Mechanical energy-induced CO2 conversion using liquid metals


 We report a green carbon capture and conversion technology offering scalability and economic viability for mitigating CO2 emissions. The technology uses suspensions of gallium liquid metal to reduce CO2 into carbonaceous solid products and O2 at near room temperature. The nonpolar nature of the liquid gallium interface allows the solid products to instantaneously exfoliate, hence keeping active sites accessible. The solid co-contributor of silver-gallium rods ensures a cyclic sustainable process. The overall process relies on mechanical energy as the input, which drives nano dimensional triboelectrochemical reactions. By altering the secondary solvent and changing the reactor height, the dissolution and conversion efficiency can be tuned. The optimum reactor height is only 27 cm, when gallium/silver fluoride mix at 7:1 mass ratio is employed as the reaction material. At CO2 input of ~8 sccm, 92% efficiency was obtained at the record low input energy of 228.5 kW∙h for the capture and conversion of a tonne of CO2. The potential impact of this green technology is remarkable, likely benefiting a variety of industries and offering an economical solution for CO2 capture and conversion.

gallium/silver uoride mix at 7:1 mass ratio is employed as the reaction material. At CO 2 input of ~8 sccm, 92% e ciency was obtained at the record low input energy of 228.5 kW•h for the capture and conversion of a tonne of CO 2 . The potential impact of this green technology is remarkable, likely bene ting a variety of industries and offering an economical solution for CO 2 capture and conversion.

Main Text
Innovative technologies are urgently demanded for capturing and converting CO 2 into value-added species, at low input energy, to mitigate the negative effects of this greenhouse gas and support a sustainable carbon cycle 1,2 . Activating CO 2 into CO 2 •radicals or other intermediates is a crucial step for CO 2 conversion, while the inertness of CO 2 molecules imposes a signi cant challenge 3 . To this end, external energy is required, and catalytic systems are commonly engaged to lower the energy barrier for CO 2 converting. The choices of both the applied energy and functional materials, and how they are pooled together for CO 2 conversion govern the e ciency balances. To date, light and electricity have been extensively used as the triggering energies together with many well-known catalytic systems [4][5][6][7][8] . However, in general, the existing electro-or photo-empowered CO 2 reduction systems suffer from sluggish reaction rates and high energy consumption. Additionally, there has also been no magical de nitive catalyst, that could synergistically operate with these energy types to carry out the conversion at low costs and high e ciencies.
Mechanical energy is easy to generate and often a waste source of energy. Except for organic synthesis and polymerisation 9, 10 , mechanical energy has received little attention in catalytic systems and has not yet been utilised for CO 2 reduction. In conductive particles suspensions, the introduction of mechanical stimuli can both increase the temperature 11 and generate triboelectri cation, as a result of the frictional contact and modulation of gaseous content solubility, respectively [12][13][14] .
Solid metallic catalysts have exhibited excellent performance for electrocatalytic or photocatalytic CO 2 reduction [15][16][17] . However, the active sites of solid metallic catalysts can deteriorate under intense mechanical stimuli and/or can be deactivated when carbonaceous materials adhere onto the catalytic sites during CO 2 reduction. In principle, the use of liquid metals, and speci c solid structures, can solve these problems. Gallium (Ga)-based liquid metals have shown intriguing properties for catalysis, including tunability by the incorporation of other elements, and remarkable resistance to coking [18][19][20][21] and also excellent mechanical tolerance. Additionally, it is known that Ga (0) can be oxidized to Ga (I) in the presence of organic materials 22 . In this work, we explore whether the same effect can also apply to CO 2 and whether we can nd a process to reduce Ga(I) back to Ga (0) in a closed cycle stimulated by mechanical agitation. We envision that by employing liquid metal mixes of Ga and a compound of Ag, a closed cyclic catalytic system can be developed. This system can then covert CO 2 into value-added species in the reversible Ga-Ga + cycle, which will be fully explored.

Assessment of CO 2 conversion for various scenarios
We exploited a suspension of Ga and Ag (I) salt mixes as the precursors of the co-catalysts and ultrasound was initially employed to stimulate the CO 2 reduction. Dimethylformamide (DMF), with good stability during mechanical agitation and high CO 2 solubility of 0.14 M at 40 °C (to assure the liquid state of Ga), was chosen as the solvent 23 . We observed that during the reaction, the CO 2 molecules near the interface of the suspended particles were reduced to form carbonaceous sheets in a process that will be detailed later.
