unique capabilities to the field of swimming motors. This review aims to provide an overview of the recent advances of LM motors and compare the difference in LM macro and micromotors from fabrication, propulsion, and application. Here, LM motors below 1 cm, named mini/micro/nano scale liquid metal motors (MLMTs) will be discussed. This work will present physicochemical characteristics of LMs and summarize the state-of-the-art progress in MLMTs. Finally, future outlooks including both opportunities and challenges of mini/micro/nano scale liquid metal motors are also provided.

Synthesis and application of core-shell liquid metal particles: a perspective of surface engineering

Liquid metal micro/nano particles (LMPs) from gallium and its alloys have attracted tremendous attention in the last decade due to the unique combination of their metallic and fluidic properties at relatively low temperatures. Unfortunately, there is limited success so far in realizing the highly controllable fabrication and functionalization of this emerging material, posing great obstacles to further promoting its fundamental and applied studies. This review aims to explore solutions for the on-demand design and manipulation of LMPs through physicochemically engineering their surface microenvironment, including compositions, structures, and properties, which are featured by the encapsulation of LMPs inside a variety of synthetic shell architectures. These heterophase, core-shell liquid metal composites display adjustable size and structure-property relationships, rendering improved performances in several attractive scenarios including but not limited to soft electronics, nano/biomedicine, catalysis, and energy storage/conversion. Challenges and opportunities regarding this burgeoning field are also disclosed at the end of this review.

Graphene-based encapsulation of liquid metal particles

Liquid metals are a promising functional material due to their unique combination of metallic properties and fluidity at room temperature. They are of interest in wide-ranging fields including stretchable and flexible electronics, reconfigurable devices, microfluidics, biomedicine, material synthesis, and catalysis. Transformation of bulk liquid metal into particles has enabled further advances by allowing access to a broader palette of fabrication techniques for device manufacture or by increasing area available for surface-based applications. For gallium-based liquid metal alloys, particle stabilization is typically achieved by the oxide that forms spontaneously on the surface, even when only trace amounts of oxygen are present. The utility of the particles formed is governed by the chemical, electrical, and mechanical properties of this oxide. To overcome some of the intrinsic limitations of the native oxide, it is demonstrated here for the first time that 2D graphene-based materials can encapsulate liquid metal particles during fabrication and imbue them with previously unattainable properties. This outer encapsulation layer is used to physically stabilize particles in a broad range of pH environments, modify the particles' mechanical behavior, and control the electrical behavior of resulting films. This demonstration of graphene-based encapsulation of liquid metal particles represents a first foray into the creation of a suite of hybridized 2D material coated liquid metal particles.

Gallium-Based Liquid Metal Particles for Therapeutics

Gallium (Ga) and Ga-based liquid metal (LM) alloys offer low toxicity, excellent electrical and thermal conductivities, and fluidity at or near room temperature. Ga-based LM particles (LMPs) synthesized from these LMs exhibit both fluidic and metallic properties and are suitable for versatile functionalization in therapeutics. Functionalized Ga-based LMPs can be actuated using physical or chemical stimuli for drug delivery, cancer treatment, bioimaging, and biosensing. However, many of the fundamentals of their unique characteristics for therapeutics remain underexplored. We present the most recent advances in Ga-based LMPs in therapeutics based on the underlying mechanisms of their design and implementation. We also highlight some future biotechnological opportunities for Ga-based LMPs based on their extraordinary advantages.

