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Room temperature electrochemical synthesis of crystalline GaOOH nanoparticles from expanding liquid metals Benchaporn Lertanantawong, James D. Riches, and Anthony Peter O'Mullane Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00538 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Room temperature electrochemical synthesis of crystalline GaOOH nanoparticles from expanding liquid metals Benchaporn Lertanantawong1, Jamie Riches2,3 and Anthony P. O’Mullane*2 1

Nanoscience and Nanotechnology Graduate Program, King Mongkut’s University of Technology Thonburi, Bangkok 10150, Thailand

2

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia

3

Institute for Future Environments, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia

ABSTRACT

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 synthesise 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 to 8.4 with NaOH. This electrochemical approach employed under ambient conditions

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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.

INTRODUCTION

Room temperature liquid metals have received a significant amount of attention given their interesting bulk and surface properties. Although mercury has been investigated in detail, gallium based liquid metals as non-toxic alternatives1 have seen an upsurge in the number of studies focused on their inherent chemical and physical properties as well as numerous applications. Depending on the composition, i.e. pure Ga or mixtures with In (GaIn) or In and Sn (galinstan, alloy of 68.5% Ga, 21.5% In and 10.0% Sn) the melting point of the liquid metal can be determined. Applications of liquid metals are quite varied2-6 and have included plasmonics,7-8 use as coolants,9 reconfigurable electronics,10 soft electronics,11 acceleration sensors,12 fluidic antennas,13 self-fueled motors14 and oscillators,15 heavy metal ion sensors,16-17 gas sensors18 and


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as catalysts.19-20 An interesting effect is the ability to actuate liquid metals under an electric field applied between two electrodes in aqueous electrolytes which has led to applications in pumps with no moving parts,21 generating chaotic advection,22 miniaturized vehicle propulsion,23 or using ionic concentration gradients in solution for self-propulsion.24-25 In all of these cases the liquid metal experiences an oxidising potential on one pole which affects its surface chemistry and contributes to the movement of the liquid metal. These applications are certainly intriguing; however a new field is emerging where liquid metals can be used as a reaction environment/solvent to create other materials. Examples of this include the galvanic replacement at the surface of liquid galinstan to create gold and silver nanomaterials,26 generation of 2D oxides via expulsion of HfO2 from an alloy of galinstan and Hf into an aqueous solution whereby sheets of atomically thin HfO2 are liberated,27 reactive substrate during chemical vapour deposition to produce layered chalcogenides,28 and the dissolution of the Ga component from Galinstan to yield InSn nanoparticles.29 In all of these cases the liquid metal is playing a significant role in the fabrication of these nanomaterials. A key question in all of these processes is the competing surface oxidation of reactive gallium. Therefore an electrochemical study of the oxidation of liquid metals based on gallium is of interest. Oxidised gallium can exist in forms such as Ga2O3 and GaOOH which are semiconductors that have been used in applications such as photocatalysis.19-20,

30-32

Gallium

oxyhydroxide (GaOOH) is a wide band gap material and has been investigated by several groups and shown to be active in the UV region for the degradation of dyes such as methylene blue.30 In addition to photocatalysis, GaOOH nanoparticles functionalized with β-cyclodextrin have shown biological cellular uptake into cancerous HeLa cells with lethal effect. The possibility of surface


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functionalization with a short lived radiotracer such as 68Ga may also allow for positron emission tomography of cancer cells.33 GaOOH/Matrigel composites have been used to modulate the behavior of cells on surfaces with variable stiffness containing a radiosensitizing material.34 Such studies allow one to understand how to enhance the biological response on a matrix with variable stiffness under therapeutically relevant conditions such as X-ray radiation. Gallium compounds have shown photocatalytic activity, bioavailability and Ga3+ ions are antiproliferative to pathologically proliferating cells such as cancer cells. In addition, the presence of scintillator materials such as GaOOH can trigger different cellular responses. All of these properties therefore makes the fabrication of GaOOH nanoparticles with well-defined morphology of significant interest. There have been many approaches used to create GaOOH nanoparticles including wet chemical synthesis,35 sonochemical hydrolysis,36 hydrothermal,37 microwave,31 continuous flow cell reactors,38 and laser ablation.39 Another reason for creating GaOOH nanomaterials is as a precursor for the large scale synthesis of crystalline Ga2O3 with defined architecture via a thermal treatment process. Interestingly an electrochemical approach has not been used to create gallium oxide based nanomaterials which is slightly surprising given the ease with which the oxidation of metals can be undertaken using this method. In this work we explore both Ga and galinstan liquid metals using chronoamperometry to induce the oxidation of the surface in aqueous solution at room temperature. We demonstrate that electrochemical oxidation of Ga and galinstan results in the expansion of the liquid metals and the same product being expelled into the electrolyte, namely nanoparticles of crystalline GaOOH with a highly layered morphology.