The best outcomes were obtained when Ga and AgF were mixed in a DMF solution which also contained 0.10 M HCl to remove the native oxide on the surface of Ga. Ga and AgF were sonicated together (Fig. 1a -using a probe sonicator for 30 min) to generate sub-micron Ga droplets of 230 nm median diameters and Ag 0.72 Ga 0.28 rods of micron/sub-micron lengths and median diameters of 160 nm ( Fig. 1b and Supplementary Fig. 1). The characterisation outcomes revealing the elemental composition will be presented in a later section.
In the reactor, CO 2 was bubbled into and dissolved in DMF through a diffuser (Fig. 1c). The dissolved CO 2 is reduced to solid carbonaceous materials at the interface of the Ga droplets. The mechanically enforced CO 2 conversion can be scaled up using a variety of mechanical sources that produce frictional contact.
To examine this prospect, CO 2 conversion by an overhead mixer was also performed and validated (Fig.   1d).
Due to the ultra-smooth nature of the liquid metal droplets, the produced carbonaceous materials on the surface are in the form of sheets 18,24 . These low dimensional sheets, on the non-polarized liquid metal surface, are exfoliated during mechanical stimulation (Fig. 1e) 18,24 . Most importantly, the carbon sheets migrate to the top of the reactor and can be isolated due to the density difference with reference to that of metallic components (Fig. 1c,d).
The qualitative and quantitative analyses of the production of carbon, when the Ga/AgF (7.0 to 1.0 mass ratio) suspension in DMF is utilised in a 20 mL reactor, are presented in Fig. 1f and Supplementary Fig. 2. The 7.0 to 1.0 mass ratio and the reaction temperature were chosen according to previously optimised data for C-C bond formation reactions 22 . Additionally, the performance of the system formed by direct alloying of Ga with silver (50 to 1.0 or 20 to 1.0 mass ratio of Ga/Ag, Supplementary Fig. 3e,f) and different silver salts ( Fig. 1g-k), including AgCl, AgBr, AgI, AgOTf, AgNO 3 (also 7.0 to 1.0 mass ratio) were compared. The homogeneous mixture (20 μL) was drop-casted onto a glass substrate and dried for Raman analysis, with the whole drop-cast region included during the Raman spectroscopy measurement (Supplementary Fig. 3a-d). The changes in the intensity of the carbon D and G bands at 1350 and 1600 cm -1 , respectively, were employed to obtain the overall picture. Thermal gravimetric analyses (TGA) and gas chromatography (GC) were also conducted for comparative quantitative assessment of the solid carbon and gaseous products (Supplementary Table 1).
For the Ga/AgF system that exhibited the best performance, the production of carbon was observed in < 1 hour of reaction ( Fig. 1f) and increased continuously over time according to TGA. The TGA showed that 4.95 mg of carbonaceous materials were produced per hour in a 20 mL reactor at a CO 2 ow rate of ~10 sccm ( Supplementary Fig. 2). In comparison, the AgCl, AgBr, AgI, and AgOTf mixes also presented CO 2 conversion capability, but they were not as e cient as the AgF system ( Fig. 1g-j, for brevity only Raman spectra are shown and not TGA). With no emerging D and G bands after 5 hours of reaction ( Fig. 1k and Supplementary Fig. 3e,f), Ga/AgNO 3 and Ga-Ag alloys were found to be ineffective for CO 2 reduction. The reason for these different conversion activities will be discussed later.
To verify that the CO 2 reduction relies on the synergism of Ga and AgF, experiments were conducted by employing Ga and AgF separately ( Supplementary Fig. 3g,h), both of which resulted in undetectable carbon production. We note that our attempts with other types of salts (e.g. KCl and NaCl) ( Supplementary Fig. 3i,j) and magnetic stirring (less powerful in comparison to ultrasonication and overhead stirring) ( Supplementary Fig. 3k), showed no carbon formation, indicating the crucial role of the chosen silver salt (AgF) and a threshold for mechanical energy input into the reaction process. Controlled N 2 bubbling also did not show any formation of carbonaceous products ( Supplementary Fig. 3l).
