Fabrication of Shape-Controlled MXene Adsorbents

5.3.1. Removal of metal ions and confirmation of the adsorption capacities

We checked the adsorption performance of MXene using copper ions. To create a circumstance in which heavy metal (copper) ions are dissolved, we used CuBr2 powder. Following the chemical equation of CuBr2(s)→Cu²⁺(aq)+2Br^-(aq), H2O containing copper ion in excess of the maximum contaminant level (by U.S. Environmental Protection Agency) for drinking water, 1.3 mg L^-1 was prepared.²⁰⁸ First, a filter test was performed by filtering the H2O through the MXene membrane and comparing the heavy metal concentration of the feed and permeation. As shown in Figure 5.1a, the H2O flowed through the hybrid MXene/PP membrane made in the same way as Chapter 2. The amount of MXene used was 50 mg, and the concentration of Cu ion was measured by ICP-OES. The MXene/PP membrane showed excellent filter ability while maintaining stability during two tests, and the Cu(II) concentrations were reduced from 14.8 to nearly 0 and 14.9 to 0.192 mg/l , respectively (Figure 5.1b).

To find out the adsorption ability of the MXene flake itself, a Cu(II) adsorption test using MXene dispersion was also performed (Figure 5.1c). By using a solution of known concentration, the amount of removed Cu(II) per MXene usage was measured. Stirring was carried out for 10 min to make the Cu ion react steadily with MXene. After the reaction, the mixed solution showed aggregation, suggesting that the negatively charged surface of MXene was neutralized by the Cu cation. Removed Cu was increased with MXene usage, and the absolute adsorption capacity (i.e., removed Cu per MXene adsorbent usage) was calculated to be 65.15 mg g^-1, which exceeds the ordinary absorbent such as GO or active carbon (Figure 5.1d).²⁰⁷

To analyze the adsorption capacities of the MXenes according to their forms, a Ti3C2Tx MXene powder adsorbent was produced. First, a thick ink with a concentration of 45 mg ml^-1 was prepared via a process to concentrate pure delaminated MXene flakes. The ink was then dried at ≈60 °C under N2

gas flow and subsequently ground to obtain a powder form (Figure 5.2a). The layered structure was observed in SEM micrographs, revealing that the powder has a high surface area of continuous 3D connects of nanosheets (Figure 5.2b). EDX spectrum measured on Ti3C2Tx MXene powder showed the elemental composition of Ti and C and O, F, and Cl as components of the surface functional groups (Figure 5.2c). The adsorption tests for various precious metals were performed using their feed solutions, which are aqueous and acidic (Figure 5.2d). The weighted MXene powder, as an adsorbent, was applied in the solution using ultrasonication for 10 min. After the adsorption reaction for 24 h, the MXene powder with adsorbed metal ions was filtrated using a polyvinylidene fluoride syringe filter having a pore dimension of 200 nm. Then the adsorption capacities were measured by comparison of the amount of ions in the feed and final solutions using ICP-OES. In the adsorption reactions, the Ti3C2Tx MXene was expected to enable the spontaneous donation of electrons to the precious metal ions

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following the reduction reactions on the surface, as reported in previous studies (Figure 5.2e).156, 209-211

To further analyze the structural and chemical alteration in Ti3C2Tx MXene and its interaction with adsorbed ions, XPS analysis is explained in the next section. The adsorption capacities of our Ti3C2Tx

MXene powder were greater than values of the lately published different adsorbents or compared to them depending on the adsorbates (Figure 5.2f).156, 195, 210, 212We considered that the reason for the lower adsorption of Pt and Ir than Au and Ag is that precious metal chloride complexes with low charge density and a smaller ionic radius have greater Coulombic interactions with their counter ions (Figure 5.2g). The MXene adsorbent showed the highest adsorption capacity for Au because Au³⁺ is a soft metal ion that has a high affinity for donor atoms on the surface groups. Low adsorption ability for Ir was ascribe to the presence of H2O inside the complexes formed by Ir³⁺ which suppresses the interaction between MXene and Ir³⁺ species, as previously reported.²¹³

Figure 5.1. Cu(II) filter and adsorption test using MXene adsorbent. (a) Photograph of flow system with MXene/PP membrane for the filtering test. (b) Comparison of Cu concentration before (feed) and after (permeation) being filtered through MXene membrane. (c) Appearance of Cu ion solution after MXene dispersion addition (left), aggregated MXene after stirring (middle), and filtrated solution after adsorption (right). (d) Mass of removed Cu with different MXene usage.

