Ultrasonics - Sonochemistry 70 (2021) 105292 Contents lists available at ScienceDirect Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson Ultrasound-assisted exfoliation of a layered 2D coordination polymer with T HER electrocatalytic activity Noemí Contreras-Peredaa, Faezeh Moghzib, Javier Baselgaa, Haixia Zhongc, Jan Janczakd, ⁎ Janet Soleimannejadb, Renhao Dongc, Daniel Ruiz-Molinaa, a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain b School of Chemistry, College of Science, University of Tehran, PO Box 14155-6455, Tehran, Iran c Center for Advancing Electronics Dresden (Cfaed) and Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany d Institute of Low Temperature and Structure Research, Polish Academy of Science, Okólna 2, 50-950 Wroclaw, Poland A R T I C LE I N FO A B S T R A C T Keywords: Large blue rectangular crystals of the 2D layered coordination polymer 1 have been obtained. The interest for 2D MOF this complex is two-fold. First, complex 1 is made of 2D layers packing along the (0–11) direction favored by the Coordination polymer presence of lattice and coordinated water molecules. And second, nanostructures that could be derived by de- Nanomaterials lamination are potentially suitable for catalytic purposes. Therefore it represents an excellent example to study Delamination the role of interlayer solvent molecules on the ultrasound-assisted delamination of functionally-active 2D metal- Ultrasound organic frameworks in water, a field of growing interest. With this aim, ultrasound-assisted delamination of the Catalysis crystals was optimized with time, leading to stable nanosheet colloidal water suspensions with very homo- geneous dimensions. Alternative bottom-up synthesis of related nanocrystals under ultrasound sonication yielded similar shaped crystals with much higher size dispersions. Finally, experimental results evidence that the nanostructures have higher catalytic activities in comparison to their bulk counterparts, due to larger metallic center exposition. These outcomes confirm that the combination of liquid phase exfoliation and a suitable synthetic design of 2D coordination polymers represents a very fruitful approach for the synthesis of functional nanosheets with an enhancement of catalytic active sites, and in general, with boosted functional properties. 1. Introduction including higher surface areas and sufficient accessible active sites, and their nanoscale thicknesses provide short ion/electron migration length Metal-organic frameworks (MOFs), and overall coordination poly- owing to the quantum confinement effect [12]. Nonetheless, the syn- mers, represent a class of crystalline materials embracing metal ions/ thetic methodology for NSs is in its fledgling stage, with many re- clusters connected by organic ligands [1]. Profiting from their well- searchers actively endeavouring nowadays novel approaches to tune defined structures, high specific surface area and structural/chemical the structure/property interface. This is crucial before any application diversity, these materials have attracted great attention for gas storage/ becomes a reality. separation [2,3], catalysis [4], and sensor applications [5]. They can Between them, the development of thin NSs with enhanced elec- also exhibit abundant active sites and tuneable redox states, which lay trochemical energy storage/conversion performance is an attractive the foundation for their applications in electrochemical energy storage/ opportunity. To date, a great effort has been dedicated to synthesize conversion, such as batteries [6], supercapacitors [7,8], water splitting ultrathin NSs [13,14], including top-down physical [15] and chemical [9] and oxygen reduction reactions [10]. However, though successful, delamination [16,17], as well as bottom-up interface-assisted synthesis most of these materials are generally synthesized as a bulk powder. [18,19] and template-assisted synthesis [19]. Nevertheless, despite the Accordingly, a mass of active sites are buried and inaccessible for success, these synthetic approaches still reveal limitations such as low charge carriers, leading to sluggish ion diffusion and low utilization of yields or require of additional cleaning or annealing steps. Recently, we active sites [11]. The apparent electrochemical performance has been reported the synthesis of NSs, from 2D coordination polymers with therefore limited compared with predicted theoretical values. In con- interlayer solvent molecules, in high yields and control over their size trast, thin nanosheets (NSs) possess a range of fascinating attributes and morphology using a Liquid Phase