We explored the minimum co-catalyst mass required in the system to maintain the e ciency of CO 2 conversion. Diluting the catalyst by 10 times offered nearly the same conversion e ciency, still achieving an equivalent production of 4.75 mg of carbonaceous materials per hour at ~10 sccm CO 2 bubbling rate ( Fig. 1l and Supplementary Fig. 2), whereas the output was dramatically reduced for dilutions of 50 or 100 times ( Supplementary Fig. 3m,n. TGA pro les are not shown for brevity).
The amount of CO 2 dissolved in solution also signi cantly in uenced the e ciency of CO 2 conversion.
Ethanolamine (ETA) is a suitable choice for increasing this amount since CO 2 solubility is 5.6 M in pure ETA 25 in comparison to 0.14 M in DMF at 40 °C. With the addition of 10 vol% ETA in DMF (referred to as DMF+ETA hereafter), CO 2 was continuously reduced with a higher e ciency, producing 7.95 mg of carbonaceous materials per hour in the same reactor at ~10 sccm CO 2 bubbling rate ( Fig. 1m and Supplementary Fig. 2). Interestingly, 22.2 cm 3 CO was also produced in one hour ( Supplementary Fig. 4).
In contrast, when DMSO or H 2 O were used, the e ciency was very low and carbon products could not be quanti ed by TGA, owing to their limited CO 2 solubility ( Supplementary Fig. 3o,p).

Carbonaceous materials characterisation
Carbonaceous materials produced from CO 2 were isolated for further characterisation. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis ( Fig. 2a and Supplementary  Fig. 5a) of the isolated carbonaceous materials reveal that the solid product consists of carbon and oxygen before any secondary washing, with trace quantities of the metallic species that can be easily (SAED) images demonstrate that part of the carbonaceous material is akin to that of slightly crystalline graphene oxide (Fig. 2b), and a certain proportion of the product exists in the amorphous state ( Supplementary Fig. 5f).

E ciency and energy requirement for CO 2 conversion
The CO 2 conversion e ciencies under different con gurations were determined using TGA and GC measurements as summarised in Supplementary Table 1 (see Supplementary Information). The conversion e ciency is de ned as (captured and reduced CO 2 /total input CO 2 )´100%, which are all obtained from the optimum mix of Ga/AgF (to be explained further in the next section).
In the small-volume reactor of 20 mL and height of 4.5 cm, which was used as the characterisation unit in the previous sections, the conversion e ciencies are 1.5% and 6.2%, for DMF and DMF+ETA cases, respectively (Fig. 2c). To demonstrate the scalability, we increased the dimensions of the reactor (Fig. 2d). When the height of the reactor was increased to 40 cm for only DMF solvent (volume of 500 mL), 27% of the input CO 2 at the ow rate of ~8.6 sccm could be continuously captured and converted (Fig. 2c). Thus, when the total height of the reactor would be 148 cm, the CO 2 conversion could reach the full capacity.
The height of the reactor for near-full conversion could be signi cantly decreased when DMF+ETA was used as the solvent as this combination could signi cantly increase the CO 2 solubility. The conversion e ciency reached the unprecedented value of 92% (at the ow of ~8.0 sccm CO 2 ) in this case for a reactor as small as 27 cm in height and 330 mL in volume ( Fig. 2c and  ). An overall estimation based on the current price of electricity suggests that the operational cost of CO 2 capture and conversion using DMF+ETA is lower than any other state-of-the-art technologies [27][28][29][30][31] .

Analysis of the suspensions
To elucidate how the functional materials are formed during the probe sonication stage, the reactions between Ga and silver salts were investigated by characterising the sonication products. Sonicating Ga with AgF (as an exemplar) forms an intermetallic phase Ag 0.72 Ga 0.28 (Fig. 3a) and GaF 3 . The existence of metallic Ag (as Ag 0.72 Ga 0.28 ) is con rmed by the Ag3d XPS peaks at 367.8 and 373.8 eV (Fig. 3b) 32 . The metallic uorides can be veri ed by the F1s XPS peak at 684.3 eV (Fig. 3c) 33 .