Non-Absorbing Dielectric Materials for Surface-Enhanced Spectroscopies and Chiral Sensing in the UV

Low-loss dielectric nanomaterials are being extensively studied as novel platforms for enhanced light-matter interactions. Dielectric materials are more versatile than metals when nanostructured as they are able to generate simultaneously electric- and magnetic-type resonances. This unique property gives rise to a wide gamut of new phenomena not observed in metal nanostructures such as directional scattering conditions or enhanced optical chirality density. Traditionally studied dielectrics such as Si, Ge or GaP have an operating range constrained to the infrared and/or the visible range. Tuning their resonances up to the UV, where many biological samples of interest exhibit their absorption bands, is not possible due to their increased optical losses via heat generation. Herein, we report a quantitative survey on the UV optical performance of 20 different dielectric nanostructured materials for UV surface light-matter interaction based applications. The near-field intensity and optical chirality density averaged over the surface of the nanoparticles together with the heat generation are studied as figures of merit for this comparative analysis.

Synergistic Effects of Carbon Dots and Palladium Nanoparticles Enhance the Sonocatalytic Performance for Rhodamine B Degradation in the Absence of Light

Carbon dot (CD) and palladium nanoparticle (Pd NP) composites are semiconducting materials having tremendous applications in catalysis with suitable band gaps. However, their combination with a suitable polymer matrix in sonophotocatalysis has not been explored. Herein, we have synthesized and characterized a new nanohybrid catalyst from a polyamide cross-linked CD-polymer and subsequent deposition of Pd NPs. A sonocatalytic activity of 99% rhodamine B dye degradation was achieved in mere 5 min in the dark. A model catalyst replacing CDs with benzene and other control studies revealed that the synergistic effects of CDs and Pd NPs enhance the sonocatalytic activity of the nanohybrid catalyst. Interestingly, visible light did not influence the activity significantly. Mechanistic investigations suggest that generation of reactive oxygen species on the surface of the CD-polymer initiated by ultrasound, which is further facilitated by Pd NPs, is the key for remarkable catalytic activity (a rate constant of 0.99 min-1). Recyclable heterogeneous catalysts under ambient conditions are promising for exploring sono-assisted dark catalysis for several avenues.

A Simple and Cost-Effective Method for Producing Stable Surfactant-Coated EGaIn Liquid Metal Nanodroplets

Liquid metals show unparalleled advantages in printable circuits, flexible wear, drug carriers, and electromagnetic shielding. However, the efficient and large-scale preparation of liquid metal nanodroplets (LM NDs) remains a significant challenge. Here, we propose a simple and efficient method for the large-scale preparation of stable eutectic gallium indium nanodroplets (EGaIn NDs). We compared different preparation methods and found that droplets with smaller particle sizes could quickly be produced using a shaking technique. The size of EGaIn NDs produced using this technique can reach 200 nm in 30 min and 100 nm in 240 min. Benefiting from the simple method, various surfactants can directly modify the surface of the EGaIn NDs to stabilize the prepared droplets. In addition, we discovered that shaking in an ice bath produced spherical nanodroplets, and after shaking for 30 min in a non-ice bath, rod-shaped gallium oxide hydroxide (GaOOH) appeared. Furthermore, the EGaIn NDs we produced have excellent stability-after storage at room temperature for 30 days, the particle size and morphology change little. The excellent stability of the produced EGaIn NDs provides a wider application of liquid metals in the fields of drug delivery, electromagnetic shielding, conductive inks, printed circuits, etc.

Attributes, Fabrication, and Applications of Gallium-Based Liquid Metal Particles

This work discusses the attributes, fabrication methods, and applications of gallium-based liquid metal particles. Gallium-based liquid metals combine metallic and fluidic properties at room temperature. Unlike mercury, which is toxic and has a finite vapor pressure, gallium possesses low toxicity and effectively zero vapor pressure at room temperature, which makes it amenable to many applications. A variety of fabrication methods produce liquid metal particles with variable sizes, ranging from nm to mm (which is the upper limit set by the capillary length). The liquid nature of gallium enables fabrication methods-such as microfluidics and sonication-that are not possible with solid materials. Gallium-based liquid metal particles possess several notable attributes, including a metal-metal oxide (liquid-solid) core-shell structure as well as the ability to self-heal, merge, and change shape. They also have unusual phase behavior that depends on the size of the particles. The particles have no known commercial applications, but they show promise for drug delivery, soft electronics, microfluidics, catalysis, batteries, energy harvesting, and composites. Existing challenges and future opportunities are discussed herein.