Exfoliation (LPE) process [20]. ⁎ Corresponding author. E-mail address: dani.ruiz@icn2.cat (D. Ruiz-Molina). https://doi.org/10.1016/j.ultsonch.2020.105292 Received 9 May 2020; Received in revised form 10 July 2020; Accepted 25 July 2020 Available online 30 July 2020 1350-4177/ © 2020 Elsevier B.V. All rights reserved. N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 This result, together with other studies [16,17,21–24], has clearly de- vacuum and chemically characterized as complex 1 by Fourier-Trans- monstrated the importance of properly selecting the mixture of solvent formed Infrared spectroscopy (FT-IR) and powder X-Ray diffraction and ultrasounds for a good delamination process. Overall, seemly (PXRD) (see Fig. S1, Supporting information). The rectangular shaped candidates for LPE processes present weak interactions between layers crystals though have lateral dimensions inferior to 10–15 μm, too small due to occluded interlayer molecules that can be easily uninvolved to be subjected to the LPE process (Fig. 2). Serendipitously we found under sonication in the appropriate solvent [25]. that addition of the Ni(NO3)2·6H2O salt to the aqueous solution of Herein we do demonstrate how such information can be put into 2,5pydc and triethylamine previous reaction with the Cu practice to obtain stable colloidal dispersions of NSs in water with en- (CH3COO)2·4H2O aqueous solution at room temperature for one day hanced catalytic properties. As a proof-of-concept, we have selected the yielded much larger blue crystals susceptible to delamination, as shown previously reported complex [Cu(2,5pydc)(H2O)]n·2H2O (1), where in Fig. 2 (for experimental details see Materials section). Single crystal − 2,5pydc stands for 2,5-pyridine dicarboxylic acid [26]. The structure is X-Ray diffraction (SXRD) confirmed the triclinic space group P1 and the made of Cu(II) ions coordinated to the carboxylic and pyridine groups lattice parameters a = 4.7886 (6) Å, b = 7.6216 (5) Å, c = 13.7293 (8) of the 2,5pydc ligand forming 2D layers while water solvent molecules Å, α = 99.390 (6)°, β = 98.762 (5)° and γ = 103.495 (7)°, char- help to stabilize the 3D packing through hydrogen bonds. On top of acteristic of complex 1. Moreover, the PXRD pattern of a bulk sample that, complex 1 has a high density of metallic centres suitable for washed and dried is in perfect agreement with the simulated from the electrocatalytic activity. Initially, effort was put to obtain large crystals SXRD of complex 1 as well as the corresponding FT-IR spectrum (Fig. of complex 1 suitable for delamination using a complementary nickel S2, Supporting information). Further, the presence of Cu as metallic salt as a template. Subsequent ultrasound-assisted exfoliation resulted center (with no or residual of nickel ions) was confirmed by Energy- in a suspension of rectangular shape and micrometre-long lateral sized dispersive X-ray spectroscopy (EDX) (Fig. S3a, Supporting information). nanosheets with a quite monodispersed size distribution. Such good And last but not least, thermal stability in ambient conditions was as- distribution was impossible to obtain using bulk synthetic approaches sessed with Thermogravimetric Analysis (TGA) (Fig. S3b, Supporting under sonication. Finally, the catalytic activity in the hydrogen evolu- information). Complex 1 showcase a high thermal stability from room tion reaction (HER) of the resulting NSs was tested [9,27]; enhance- temperature to 100 °C. After exceeding 100–140 °C, a weight loss of ment on the density of exposed active sites after exfoliation boosts a 12.74% is observed corresponding to the evaporation of solvation water higher HER electrocatalytic activity in base electrolyte compared to the molecules in complex 1. It is only beyond 260–270 °C that complex 1 bulk candidates. A schematic representation of both the exfoliation thermally decomposes. process and the catalytic activity of the resulting NSs is given in Fig. 1. Interestingly, reaction of the Ni(NO3)2·6H2O salt with an aqueous solution of 2,5pydc and triethylamine, exactly under the same experi- 2. Results and discussion mental conditions previously described but in the absence of the copper salt (experimental details are given in the Experimental Section), 2.1. Synthesis and characterization of large crystals of complex 1 yielded the new monomeric complex 2. Complex 2 crystallizes in the P21/c group where discrete molecular units establish a 3D supramole- First attempts to obtain crystals of complex 1 large enough to be cular network through weak hydrogen bonds (Fig. 3). The crystal- delaminated were done following a methodology