The compositions and morphologies of the materials were investigated and correlated with the CO 2 reduction performance. As illustrated in Fig. 3a, XRD patterns of Ga mixed with silver salts, which lead to AgF) or a combination of both ( Fig. 3h for AgCl) together with the Ga spheres. The presence of only rodshaped morphology (Fig. 3i for AgF) is found to associate with an enhanced CO 2 catalytic capability. The Ga/AgF system, which generated the highest e ciency for CO 2 conversion, showed the rod-shaped Ag 0.72 Ga 0.28 , while Ag 0.72 Ga 0.28 with non-rod morphology from other silver salts (or limited rod morphology for AgCl) exhibited limited catalytic abilities. The high-resolution TEM images, SAED pattern (Fig. 3j,k) and the TEM-based EDS mapping (Fig. 3l-n) further con rm the existence of the Ag 0.72 Ga 0.28 rods and their growth direction along the [201] plane (Fig. 3k). As shown in Fig. 3o-q, the native oxide layer on the surface of the Ga droplets can be observed when dried for analysis. Furthermore, there were no obvious changes to the Ag 0.72 Ga 0.28 structures after 5 hours of reaction according to both XRD ( Supplementary Fig. 9x) and SEM ( Supplementary Fig. 10), indicating that the Ag 0.72 Ga 0.28 rods were resilient towards mechanical agitation.
The concentration of gallium and silver ions in solution during the reaction was measured by inductively coupled plasma mass spectrometry (ICP-MS) ( Supplementary Fig. 11). The ion concentrations uctuated without showing any increasing or decreasing trend, indicating that the catalysts are not consumed, and that the system is stoichiometrically stable.

Reaction mechanism
Based on the con guration of the catalytic process, we can consider a scenario in which the contact of the Ga/DMF interface is altered by the interfacial formation of CO 2 bubbles. CO 2 bubbles are formed as the Ga/DMF interface becomes warmer due to localised friction 11 . As such, the interfacial solubility of The CO 2 reduction in our system is completed through a reversible Ga-Ga + cycle. Cyclic voltammetry was conducted to provide an insight into the catalytic mechanism of the Ag 0.72 Ga 0.28 rods via the suggested route. The results showed that, for the working electrode containing Ga droplets and Ag 0.72 Ga 0.28 rods as the co-contributors, Ga is oxidized to Ga + at 0.18 V and then reduced to elemental gallium at -0.31 V (Fig.   4a) 34 . Considering that the triboelectric process generates time-dependent voltages of several volts, the carbonaceous sheets are rapidly produced on the surface of liquid metals as we have previously demonstrated 35 . We also note that Ga + reduction was not observed when either Ga droplets (Inset in Fig.  4a) or Ga droplets with non-rod morphology Ag 0.72 Ga 0.28 were used as the working electrode ( Supplementary Fig. 13), showing the importance of the long rods for the cyclic reaction.
To investigate the reaction mechanism, we performed a detailed assessment of the reaction intermediates and the by-products generated during CO 2 reduction. The overall reaction process in DMF is described by chemical reaction equations (1-6). The equations are separated into 'liquid metal components' reactions (equations (1-4)) and 'solid components' reactions (equations (5,6)). For the description of the liquid metal component reactions, a series of characterisations were conducted.
Nuclear magnetic resonance (NMR) analysis showed that the solvent DMF was not involved in the reaction (Supplementary Fig. 14a). The CO 2 reduction is realised via the voltage provided by the nano triboelectrochemical process on the surface of Ga liquid droplets that turn Ga into Ga + , while CO 2 is activated into the CO 2 •radical (equation (1)). The existence of the CO 2 •radical during the reaction is demonstrated by electron paramagnetic resonance (EPR), which uses 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical trapping agent to form DMPO-CO 2 •adduct for spectroscopic analysis (Fig. 4b, see Methods) 36 . The CO 2 to CO 2 •process is followed by the generation of the intermediates CO and O 2radicals (equation (2)) 37 . The former is further converted to carbonaceous materials on the liquid metal surface (equation (3) which is the optimum case and it can be altered according to the C to O ratio in the obtained carbonaceous solid). The equations are as follows: The description of the 'solid components' reactions is as follows. According to the cyclic voltammetry results, the oxidized Ga + can be reduced to elemental Ga by receiving an electron from the Ag 0.72 Ga 0.28 and the Ag 0.72 Ga 0.28 turns into Ag 0.72 Ga 0.28 + (equation (5)). The catalytic cycle is closed by the electrons provided from the O 2to O 2 (equation (4)) to reduce Ag 0.72 Ga 0.28 + back to Ag 0.72 Ga 0.28 (equation (6)), where the existence of O 2 can be con rmed through GC (Supplementary Fig. 4 and Supplementary Table   1).