Liquid Metal-Based Route for Synthesizing and Tuning Gas-Sensing Elements

There is a strong demand for developing tunable and facile routes for synthesizing gas-sensitive semiconducting compounds. The concept of synthesizing micro- and nanoparticles of metallic compounds in a tunable process, which relies on liquid metals, is presented here. This is a liquid-based ultrasonication procedure within which additional metallic elements (In, Sn, and Zn) are incorporated into liquid Ga that is sonicated in a secondary solvent. We investigate liquid metal sonication in dimethyl sulfoxide (DMSO) and water to show their impact on the size, morphology, and crystal structure of the particulated products. The synthesized materials are annealed to investigate their responses to model reducing (H₂) and oxidizing (NO₂) gas species. The preparation process in DMSO gives rise to predominantly monoclinic Ga₂O₃ crystals which are favorable for gas sensing, while the emergence of rhombohedral Ga₂O₃ phases from the water sonication process led to inactive samples. The ease of tunability without hazardous precursors during the synthesis procedure is demonstrated. The route presented here can be uniquely employed for designing and engineering on-demand functional materials for sensing applications.

Phase junction enhanced photocatalytic activity of Ga2O3 nanorod arrays on flexible glass fiber fabric

Ga₂O₃ nanostructures hold great potential applications in photocatalytic fields due to their stability, high efficiency and environmental friendliness. The construction of phase junction has been proved to be one of the most effective strategies for enhancing Ga₂O₃ photocatalytic activity. However, the influence of the formation process at the interface of the phase junction on the photocatalytic activity of Ga₂O₃ nanostructures is far less well understood. In this work, for the first time, large-area Ga₂O₃ nanorod arrays (NRAs) with controllable α/β phase junction were prepared in situ on a flexible glass fiber fabric by a facile and environmentally friendly three-step method. The α/β-Ga₂O₃ phase junction NRAs exhibit an ultra-high photocatalytic degradation rate of 97% during Ultraviolet (UV) irradiation for 60 min, which is attributed to a unique phase junction promoting efficient charge separation. However, the photocatalytic activity of α/β-Ga₂O₃ phase junction NRAs is not evident in the early phase transition, possibly due to the presence of defects acting as charge recombination centers.

Ga₂O₃ nanostructures hold great potential applications in photocatalytic fields due to their stability, high efficiency and environmental friendliness.

Ga-Based Liquid Metal Micro/Nanoparticles: Recent Advances and Applications

Liquid metals are emerging as fluidic inorganic materials in various research fields. Micro- and nanoparticles of Ga and its alloys have received particular attention in the last decade due to their non toxicity and accessibility in ambient conditions as well as their interesting chemical, physical, mechanical, and electrical properties. Unique features such as a fluidic nature and self-passivating oxide skin make Ga-based liquid metal particles (LMPs) distinguishable from conventional inorganic particles in the context of synthesis and applications. Here, recent advances in the bottom-up and top-down synthetic methods of Ga-based LMPs, their physicochemical properties, and their applications are summarized. Finally, the current status of the LMPs is highlighted and perspectives on future directions are also provided.