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EXPERIMENTAL SECTION Materials Liquid Gallium (Sigma Aldrich), Galinstan (Galinstan fluid 4 N, Geratherm Medical AG, Germany) and Sodium Nitrate (Aldrich) were used as received. All aqueous solutions were prepared using deionized water (resistivity of 18.2 MΩ.cm at 25°C) purified by use of a Milli-Q reagent deionizer (Millipore). GaOOH synthesis Electrochemical oxidation process were carried out at (25 ± 2)°C using a Metrohm (Autolab PGSTAT128N, EcoChemie, Netherlands) electrochemical analyser. A three-electrode setup was used for the electrochemical oxidation of the liquid metal, where a hanging gallium drop from a 10 mL syringe and needle filled with liquid metal was used as the working electrode, while an Ag/AgCl electrode (3 M KCl) and a platinum coil were used as reference and auxiliary electrodes, respectively (Scheme 1). All potentials were converted to the RHE scale via ERHE = EAg/AgCl + 0.059 x pH + 0.197 V. The electrochemical synthesis of GaOOH particles was performed by applying potentials ranging from 1.35 to 2.6 V vs RHE for 600 seconds in 10 mL of 0.1 M sodium nitrate (pH 7) at (30 ± 2)°C until GaOOH material was ejected from the hanging Ga or galinstan drop. The pH of the 0.1 M NaNO3 was adjusted to pH 8.4 upon the addition of NaOH. In addition, a solution of 0.5 M NaOH (pH 13.4) was also investigated for the oxidation of liquid galinstan. The solution pH was measured with a Denver Instruments UB 10 pH meter. The reaction solution was then centrifuged at 5000 rpm to collect the colloidal GaOOH particles. The sediment was then washed three times with deionized water and kept dry at room temperature.


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Characterization Material characterization was performed using a field-emission scanning electron microscopy (FESEM, Zeiss Sigma VP field emission scanning electron microscope equipped with an Oxford XMax 50 Silicon Drift energy-dispersive X-ray detector at 20 kV under high vacuum), and Xray diffraction (XRD, BRUKER AXS D8 Discover operating at 40 kV and 40 mA by using CuKa radiation with Goebel Mirror (for parallel beam)). TEM and STEM images were taken at an accelerating voltage of 200 KV using a JEOL 2100 instrument equipped with a highsensitivity silicon drift X-ray detector and a Gatan Orius SC1000 CCD camera. UV-visible spectra were obtained using spectroscopy an Agilent Cary® 50 UV- and Visible spectrophotometer. Photoluminescence was carried out at room temperature with a F-2500 Fluorescence Spectrophotometer (Hitachi - Science & Technology, UK). Prior to SEM imaging, samples were thoroughly rinsed with Milli-Q water and dried under a flow of nitrogen. The electrolyte was also characterized with inductively coupled plasma optical emission spectrometry (ICP-OES) (Horiba, JY2000, Japan) after oxidation of the liquid galinstan electrode.

Scheme 1. Schematic diagram of the electrochemical synthesis of GaOOH.


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RESULTS AND DISCUSSION A hanging drop electrode configuration (Scheme 1) was used for the electrochemical oxidation of the liquid metal. The temperature was maintained at (30 ± 2)°C for these experiments, in particular when using Ga to retain its liquid state during the experiment. This was not required for galinstan, which has a melting point of -19°C, but for comparison purposes the temperature was kept constant in all cases. Figure 1a shows linear sweep voltammograms (LSVs) recorded at Ga and Galinstan in 0.1 M NaNO3. A similar response is recorded for both materials with an onset in oxidation current at ca. 1 V followed by a sharper increase in current at 1.7 V. This response is typical of an electrodissolution process and does not show any indication of surface passivation that would cause the current to decrease. The slight difference in the LSV response for Galinstan may be due to the oxidation of the additional metals in the alloy, namely In and Sn.

