Jessica Crawfordab,
Aidan Cowmanb and
Anthony P. O'Mullane*ab
aSchool of Chemistry and Physics, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia. E-mail: anthony.omullane@qut.edu.au
bCentre for Materials Science, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia
First published on 11th August 2020
Room temperature liquid metals based on Ga can be used as a synthesis medium for the creation of metal oxide nanomaterials, however one thermodynamic limitation is that metals that are more easily oxidised than Ga are required to ensure their preferential formation. In this work we demonstrate a proof of principle approach whereby exposing the liquid metal alloyed with the required metal to acidic conditions circumvents preferential formation of Ga2O3 and allows for the formation of the required 2D transition metal oxide nanosheets. The synthesis procedure is straightforward in that it only requires bubbling oxygen gas through the liquid metal alloy into a solution of 10 mM HCl. We show that the formation of thin nanosheets of ca. 1 nm in thickness of CoO is possible. The material is characterised using transmission electron microscopy, atomic force microscopy, X-ray photoelectron and Raman spectroscopy. The electrocatalytic activity of the CoO nanosheets was investigated for the oxygen evolution reaction where the nanosheet thickness was found to be a factor influencing the activity. This proof of principle offers a route to the possible formation of many other 2D transition metal oxides from metals that are less readily oxidised than Ga by taking advantage of the interesting properties of room temperature liquid metals.
Nanostructured transition metal oxides have shown excellent activity for many electrocatalytic reactions including those associated with water splitting, i.e. the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The OER has received particular attention given the sluggish kinetics of the process which limits overall performance for electrochemical water splitting. Many transition metal oxides based on Fe, Co, Ni and Mn have been studied for alkaline electrolysis and several comprehensive reviews are available in this area.22–26 The realisation of ultrathin nanosheets of such oxides is attractive for exposing edge sites and increasing the amount of active sites available for the reaction which is highly important in the development of new electrolysers for the generation of green hydrogen. At present there is an urgent need to replace currently used expensive precious metal oxide electrocatalysts and promisingly previous work has shown that cobalt oxide/hydroxide materials in particular show activity for the sluggish OER.27–32
In this work we extend the applicability of nanosheet formation from Ga based liquid metals by producing 2D nanosheets of a material that is unfavoured, namely oxidised Co (ΔGf for CoO is −214.2 kJ mol−1 which is significantly higher than that for Ga2O3), via control of the solution pH into which the material is ejected. This material is then investigated for the OER under alkaline conditions.
ERHE = EAg/AgCl + 0.059 × pH + E0Ag/AgCl V. |
Samples were imaged using a Zeiss Sigma Scanning Electron Microscope. A JEOL 2100 transmission electron microscope was also used where the material was drop cast onto a lacey carbon copper TEM grid. Images were taken at 200 kV using diffraction mode with a selected area aperture to take diffraction patterns. A Bruker Atomic Force Microscope (AFM) using ScanAsyst in air method in contact mode was used to measure the thickness of samples. Gwyddion AFM analysis software was used to treat images and analyse all thicknesses. For every thickness measurement the mean from the low area and high area was taken and the difference calculated and rounded to 1 decimal point.
Fig. 1 The gas injection method showing Co–GaInSn in water forming gallium oxide and Co–GaInSn in hydrochloric acid forming cobalt oxide nanosheets. |
Our initial experiments were conducted with MilliQ water as the solvent using 0.5 wt% Co in Galinstan. As anticipated we did not observe the formation of any Co containing species but saw an abundance of ellipsoid shaped particles generated in water when imaged by SEM (Fig. 2a) which are consistent with the formation of Ga oxides.33–35 It is known that gallium oxides are soluble in acidic solution36 and therefore the pH of the solution was controlled via the addition of HCl at various concentrations to minimise its formation. When the solution was acidified using 10 mM HCl there was a clear change in the morphology of the material ejected from the liquid metal (Fig. 2b and c). The formation of thin nanosheets can be observed (Fig. 2b) with some areas showing multiple sheets on top of each other or folded sheets (Fig. 2c). It should be noted that the sample was allowed to settle prior to analysis to remove any residual droplets that may have formed in the supernatant.