The catalytic cycle of CO 2 reduction at the interface of Ga droplets is illustrated in Fig. 4c. This catalytic mechanism also aligns with the DMF+ETA case, and the extra by-products are produced due to the presence of ETA that promotes the process towards CO production according to equation (5) (Supplementary Fig. 4 and Supplementary Fig. 14b) 38 .
Since the reaction is activated by the triboelectric potential, other forms of mechanical stimuli can also be applied, and the system can be readily scaled up. As a demonstration, we further validate our strategy by coupling an overhead stirrer to a 50 mL reactor. We found that CO 2 conversion continuously takes place in a stable manner when the stirring speed exceeds a threshold of 200 rpm (at room temperature, Supplementary Fig. 15a) and the conversion e ciency increases along with the increase of the stirring speed (equivalently, the mechanical energy input) (Supplementary Table 1 and Supplementary Fig.  15b,c).

Conclusion
In conclusion, we demonstrated a promising mechanical energy-induced CO 2 conversion method which operates solely with mechanical stimuli. The catalytic process was found to be mechanically enabled by co-contributors containing liquid metal Ga droplets and resilient intermetallic Ag 0.72 Ga 0.28 structures. We demonstrated the crucial roles of the composition and morphology of the functional materials, as well as the importance of precursor selection for triboelectrochemical CO 2 conversion. The obtained CO 2 capture and conversion e ciency was 92% for a reactor with a height as small as 27 cm at the input CO 2 ow rate of ~8 sccm.
The prospects of further exploring the proposed system are extensive. This methodology can be extended for large-scale systems and long-term operation by employing different forms of mechanical energies. This methodology with its remarkable capabilities provides promising prospects for future practical deployment of CO 2 conversion technologies. Altogether, the novel concept provided by Preparation of functional materials. (1) Sliver salts as precursors: During a typical co-contributor preparation process (using AgF as an example), Ga (7.0 g) was rst added into a glass vial which is prelled with 5.0 mL DMF solution, followed by adding HCl solution to give a nal 0.10 M to remove the surface oxide layer of Ga. AgF (1.0 g) was then added to the mixture as the precursor.
(2) Ag (150 nm particle size) as precursors: For the preparation of Ag-Ga alloy, the silver powder was added to Ga (7.0 g) in concentrations of 2.0 wt% and 5.0 wt%, respectively. The mixtures were ground using a mortar and pestle inside a N 2 -lled glove box to minimize oxidation of the liquid metal 18 . The grinding process, typically lasts 40 min, was stop until the sample showed a smooth and re ective appearance.
(3) Probe sonication procedures: The mixture from step (1) or step (2) was sonicated with a probe sonicator (VC 750, Sonics & Materials) under the protection of N 2 . The sonication amplitude was set to 55%, corresponding to an ultrasonic power input of ~410 W. The sonicator was paused for 1 s after each 9 s sonication and the total sonication time was 30 min. CO 2 conversion experiment. (1) Bath sonication as the energy source: After the preparation of functional material, CO 2 was bubbled into the reactor through a diffuser and the ow rate of CO 2 gas (10 sccm) was controlled using a gas mass ow controller (MKS, GE50A). Bath sonication (FREQ 50Hz, Unisonics) was employed as the mechanical energy source to trigger CO 2 conversion. The temperature of the reaction solution was kept around 40 °C during the 5 hours reaction.
(2) Overhead stirrer as the energy source: When CO 2 was bubbled into the reactor through a diffuser at the same rate of 10 sccm, an overhead stirrer (DLS Digital Overhead Stirrer, 120 W) was utilized as the mechanical energy source. Different rotation speeds, including 200, 300, 400, 500 and 1000 rpm, were applied for initiating the CO 2 conversion. The experiment was performed at room temperature for 24 hours.