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Figure 5.2. Fabrication of Ti3C2Tx adsorbent powder and precious metal adsorption. (a) Photographs of highly concentrated Ti3C2Tx MXene ink and powder fabricated by drying the ink. (b) SEM micrograph of Ti3C2Tx MXene powder and (c) EDX spectrum of the powder, revealing the chemical composition of the Ti3C2Tx MXene phase. (d) Photographs of precious metal adsorption test using metal-feed solutions and Ti3C2Tx MXene powder. (e) A table showing the creation reactions of complexes containing the targeted metal ion from feed solutions and reduction of metals during adsorption on the Ti3C2Tx MXene. (f) Adsorption capacities of Ti3C2Tx MXene adsorbent for the targeted precious metal ions. The red symbols show references from the previous reports using other adsorbent materials. (g) Adsorption capacities for the precious metal ions with their ionic radii and complexes formed in the feed solutions.

5.3.2. Development of MXene nanostructures for the surface area and dispersibility

Based on the previous results, several types of Ti3C2Tx MXene adsorbents in various forms were fabricated for workable H2O-treatment operations. Among them, a form of powder was selected to selectively adsorb precious metals with high accessibility (without pressure), easy recovery, and recyclability. The development of MXene nanostructure was performed to enhance the surface area and dispersibility, which could provide high operational sites for metal-ion adsorption on MXene flakes.

The pristine MXene powder, composed only of delaminated MXene flakes, was prepared by the approach disclosed above. Then the powder was ball-milled in a N2 gas-filled SS vial to avoid harsh oxidation. After size-selecting with 45 μm sieves, a BM powder was achieved (Figure 5.3a). SEM micrographs showed that the films of piled MXene powder were broken into micrometer-sized particles, exposing the lateral structure of MXene. As previously reported, a TEM micrograph of exfoliated flakes from the BM revealed a layered and porous structure with a clean surface and edge, demonstrating that ball milling not only plays a key role in breaking the MXene powder but also in the creation of the flake's porous morphology (Figure 5.3b).²¹⁴The Ti 2p spectrum of the BM indicate the presence of Ti, C, O and F components without high oxidation (TiO2) or any degradation after the ball-milling process

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(Figure 5.3c).It was also confirmed through XRD analysis that the flakes maintained a similar inter- layer spacing for ND films and that the degree of stacking order of Ti3C2Tx layers was decreased after the ball-milling process (Figure 5.3d). When the MXene adsorbent powders were dispersed, the difference in dispersibility was clearly observed (Figure 5.4).Large fragments of ground ND powder were sedimented immediately by gravitational force, while the dispersion of BM powder remained stable after several hours (Figure 5.5). Dispersed flakes exfoliated from the ND powder showed large lateral sizes and thick morphology, which may be due to the strong interaction between the staked flakes (Figure 5.4a and c). In contrast, the flakes exfoliated from the ball-milled MXene powder had small lateral sizes of several hundred nanometers and a thin morphology, presumably due to the partial oxidation that appeared during the ball-milling, which reduced the size of the flakes and weakened the inter-layer interaction, forming the porous morphology (Figure 5.4b and d).²¹⁵The size and thickness distribution of flakes observed through SEM micrographs and AFM mapping micrographs exhibited reduction of flake size by ball milling, indicating the increased yield for dispersion of MXene flakes (Figure 5.3e and f).The BET surface area calculated by the nitrogen adsorption and desorption test at 77 K showed increased surface area by ball milling due to the increased accessibility of molecules between the small particles (Figure 5.3g). Calculated BJH desorption pore distribution also exhibited increased pore volume, in agreement with the surface area measurement (Figure 5.3h).

Figure 5.3. Synthesis and structural characterization of ball-milled MXene adsorbent. SEM micrograph of (a) ball-milled MXene powder (BM). The inset shows a photograph of BM. (b) TEM micrograph of BM flakes on a Cu grid. (c) High-resolution XPS spectra of the Ti 2p region of BM. (d) XRD patterns of N2 dried film (ND) and BM showing the different structure of piled MXene nanosheets.

Distributions of (e) in-plane dimension and (f) thickness of BM and ND flakes. (g) Nitrogen (77 K) adsorption-desorption isotherms and (h) BJH pore size distributions of BM and ND. Calculated surface areas and pore volumes are included in (g) and (h), respectively.

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Figure 5.4. Structural characterization of ND and BM flakes. SEM micrographs of (a) ND and (b) BM flakes on SiO2/Si substates. AFM mapping micrograph of the (c) ND and (d) BM flakes.

Figure 5.5. Dispersion stability of BM flakes. Photographs of stability test of ND and BM flakes with various concentrations. BM flakes were well dispersed and there was little aggregation or settling in comparison with ND flakes.