Electron diffraction measurements indicated that the material is amorphous (Fig. 2b inset). The individual sheets can be up to 500 nm in length whereas the folded sheets are larger. The thickness of the sheets was measured by atomic force microscopy (AFM) where an individual sheet is shown in Fig. 2d. The thickness of 50 different sheets was then measured and the histogram showing the distribution in thickness values (Fig. 2e) indicates that the sheets show that a high proportion are ca. 1 nm thick, indicating 2D nanosheet formation, with very few sheets as thick as 4.5 nm. The latter observation is due to the presence of sheets on top of each other or folded sheets as seen in the TEM images (Fig. 2c).
Raman spectroscopy was then undertaken and a typical Raman spectrum of the 2D nanosheets is shown in Fig. 3a. The peaks at (195.7, 619.7), 484.3 and 691.8 cm−1 can be assigned to the F2g, Eg and A1g active modes of CoO respectively.37 The intense sharp peaks at 516.1 and 526.6 cm−1 are due to the underlying Si substrate that was used for AFM imaging purposes. X-ray photoelectron spectroscopy (XPS) analysis confirmed the formation of CoO. The Co 2p spectrum (Fig. 3b) can be fitted to the Co2+ oxidation state while the spin orbit splitting of 15.5 eV is also indicative of Co2+.38 The O 1s spectrum shows a peak at 531.8 eV which is generally consistent with the presence of hydroxide groups,39 however there have been many reports which show that CoO nanomaterials exhibit a O 1s peak at a higher binding energy of 531.2 eV.40,41 In particular it has been reported that this higher binding energy peak for CoO compared to that normally seen for lattice oxygen is due to a large number of defects which is encountered in very thin films.40 It should also be noted that there was no evidence of gallium in the sample when analysed using XPS.
Fig. 3 (a) Raman spectrum, XPS spectra of nanosheets (b) Co 2p and (c) O 1s fabricated from 0.5% Co–GaInSn exposed to 10 mM HCl. |
Therefore the use of 10 mM HCl was successful in minimising the formation of gallium oxide while allowing for the oxidation of metallic Co to occur which resulted in the generation of 2D nanosheets of CoO with a thickness of ca. 1 nm that contain a significant number of defects and is denoted as CoOx. This value is consistent with the ultrathin nature of nanosheets that can be produced using a similar approach from metals alloyed with galinstan reported previously that included the much more reactive metals of Gd, Hf and Al which all produced nanosheets less than 1.1 nm in thickness,21 and Ti with nanosheets of 2 nm thickness.42 Even though thermodynamically the oxidation of Ga is preferred, as ΔGf of CoO is much higher at −214.2 kJ mol−1, the presence of 10 mM HCl results in the dissolution of any gallium oxides that are formed which subsequently promotes CoOx formation. The conditions are also appropriate for the existence of CoOx in this solution as previous work has shown that CoO only begins to dissolve to a significant extent at concentration of 0.5 M HCl.43 Therefore the mechanism of formation is due to oxygen bubbles travelling rapidly through the liquid metal causing rapid interfacial oxidation at the air/liquid metal/electrolyte interface due to the Cabrera–Mott oxidation process which limits the growth of the oxide layer at the Co–GaInSn surface to the several Å to nm scale. The nanosheets are then ejected into solution where any Ga2O3 that would have formed dissolves allowing for the accumulation of CoOx nanosheets in solution. The concentration of HCl is enough to dissolve any Ga2O3 but allows for CoOx to exist stably in solution. This will result in some consumption of the Ga component, however the liberated Ga3+ ions that are generated can be in principle be recovered via electrochemical reduction to Ga.44 An advantage of this approach is that it can be done in a single step in a relatively straightforward manner. Other approaches to creating isolated nanosheets of 2D metal oxides require layered materials to be initially synthesised followed by various exfoliation methods which can produce polydispersity in sheet thickness. Another report has shown that indium tin oxide nanosheets can be printed from a liquid InSn alloy held at 200 °C via sandwiching the alloy between two substrates which when removed results in ITO nanosheets deposited on the top substrate.45 The method presented here produces a high proportion of very thin nanosheets in a relatively straightforward process at room temperature.