Figure 1. Cyclic voltammograms recorded at a scan rate of 50 mV s-1 Gallium (red line) and Galinstan (black line) (a). Current time responses for a Ga (b) and Galinstan (c) liquid droplet recorded at 1.35 V (black line), 1.6 V (red line) and 2.1 V (blue line) for 600 s.


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Figure 1b shows the current time response for a Ga liquid droplet recorded at applied potentials from 1.35 to 2.1 V for 600 s. In all cases there is a slight increase in current for the first 60 s after which the response remains stable. This is in contrast to the case for Galinstan which shows a continuous increase in current which becomes more pronounced as the potential is made more positive. To understand the source of this behavior, videos were recorded during the course of the constant potential experiments. At 1.35 V it can be seen from the captured images at every 2 minutes (Figure 2) that there is a remarkable difference in the morphology of the liquid metal droplets. For Ga there is an increase in size after 2 minutes which would explain the increased current over the first minute (Figure 1b). The surface is also discoloured with a black material. With time the size of the droplet does not increase and the surface shows a similar level of coverage with material. However in the case of Galinstan the morphology of the droplet changes dramatically. After 2 minutes the liquid drop becomes elongated and also covered with a black material. With increasing time however the droplet continues to expand towards the bottom of the electrochemical cell and to generate a tear shaped metal droplet. This continuous increase in the size of the liquid drop explains the gradual increase in current seen in Figure 1c due to the increased surface area.


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Figure 2 Digital images of Ga (top row) and galinstan (bottom row) at a constant potential of 1.35 V from 0 to 10 minutes (image every two minutes is shown).

When the potential is increased to 2.1 V the same trends can be observed, however in this case the amount of material formed on the surface of the liquid metals is extensive and begins to fall from the surface of the droplets (Figure 3). For Ga, a thick layer of material is formed which results in large flakes of material falling from the drop into the electrochemical cell. For galinstan it is observed that the oxidation product becomes confined to the lower end of the expanding drop and a steady stream of fine material is continuously expelled from the base of the drop. Under these conditions the liquid metal drop almost reaches the bottom of the cell. In both cases the product begins to get ejected into solution at 4 minutes. At an applied potential of 2.6 V the amount of material generated after 10 minutes is extensive which is shown in Figure S1 and was therefore chosen for characterization purposes. At 2.6 V there was some evolution of oxygen from the electrode, as well as hydrogen production from the counter electrode, however the formation of the oxide layer severely impeded the water oxidation process.


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Figure 3 Digital images of Ga (top row) and galinstan (bottom row) at a constant potential of 2.1 V from 0 to 10 minutes (image every two minutes is shown).

Previous work on the actuation of liquid galinstan drops in a microchannel between two electrodes in NaOH solution showed that at applied potentials of 30 V the liquid metal drop became elongated with the formation of a grey precipitate at one end.40 With time the liquid metal was propelled in the direction away from the precipitate and pristine liquid metal was seen at the leading front. The movement of liquid metals in a channel under bipolar electrochemical conditions is determined by surface tension induced forces and friction between the liquid metal and the supporting electrolyte as well as the bottom of the channel.40-41 The surface tension induced forces are due to a redistribution of charges in the electrochemical double layer on the liquid metal as well chemical reactions that create an oxide on the surface. In the experimental setup in this work, direct electrical contact is made to the liquid metal drop rather than the liquid metal experiencing the influence of an applied electric field between two electrodes. Also frictional forces with an underlying material are absent and the only frictional forces are those between the liquid metal and the surrounding electrolyte. Here the liquid metals under potential


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control expand with time, which is more evident in the case of galinstan. The expansion of the liquid metal can be attributed to differences in the surface tension across the liquid metal. In the case of galinstan a precipitate accumulates at the bottom of the droplet which results in a large surface tension gradient. It has been reported previously that the formation of oxide on a gallium based liquid metal dramatically decreases the surface tension which results in a pronounced nonspherical shape.42 Therefore in response to the applied potential in this work and the continual formation of an oxide the liquid metal becomes elongated as seen previously for the actuation of a liquid metal in a microchannel.40 As discussed in previous reports a constant electrowetting effect propels the liquid metal.21,