5.3.3. Investigation of nanostructure-dependent electrochemical properties of the MXenes With the help of surface terminations which provides electronegativity, our dispersed MXene adsorbent powder exhibited high surface charges especially in BM, which may be due to the increased surface areas. The combined DLS and zeta potential analysis indicated the reduced size of dispersed flakes in DI water after ball milling, which in turn increased the electrophoretic momentum of the adsorbent in the current field, corresponding to higher zeta potential from BM (Figure 5.6a). The reduced lateral size and porous morphology of BM flakes also influence the concentration of the MXene dispersions, resulting in increase in characteristic absorbance peaks in the UV-vis spectrum (Figure

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5.6b).^{141, 216} In addition to the electrostatic states of BM and ND in the aqueous circumstance, to reveal the dependence of electrochemical features of Ti3C2Tx MXene on surface area, such as galvanic displacement for metal-ion adsorption, ion transport and charge transfer resistance was estimated from flake size-dependent EIS.Spectroscopy was measured using a three-electrode system with half-cell set up. Ag/AgCl and graphite rod were used as the reference and counter electrodes (Figure 5.7a). Working electrodes for the test were fabricated by depositing MXene powder on different carbon base materials (i.e., CC and CP).SEM micrographs of BM powder on CP showed extruded particles compared to ND, which may lead to easier electrolyte penetration and contact with enormous operational sites (Figure 5.7b and c).The slope of Nyquist diagrams at high frequency showed the difference in charge transfer resistance. The lower resistance of BM than ND on conducting CC could be attributed to a smaller flake size and easy ion diffusion, which is consistent with the previous surface-area measurement (Figure 5.6c). Therefore, smaller flake size enables better electrolyte accessibility to more operational sites, which may be responsible for the electrochemical performance of Ti3C2Tx MXene as reported by previous studies.^{71, 217} Furthermore, a distinct tendency toward decreased impedance as the voltage increases shows a charge transfer to metallic MXene flakes and a reduction reaction of ions on a surface, suggesting metal adsorption by reduction mechanism (Figure 5.6d).

Figure 5.6. Electrochemical properties of BM and ND. (a) Zeta potential analysis for BM and ND.

The inset shows the DLS analysis for BM and ND. (b) UV-vis spectra of aqueous dispersion of BM and ND. (c) Nyquist diagrams from electrochemical impedance spectroscopy (EIS) for conductors produced with BM and ND at -0.8 V vs. Ag/AgCl. The inset shows the equivalent circuit diagram. (d) EIS performance of BM-coated CC electrode (the inset) obtained at different potentials.

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Figure 5.7. Morphologies of the ND and BM electrodes. SEM micrographs of CP base materials drop-casted with (a) ND and (b) BM. Insets in (a) and (b) show morphologies of each individual MXene flake dispersed and dropped on SiO2/Si wafer.

Figure 5.8. Size-dependent ion accessibility of MXene flakes on the different substrate. (a) EIS performance of BM and ND-based conductors on CP with insets showing a photograph of the specimen.

(b and c) EIS performance of BM and ND/CP conductors obtained at different potentials.

Our MXene adsorbent in forms of BM and ND showed the same tendency of EIS on another conducting CP that has different base material morphology, indicating the high quality of MXene flakes and their high reductive property (Figure 5.8). Notably, the prepared high-quality Ti3C2Tx MXene flakes exhibit remarkable atmospheric stability even after 4 months of storing in air, in contrast to typical aqueous Ti3C2Tx dispersions, which can only be stored for a few days. The XRD spectrum of BM that has been stored for 4 months is nearly identical to the spectrum of fresh specimen (Figure 5.9a).

Furthermore, zeta potential of the material has not decreased dramatically, showing that the particle did not break or aggregated due to oxidation during storing (Figure 5.9b). We observed the BM after 4 months of aging by Raman spectroscopy, and it kept its intact surface function groups with the Eg

vibrational modes in Tx and C regions (Figure 5.9c).¹⁴⁰ After a simple manual shake or sonication, the stored BM could be re-dispersed and formed a dispersion, showing a high degree of reversibility.

Consequently, BM concededly retained the enhanced electrochemical characteristics and demonstrated comparable EIS, even after 4 months, as the compact structure prevented the inner flakes from interacting with humid air after solvent removal (Figure 5.9d).

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Figure 5.9. Ambient stability and electrochemical property of BM after being stored for 4 months.

(a) XRD patterns of fresh and stored 4 months of BM. (b) Zeta potential analysis for fresh and stored 4 months of BM showing the similar surface charge. The inset shows the DLS results. (c) Raman spectrum of BM after being stored for 4 months. (d) Nyquist diagrams from EIS for conductors produced with the fresh and stored BM at -0.8 V vs. Ag/AgCl.

5.4. Measurements of Adsorption Efficiency and Precious Metal Recovery