Crystal Chemistry, Band-Gap Red Shift, and Electrocatalytic Activity of Iron-Doped Gallium Oxide Ceramics

This work for the first time unfurls the fundamental mechanisms and sets the stage for an approach to derive electrocatalytic activity, which is otherwise not possible, in a traditionally known wide band-gap oxide material. Specifically, we report on the tunable optical properties, in terms of wide spectral selectivity and red-shifted band gap, and electrocatalytic behavior of iron (Fe)-doped gallium oxide (β-Ga₂O₃) model system. X-ray diffraction (XRD) studies of sintered Ga_2-x Fe _x O₃ (GFO) (0.0 ≤ x ≤ 0.3) compounds provide evidence for the Fe³⁺ substitution at Ga³⁺ site without any secondary phase formation. Rietveld refinement of XRD patterns reveals that the GFO compounds crystallize in monoclinic crystal symmetry with a C2/m space group. The electronic structure of the GFO compounds probed using X-ray photoelectron spectroscopy data reveals that at lower concentrations, Fe exhibits mixed chemical valence states (Fe³⁺, Fe²⁺), whereas single chemical valence state (Fe³⁺) is evident for higher Fe content (x = 0.20-0.30). The optical absorption spectra reveal a significant red shift in the optical band gap with Fe doping. The origin of the significant red shift even at low concentrations of Fe (x = 0.05) is attributed to the strong sp-d exchange interaction originated from the 3d⁵ electrons of Fe³⁺. The optical absorption edge observed at ≈450 nm with lower intensity is the characteristic of Fe-doped compounds associated with Fe³⁺-Fe³⁺ double-excitation process. Coupled with an optical band-gap red shift, electrocatalytic studies of GFO compounds reveal that, interestingly, Fe-doped Ga₂O₃ compound exhibits electrocatalytic activity in contrast to intrinsic Ga₂O₃. Fe-doped samples (GFO) demonstrated appreciable electrocatalytic activity toward the generation of H₂ through electrocatalytic water splitting. An onset potential and Tafel slope of GFO compounds include ∼900 mV, ∼210 mV dec^-1 (x = 0.15) and ∼1036 mV, ∼290 mV dec^-1 (x = 0.30), respectively. The electrocatalytic activity of Fe-doped Ga-oxide compounds is attributed to the cumulative effect of different mechanisms such as doping resulting in new catalytic centers, enhanced conductivity, and electron mobility. Hence, in this report, for the first time, we explored a new pathway; the electrocatalytic behavior of Fe-doped Ga₂O₃ resulted due to Fe chemical states and red shift in the optical band gap. The implications derived from this work may be applicable to a large class of compounds, and further options may be available to design functional materials for electrocatalytic energy production.

Galvanic replacement of liquid metal Galinstan with copper for the formation of photocatalytically active nanomaterials

Galvanic replacement of liquid metal Galinstan under mechanical agitation with copper creates a multi-elemental system that is photocatalytically active for the degradation of organic dyes where reuseability is achieved via immobilisation on a solid support.

Chameleon Metals: Autonomous Nano-Texturing and Composition Inversion on Liquid Metals Surfaces

Studies on passivating oxides on liquid metals are challenging, in part, due to plasticity, entropic, and technological limitations. In alloys, compositional complexity in the passivating oxide(s) and underlying metal interface exacerbates these challenges. This nanoscale complexity, however, offers an opportunity to engineer the surface of the liquid metal under felicitous choice of processing conditions. We inferred that difference in reactivity, coupled with inherent interface ordering, presages exploitable order and selectivity to autonomously present compositionally biased oxides on the surface of these metals. Besides compositional differences, sequential release of biased (enriched) components, via fractal-like paths, allows for patterned layered surface structures. We, therefore, present a simple thermal-oxidative compositional inversion (TOCI) method to introduce fractal-like structures on the surface of these metals in a controlled (tier, composition, and structure) manner by exploiting underlying stochastic fracturing process. Using a ternary alloy, a three-tiered (in structure and composition) surface structure is demonstrated.