Sample characterisations. Raman spectra were collected via a Raman spectrometer (Via Raman microscope, Renishaw) utilizing the 532 nm laser source. XPS was performed on a Thermo Scienti c Kalpha X-ray spectrometer. The carbon product was studied using micro-FTIR spectroscopy, on a PerkinElmer Spectrum 100 FTIR Spectrometer which is coupled to a Spotlight 400 FTIR Imaging System Cyclic voltammetry experiments: Cyclic voltammetry experiments were conducted to investigate the roles of the Ag 0.72 Ga 0.28 rods during the CO 2 reduction. Ga droplets and Ag 0.72 Ga 0.28 rods were obtained from the Ga and AgF precursors. Then, we painted the Ga/Ag 0.72 Ga 0.28 (Ag 0.72 Ga 0.28 in the shape of a rod) on uorine doped tin oxide (FTO) and baked it until the material dried and immobilised on the FTO as the working electrode. A calomel reference electrode and a gold counter electrode were used to set up a threeelectrode con guration. DMF+ETA solution containing 0.10 M HCl was utilized as the electrolyte to keep the condition consistent to the reaction situation. As a comparison, Ga droplets and Ga/Ag 0.72 Ga 0.28 (Ag 0.72 Ga 0.28 with non-rod morphology, using Ga/AgI) were painted on FTO as the working electrode respectively and all other parameters were kept identical.
EPR experiments: To provide mechanistic insights, EPR experiments were conducted to con rm the existence of the activated CO 2 . DMPO is known as a standard CO 2 •radical captor and the combination of DMPO with the CO 2 •radical shows characteristic signals 36, 39 . During the experiments, 20 mg DMPO was added and dissolved into 5.0 mL Milli-Q water and then 1.0 mL DMPO solution was further added into the reaction system. After bubbling CO 2 into the solution in the presence of bath sonication for 30 min, 1.0 mL reaction solution was centrifuged to remove all the suspended materials for EPR measurement. As a control, 1.0 mL DMPO solution was added into the reaction system in the process of bath sonication without bubbling CO 2 .
To verify that the EPR signal obtained from our reaction system was due to the formation of CO 2 •-, an independent reaction involving CO 2 •formation is further conducted. It is well established that the CO 2 •radical can be generated from NaHCO 2 and H 2 O 2 under the irradiation of ultraviolet light 36 . NaHCO 2 and 30% H 2 O 2 were dissolved in 5.0 mL Milli-Q water in the concentration of 100 mM and 100 μM, respectively, with the DMPO concentration being 50 mM. After irradiation with ultraviolet light for 10 min, we mixed 1.0 mL photolytic solution with 20 mL DMF solution for EPR analysis to keep the condition consistent with the conditions of our CO 2 conversion reaction. By comparison, the EPR spectra from the reaction system and photolytic solution were in good agreement, thereby validating the existence of CO 2 •radical during our CO 2 conversion reaction (Fig. 4c).

Data availability
The datasets generated during the current study are available from the corresponding authors upon reasonable request. Figure 1 Schematics and Raman spectra of solid carbon produced from CO2 using liquid metal. a-d, Schematic illustrations for the preparation of a suspension of catalyst (a,b) and the CO2 reduction process using different mechanical energy inputs (c,d). e, Schematic illustration of the formation and detachment of carbon akes on the surface of Ga droplets in the presence of the solid rods. f-k, Raman spectra of the samples obtained from the reaction mixes of Ga with different silver salts as precursors in DMF: AgF (f, versus time), AgCl (g), AgBr (h), AgI (i), AgOTf (j) and AgNO3 (k). The D and G bands at 1350 and 1600 cm-1, respectively, emerged after the reactions occur. l,m, Raman spectra (versus time) from the surface of mixtures from the 10-times diluted reaction system (Ga and AgF mix) by employing DMF (l) and DMF+ETA (m) as the reaction solutions. The blue and red curves in f-m are Raman spectra for the samples before and after reaction, respectively.  Characterisation of the functional materials. a, XRD patterns after probe sonication by employing different silver salts with Ga as the precursors. Except for Ga/AgNO3, the other silver salts and Ga were converted into Ag0.72Ga0.28. b,c, XPS analysis of the state of silver and uoride on the surface of the mixtures after probe sonication. d-i, SEM images of the materials after probe sonication when different silver salts were used as precursors as Ga/AgNO3 (d), Ga/AgOTf (e), Ga/AgBr (f), Ga/AgI (g), Ga/AgCl (h), and Ga/AgF (i). Ag0.72Ga0.28 was found in the shape of rods only when AgF was used as the precursor, and some rods were also seen for the AgCl case, while Ag0.72Ga0.28 from other silver salts have non-rod morphologies. j,k, TEM and HRTEM of Ag0.72Ga0.28 nanorods with SAED images inserted in j. l-q, TEM and corresponding EDS images of Ag0.72Ga0.28 rods and Ga droplets.