40

This is also likely to be the case here for Galinstan and

explains the plume of material ejected from the bottom of the droplet. This process becomes more pronounced at more positive voltages. In the case of pure Ga a precipitate is formed over the entire surface of the drop and expansion is minimized while also being more uniform when compared to galinstan. This is consistent with Liu’s work where it was reported that extensive oxide formation on liquid gallium prevents the deformation process from occurring.42 The precipitate also flakes off randomly from most areas of the droplet due to poor attachment rather than expulsion as a fine powder from the bottom of the droplet. It is also unlikely that gravity explains the elongation of galinstan as a similar effect should be observed for Ga. An interesting effect then occurred when collecting the sample after the experiment. Once the precipitate was allowed to settle in the electrochemical cell it was found that it turned white after ca. 3 h (for sample shown in Figure S1). This time was dependent on the amount of material generated, whereby smaller amounts required less time. The centrifuged and washed dispersion was then dropcast onto a substrate and imaged with SEM where an example of the product from Ga oxidation is shown in Figure 4. A dense layer can be seen in Figure 4a but


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consists of individual particles that are not coalesced together as observed more readily in Figure 4b. They have an oblong shape ca. 500 nm in length with a width of ca. 350 nm where each particle consists of layers of nanoplates (Figure 4c and d). STEM imaging was then undertaken (Figure 5) where the layered morphology can be seen more clearly. EDS mapping revealed the presence of Ga and oxygen in the sample indicating the formation of an oxide of gallium. To identify the material, X-ray diffraction was undertaken where the diffraction pattern is shown in Figure 6. The peaks can be indexed to the orthorhombic α-GaOOH phase.35, 43

Figure 4. SEM images showing the morphology of samples synthesized by electrochemical oxidation of gallium at pH 7.

Figure 5 STEM image and EDS mapping of electrochemically synthesized GaOOH from Ga.


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The product of the electrochemical oxidation of galinstan was also investigated to see if it was comparable to that of Ga or whether the presence of In and Sn in the alloy affected the composition of the product. From the X-ray diffraction data (Figure 6a) there is no discernible difference between the materials and indicates that In and Sn do not influence the formation of αGaOOH. This is consistent with the LSV data (Figure 1) which shows a very similar electrochemical oxidation process and therefore the difference in behavior is probably due to the increasing surface area of the drop during the course of the experiment. The optical characteristics of GaOOH produced from both Ga and Galinstan were also investigated. The UVvisible absorption spectra for both samples is shown in Figure 6b where it can be seen that the same absorption profile was attained but with slightly higher absorption across the wavelength range of interest for the galinstan sample. There is strong absorption in the UV region which is consistent with previous reports on this wide band gap material.30-31 The room temperature photoluminescence spectra for both samples are shown in Figure S2 where an emission peak can be seen at 300 nm. The intensity of the peak is slightly higher from the sample obtained via electrochemical oxidation of galinstan. The emission in the UV region is consistent with previous work on GaOOH nanoparticles modified with cyclodextrin.33 The generation of emission in the UV region has been attributed by Yeh to a self-trapped exciton from a detrapped electron emitting a UV photon.39


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Figure 6. (a) XRD patterns and (b) UV-visible absorption spectra of GaOOH synthesized from Ga (Red line) and galinstan (black line). Also illustrated are the peak positions for JCPDS card 00-054-0910.

TEM imaging showed that particles produced via the electrochemical oxidation of galinstan are also oblong shaped (Figure 7a). At higher resolution a layered structure (Figure 7b) is also evident as seen for GaOOH produced via the electrochemical oxidation of Ga. At higher resolution at the edge of the particle some lattice planes can be observed (Figure 7c) which is supported by the fast Fourier transform (FFT) image shown in the inset. The electron diffraction pattern (Figure 7d) is also consistent with the formation of GaOOH (04-010-9861). It should be noted that on parts of the TEM grid an additional material was found to surround some of the GaOOH particles (Figure 7a). This material was found to be rich in In and Sn with also oxygen present and indicates the formation of some oxides of In and Sn as seen in the EDS mapping experiment (Figure S3). In addition the electrolyte was analysed for dissolved In and Sn ions where their presence was confirmed by ICP-OES (Figure S4). Therefore, in the case of galinstan all three elements are electrochemically oxidised in this process, however In and Sn are not