Native Surface Oxides Featured Liquid Metals for Printable Self-Powered Photoelectrochemical Device

Constructing high-performance photo-electrodes by patterning the photo-active semiconducting components with desirable texture and architecture is one of the most promising approaches to achieve the practical and scale-up application of photo-electric or photoelectrochemical (PEC) devices. However, it is a still big challenge to efficiently and effectively handle nano-structural semiconducting materials into intergraded circuit devices, displaying good electric-contact and stability. Here, a facile manufacture strategy for fabricating native metal-oxides based photo-electrodes by directly printing Ga-based liquid metals is explored. The PEC device, functionalized by the native Ga-oxide functional layer, exhibits self-powered photo-detection behaviors and presents fast photo-electric responsibility in response to the simulated Sunlight illumination. This printable PEC device shows good potential for high sensitive self-powered photo-detector and provides a flexible and versatile approach for the design and fabrication of novel electrode structures.

Epitaxy of III-Nitrides on β-Ga2O3 and Its Vertical Structure LEDs

β-Ga₂O₃, characterized with high n-type conductivity, little lattice mismatch with III-Nitrides, high transparency (>80%) in blue, and UVA (400–320 nm) as well as UVB (320–280 nm) regions, has great potential as the substrate for vertical structure blue and especially ultra violet LEDs (light emitting diodes). Large efforts have been made to improve the quality of III-Nitrides epilayers on β-Ga₂O₃. Furthermore, the fabrication of vertical blue LEDs has been preliminarily realized with the best result that output power reaches to 4.82 W (under a current of 10 A) and internal quantum efficiency (IQE) exceeds 78% by different groups, respectively, while there is nearly no demonstration of UV-LEDs on β-Ga₂O₃. In this review, with the perspective from materials to devices, we first describe the basic properties, growth method, as well as doping of β-Ga₂O₃, then introduce in detail the progress in growth of GaN on (1 0 0) and (−2 0 1) β-Ga₂O₃, followed by the epitaxy of AlGaN on gallium oxide. Finally, the advances in fabrication and performance of vertical structure LED (VLED) are presented.

Inorganic Nanoshell-Stabilized Liquid Metal for Targeted Photonanomedicine in NIR-II Biowindow

Gallium and gallium-based alloys, typical types of liquid metals with unique physiochemical properties, are emerging as a next generation of functional materials in versatile biomedical applications. However, the exploration of their biomedical performance is currently insufficient, and their intrinsic low oxidative resistance is a key factor blocking their further clinical translation. Herein, we report on the surface engineering of liquid metal-based nanoplatforms by an inorganic silica nanoshell based on a novel but facile sonochemical synthesis for highly efficient, targeted, and near-infrared (NIR)-triggered photothermal tumor hyperthermia in the NIR-II biowindow. The inorganic silica-shell engineering of liquid metal significantly enhances the photothermal performance of the liquid metal core as reflected by enhanced NIR absorption, improved photothermal stability by oxidation protection, and abundant surface chemistry for surface-targeted engineering to achieve enhanced tumor accumulation. Systematic in vitro cell-level evaluation and in vivo tumor xenograft assessment demonstrate that (Arg-Gly-Asp) RGD-targeted and silica-coated nanoscale liquid metal substantially induces phototriggered cancer-cell death and photothermal tumor eradication, accompanied by high in vivo biocompatibility and easy excretion out of the body. This work provides the first paradigm for surface-inorganic engineering of liquid metal-based nanoplatforms for achieving multiple desirable therapeutic performances, especially for combating cancer.

Synthesis of LaFeO3/Ag2CO3 Nanocomposites for Photocatalytic Degradation of Rhodamine B and p-Chlorophenol under Natural Sunlight