Electrochemical water splitting in alkaline conditions using various types of cobalt oxides/hydroxides, have been studied previously at a variety of morphologies, including nanosheets, however they have been significantly thicker than reported here at around 6 monolayers per sheet for Co(OH)2.31 Therefore the 2D CoOx nanosheets fabricated here were studied for the OER in 0.1 M NaOH. As mentioned previously the effect of the pH of the solution into which the material is ejected is expected to play a critical role. Therefore 1 mM, 10 mM and 50 mM HCl solutions were used to synthesise CoOx which was then investigated for the OER (Fig. 4a). From this data the material synthesised from 10 mM HCl shows the best performance in terms of current density where values as high as 70 mA cm−2 were reached. For the 1 mM HCl solution the formation of gallium oxide is still likely to occur inhibiting the extensive formation of CoOx while for the 50 mM HCl solution dissolution of CoOx will be a complicating factor, which results in poor performance due to a lack of material. The amount of Co in the Co–GaInSn mixture was then increased to 1 wt% (Fig. 4b) and 1.5 wt% (Fig. 4c) as a means to increase OER performance. However, in both cases inferior OER current densities were attained compared to 0.5 wt% Co although a slightly better onset potential was achieved for the 1.5 wt% sample. It was also found that the 10 mM HCl solution used in the synthesis was the optimum solution at all weights of Co that was used.
As the 10 mM HCl solution provided the best results, the different weights of Co that were used were compared under these conditions (Fig. 4d). Here the redox processes for CoOx prior to the onset of the OER are shown. Over the potential range of 1.0 to 1.5 V vs. RHE the magnitude of the redox process increases upon increasing the Co content used for the synthesis. The peak that is evident at 1.2 V is due to the oxidation of CoO into CoOOH which is followed by a broader peak at 1.38 V attributed to the oxidation of CoOOH into CoO2 where the Co4+ species is regarded as the active component for the OER.46,47 From this data it is seen that upon increasing the Co content the magnitude of these redox processes increases prior to the OER, which indicates the formation of more CoOx, however this does not translate into improved activity.
This is also reflected in the Tafel slope values for the 0.5, 1.0 and 1.5 wt% Co samples which were determined to be 80, 105 and 100 mV dec−1 respectively. Therefore, the optimised conditions are gas injection into a Galinstan droplet containing 0.5 wt% Co immersed in a 10 mM HCl solution. It was also found that after 30 min of oxygen gas bubbling that the expulsion of CoOx nanosheets ceased. The stability of these 2D nanosheets was then investigated at a constant current of 10 mA cm−2 where a consistent potential of 1.62 V was maintained for 8 h. This overpotential value of 390 mV is comparable to liquid exfoliated layered Co(OH)2 that was ultrasonicated in aqueous surfactant solution for 4 h which gave an overpotential value of 440 mV.31 It should be noted that the inherent activity of Co(OH)2 is lower than many other OER electrocatalysts but can be improved by modifying the surface chemistry to carboxylate groups,48 incorporating S atoms,49 doping with Fe,50 adding graphene51 or gold nanoparticles32 which provides better activity than reported here. However in terms of the less studied CoOx material, the performance is slightly better than CoO nanofibers52 in terms of overpotential at 10 mA cm−2 and comparable to previous reports of CoO nanomaterials which were investigated in a more concentrated alkaline electrolyte of 1 M KOH, compared to 0.1 M used here, which gave values of 400 mV for both nanoparticles53 and nanoplates.54 Again the activity of CoO can be improved via doping with a second metal such as Fe52 and Zn54 but this was not the goal of the current study.
Finally, the better performance of the 0.5 wt% Co sample is attributed to the thin nature of the nanosheets that in principle exposes more active sites for the OER. Therefore, to investigate this phenomenon the effect of sheet thickness was investigated. It was found that if the 2D CoOx nanosheet suspension was allowed to age for 1 week then the thickness of the nanosheets increased to a range of 2.5 to 4 nm as seen from AFM images and the corresponding histogram for sheet thickness (Fig. S1 and S2†). The electrochemically active surface area (ECSA) was determined using background capacitive measurements (Fig. S3†) where the value decreased by over a factor of 4 from 0.33 mF cm−2 to 0.073 mF cm−2 which is a result of the increased sheet thickness. This is reflected in a decrease in the OER activity where the OER current density reduced to a value of 25 mA cm−2 at 1.70 V (Fig. S4†). This observation is consistent with previous reports showing that increasing the thickness of 2D transition metal oxides decreases electrocatalytic activity.31
Footnote |
† Electronic supplementary information (ESI) available: AFM images, OER activity of aged samples, electrochemical surface area measurements. See DOI: 10.1039/d0ra06010k |
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