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incorporated in the oxidised gallium product which is the same as that produced for pure gallium. This can also explain why the drop remains liquid, apart from the oxidised product at the surface, as all three elements are being ejected from the liquid metal thereby not creating an enriched In/Sn material that would not be liquid.29 Even though In and Sn products were observed in the TEM it is clear that there is preferential formation of GaOOH which is not unexpected given the larger amount of Ga present in galinstan. However it does illustrate an interesting phenomenon when studying oxidation processes in liquid metals. Recent work has shown that Hf incorporated into galinstan shows preferential formation of HfO2 at the surface (through oxidation with air) rather than an oxide of gallium due the thermodynamics associated with oxide formation, whereby the formation of HfO2 is thermodynamically more feasible than that for gallium oxide.27 In our work the thermodynamic driving force for gallium oxide formation is significantly greater than that for In and Sn and shows that this environment is an interesting medium for generating metal oxide materials. Measuring the particle sizes from TEM gave values for GaOOH produced from Ga of major axis: 597 ± 106 nm, minor axis: 429 ± 74 nm and from Galinstan, major axis: 518 ± 157 nm, minor axis: 251 ± 99 nm.


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Figure 7 Bright field TEM images of GaOOH particles derived from Galinstan show the particle morphology (a and b) with a higher resolution image at the edge of a single particle (c) (inset is a FFT pattern, and a selected-area electron diffraction (SAED) pattern in (d).

The electrochemical formation of GaOOH most likely proceeds via the same mechanism as the chemical synthesis of GaOOH whereby Ga(OH)3 is formed as a precursor to GaOOH. Once Ga is electrochemically oxidized to the stable Ga3+ form it will react with hydroxide ions to form Ga(OH)3. At pH 7 it has been reported that Ga3+ ions react with OH- ions to form amorphous Ga(OH)3.43 Previous work has shown that the conversion of Ga(OH)3 into GaOOH is fast35 where the overall process can be described as follows:

Ga3+ + 3OH- → Ga(OH)3

(1)

Ga(OH)3 → GaOOH + H2O (2)


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Therefore in principle the formation of GaOOH may occur on the surface of the liquid metal or in solution if the Ga(OH)3 form is detached from the electrode surface as a source of oxidizing potential is no longer required for this conversion process. It should be noted that if a drop of Ga is left in 0.1 M NaNO3 overnight then the formation of a precipitate was not observed, indicating that the electrochemical oxidation of Ga into Ga3+ is required. In the literature it has been reported that hydrothermal conditions are required for the formation of crystalline GaOOH30-31,

44-45

whereas here under electrochemical conditions a

crystalline product is attained at room temperature under ambient conditions. A wet chemical route, performed at 60°C for 18 h, has been reported to produce similar oblong shaped GaOOH particles to the ones shown here (Figure 4). However to ensure the formation of elongated crystals ammonia was required with a controlled pH range to direct the growth process. The presence of OH- ions plays a key role in determining the morphology of GaOOH crystals. The (0001) facet preferentially absorbs OH- ions due to differences in the surface energies of the GaOOH facets.46 Particles then grow along the (001) orientation to produce elongated nanostructures. The pH of the electrolyte was then changed to investigate if this affected the type of GaOOH produced using this approach. When the pH was changed to 8.4 and the applied potential was kept at 2.6 V vs RHE for 10 min a similar phenomenon was observed at the galinstan drop electrode in that material was continuously expelled from the liquid drop. SEM images of the resultant GaOOH material that was collected are shown in Figure 8. In contrast to the previous conditions of pH 7 this increase in electrolyte pH resulted in nanomaterials that were significantly more elongated with a higher aspect ratio with lengths of ca. 500 nm and a width of ca. 200 nm which is significantly different to the structures seen in Figure 4. However there is some consistency between the samples in that they also comprise of layers of plate like


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structures which is a characteristic feature of this material and how it assembles. Previous work on the chemical synthesis of GaOOH also indicated that pH played a key role in determining the shape of the GaOOH material with higher pH solutions promoting the growth of elongated structures.35 However it should be noted that other studies on GaOOH formation using chemical routes indicated that if much higher pH conditions were used then each facet has a similar probability of absorbing OH- ions which results in more isotropic growth and not preferential growth along the (001) orientation.45 To test this under electrochemical conditions the solution pH was increased to 13 and the same electrochemical oxidation procedure was undertaken.