Novel LaFeO3/Ag2CO3 nanocomposites are synthesized by co-precipitation method for photocatalytic degradation of Rhodamine B (RhB) and p-chlorophenol under visible light irradiation. Heterostructures between LaFeO3 and Ag2CO3 semiconductors are formed during the synthesis of these nanocomposites. Among the nanocomposites prepared with different ratios of LaFeO3 and Ag2CO3, 1% LaFeO3/Ag2CO3 shows the highest photocatalytic activity for the degradation of RhB. Maximum electron-hole pair decoupling efficiency is observed in 1% LaFeO3/Ag2CO3, which causes the greater activity of the heterostructure. Degradation efficiency of 99.5% for RhB and 59% for p-chlorophenol has been obtained under natural sunlight within 45 min. Interestingly, the stability of Ag2CO3 is improved dramatically after making nanocomposite, and no decomposition of the catalyst was observed even after several photocatalytic cycles. Reactive oxygen species scavenging experiments with p-benzoquinone, isopropyl alcohol, and ammonium oxalate suggest that a major degradation process is caused by holes. Degradation of RhB into small organic moieties is detected using LC-MS technique. Further, the efficient mineralization of the degradation products occurs during the catalytic process.

A novel n-type CdS nanorods/p-type LaFeO3 heterojunction nanocomposite with enhanced visible-light photocatalytic performance

In this work, a novel n-type CdS nanorods/p-type LaFeO₃ (CdS NRs/LFO) nanocomposite was prepared, for the first time, via a facile solvothermal method. The as-prepared n-CdS NRs/p-LFO nanocomposite was characterized by using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDX), UV-visible diffuse reflection spectroscopy (DRS), vibrating sample magnetometry (VSM), photoluminescence (PL) spectroscopy, and Brunauer–Emmett–Teller (BET) surface area analysis. All data revealed the attachment of the LFO nanoparticle on the surface of CdS NRs. This novel nanocomposite was applied as a novel visible light photocatalyst for the degradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) dyes under visible-light irradiation. Under optimized conditions, the degradation efficiency was 97.5% for MB, 80% for RhB and 85% for MO in the presence of H₂O₂ and over CdS NRs/LFO nanocomposite. The photocatalytic activity of CdS NRs/LFO was almost 16 and 8 times as high as those of the pristine CdS NRs and pure LFO, respectively. The photocatalytic activity was enhanced mainly due to the high efficiency in separation of electron–hole pairs induced by the remarkable synergistic effects of CdS and LFO semiconductors. After the photocatalytic reaction, the nanocomposite can be easily separated from the reaction solution and reused several times without loss of its photocatalytic activity. Trapping experiments indicated that ·OH radicals were the main reactive species for dye degradation in the present photocatalytic system. On the basis of the experimental results and estimated energy band positions, the mechanism for the enhanced photocatalytic activity was proposed.

A novel n–p CdS nanorods/LaFeO₃ (CdS NRs/LFO) heterojunction nanocomposite was prepared via a solvothermal route and applied as a visible-light photocatalyst for enhanced degradation of organic dye pollutants.

Room Temperature Electrochemical Synthesis of Crystalline GaOOH Nanoparticles from Expanding Liquid Metals

Gallium oxyhydroxide (GaOOH) is a wide band gap semiconductor of interest for a variety of applications in electronics and catalysis where the synthesis of the crystalline form is usually achieved via hydrothermal routes. Here we synthesize GaOOH via the electrochemical oxidation of gallium based liquid metals in solutions of 0.1 M NaNO₃ electrolyte with pH adjusted over the range of 7-8.4 with NaOH. This electrochemical approach employed under ambient conditions results in the formation of crystalline oblong shaped α-GaOOH nanoparticles from both liquid gallium and liquid galinstan which is a eutectic based on Ga, In, and Sn. The size and shape of the GaOOH particles could be controlled by the solution pH. The product is characterized with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-visible spectroscopy, and photoluminescence spectroscopy. During the electrochemical oxidation process, the liquid metal drop was found to expand significantly in the case of galinstan due to a constant electrowetting effect which resulted in the continuous expulsion of nanomaterial from the expanding liquid metal droplet. This electrochemical approach may be applicable to other liquid metals for the fabrication of metal oxide nanomaterials and also demonstrates that significant chemical reactions may be occurring at the surface of liquid metals that are actuated under an applied electric field in aqueous electrolytes.

Liquid metals: fundamentals and applications in chemistry

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.