Figure 8 SEM images showing the morphology of samples synthesized by electrochemical oxidation of gallium at pH 8.4.

However, a very interesting effect occurred once a positive potential was applied to the hanging galinstan drop electrode in a solution of pH 13. Immediately upon application of the potential the liquid metal was ejected from the bottom of the syringe into the electrochemical cell. To try and drive the oxidation of the liquid metal contact was made to the drop at the bottom of the cell and the potential was applied. This resulted in immediate deformation of the spherical


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liquid metal which flattened out to cover the bottom of the cell (Figure 9). When the applied potential was removed the liquid metal recovered its shape back to a spherical droplet in the bottom of the cell due to an increase in the surface tension of the liquid metal. However there was no evidence of extensive oxide formation on the surface of the liquid metal. This can be related to the solution pH as under conditions of pH 13, Ga(OH)3 is highly soluble and readily forms gallates (Ga(OH)4-) which are soluble.42 Liu et al also reported that in solutions of high pH that application of voltages > 10 V to a liquid gallium drop resulted in extensive deformation of the liquid metal which recovered once the potential was removed.42 The key difference compared to the previous cases at lower pH studied here is that once the oxide is formed a surface tension gradient is introduced which causes deformation of the liquid. However this oxide is dissolved in pH 13 and therefore the process continues rather than slows down. In our case at lower pH values the rapid buildup of oxide material allows for some deformation to occur but then becomes limited as the GaOOH product is not soluble and therefore the deformation process is inhibited and ultimately ceases. Interestingly for pure Ga the buildup of oxide was more extensive compared to galinstan and therefore prevented the deformation process to a greater extent.


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Figure 9 Digital images of galinstan at a constant potential of 2.1 V from 0 to 5 seconds (image every 0.5 seconds is shown).

This work shows an interesting example where crystallisation of the product is not confined exclusively to the working electrode surface, i.e. the product is continuously ejected from the surface of the liquid metal due to continuous electrowetting as discussed previously. It also differs from our previous work on the galvanic replacement of liquid galinstan with gold and silver nanomaterials which were confined to the surface.26 However in the galvanic replacement case the oxidation of gallium under those conditions was much milder (standard reduction potential of the Ag/Ag+ couple for example is 0.799 V vs SHE) which is a significantly lower driving force for Ga oxidation than the potential bias applied to the liquid metal in this work. These milder conditions (which required at least 24 h for noticeable precipitation to occur) allows the silver to nucleate and grow outwards from the surface while remaining attached to the liquid metal. The rapid and continuous generation of Ga3+ ions under electrochemical control, in


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addition to the constant electrowetting effect, in particular in the case of galinstan, does not provide such a situation and therefore the product does not remain attached to the electrode.

CONCLUSION The electrochemical oxidation of liquid metals based on Ga and Galinstan result in the formation of crystalline α-GaOOH whose size and shape are dependent on the electrolyte pH. In both cases the product is crystalline with an oblong shape and can be achieved under ambient conditions at room temperature which is advantageous compared to hydrothermal routes that require elevated temperature and/or more reaction time. The presence of In and Sn in the galinstan eutectic did not influence the final GaOOH product. Interestingly during the electrochemical experiment the liquid metal expanded once a potential was applied which became more pronounced at higher applied voltages, in particular in the case of galinstan. The oxidation of the surface results in a change in the surface tension along the droplet which results in movement of the liquid metal. Under these continuous electrowetting conditions for galinstan a continuous plume of nanomaterial is ejected from the base of the expanding droplet. This approach may be interesting to apply to other liquid metal systems for the fabrication of other metal oxide nanomaterials. ASSOCIATED CONTENT Supporting Information TEM images, EDS mapping of materials produced via the electrochemical oxidation of Galinstan and ICP-AES data obtained from the electrolyte. This material is available free of charge via the Internet at http://pubs.acs.org.


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Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The SEM, TEM and XRD data reported in this paper were obtained at the Central Analytical Research Facility operated by the Institute for Future Environments (QUT). Access to CARF is supported by generous funding from the Science and Engineering Faculty. AOM gratefully acknowledges

funding

from

the

Australian

Research

Council

(DP170102138).

BL

acknowledges funding through the Thailand Research Fund (Grant number: RSA6080050) and International Strategic Output & Outcome, KMUTT for research travel support. REFERENCES (1)

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