slightly different to lographic data and structure refinement details from SXRD are pre- that previously described [26]. In a typical experiment, an aqueous sented in Tables S1 and S2 (Supporting information). The lattice parameters of the structure are a = 5.3133(4) Å, b = 23.8147(14) Å, solution of 2,5pydc and triethylamine was mixed with an aqueous so- c = 7.2158(8) Å, α = 90°, β = 104.880(9)° and γ = 90°. The Ni lution of Cu(CH3COO)2·4H2O at room temperature for one day. Re- metallic center is coordinated to two 2,5pydc in a bidentate mode, as sulting blue crystals were washed with water and ethanol, dried under Fig. 1. (Top) Schematic representation of the ultrasound-assisted LPE process for the delamination of 1, resulting in the formation of colloidal NS suspensions of 1. (Bottom) Exfoliated NSs with LPE process present more catalytic active sites than bulk material. 2 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 Fig. 2. (Left) SEM images of crystals of complex 1 synthetized as reported; (Right) Optical image of crystals of complex 1, which can be stored over long periods of time, dried or dipped in water, without any loss of the crystalline phase. the pyridyl and the 2-positioned carboxylic acid are involved in the respectively in the spectrum of 2. Interestingly, the 1602 cm−1 band is coordination in equatorial positions. Two water molecules are co- splitting into two peaks, one of them landing at ν = 1593 cm−1 proving ordinated to the Ni in the axial positions completing an octahedral the coordination to the metal ligand is through only one of the car- coordination sphere. Hydrogen bonds between carboxylic groups and boxylic groups. lattice water molecules ensure the good packing of 2 (Fig. 3b). Lattice and coordinated water molecules and 5-positionned carboxylic groups are involved in hydrogen bonding, packing the organo-metallic mole- 2.2. Structural analysis of complex 1 cule into a three dimensional (3D) supramolecular network structure (for more information see Table S3, Supporting information). FT-IR Even though the crystal structure of complex 1 was already de- spectroscopy confirmed the coordination in 2 (Fig. S4). As seen in the scribed, herein we revise in detail the supramolecular packing of the 2D spectra, the characteristic bands of the COO− at ν = 1597 cm−1 and layers due its relevance on the choice of solvent for the ultrasound- ν = 1386 cm−1 are shifted to v = 1602 cm−1 and v = 1397 cm−1 assisted delamination. The asymmetric unit of complex 1 consists of one crystallographically independent Cu(II) atom, one 2,5pydc ligand, one Fig. 3. (a) Unit cell of complex 2; (b) Crystal packing of 2 mediated mainly by hydrogen bonds, displayed in green, between carboxylic groups and lattice water molecules. 3 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 Fig. 4. (a) Image of the unit cell of 1 showing the crystallographic axis; (b) Side view of two layers of 1 showing lattice water molecules interconnecting them. Hydrogen bonds are displayed in blue showing the zig- zag configuration (c) Image of the crystal- lographic arrangement of the 2D layers in complex 1 obtained from a side view; (d) Top view of a 2D layer displaying the one dimensional chain arrangement of Cu ions. water molecule coordinated to the metallic center and two lattice water Supporting information). First attempts to delaminate complex 1 in molecule (see Fig. 4a). As we can observe in Fig. S5 (Supporting in- acetone, DMF, ethanol and methanol were unsuccessful in spite of all formation), two different types of Cu(II) metallic centers can be found the different experiments tested. In all the cases fragmentation in in the structure with octahedral coordination. The first metallic center smaller crystals with less defined morphologies were found from where Cu1 (Fig. S5a) is coordinated to four different 2,5pydc ligands through only a small fraction of flakes was identified (see Fig. S7, Supporting their 5-positionned carboxylic groups in equatorial positions. The axial information). More successful were the different attempts to delaminate positions of Cu1 are occupied by two water molecules. On the other crystals of 1 in water. More in detail, after 10 min of sonication in water hand, the second metallic center Cu2 (Fig. S5b), is coordinated to two no NSs but initial symptoms of fragmentation are observed. After in- 2,5pydc in a bidentate mode, as the pyridyl and the 2-positioned car- creasing the sonication time to 20 min, crystals start to lose their boxylic acid are involved in the coordination, and to two 2,5pydc in a morphology while the population of micro-/nanostructures becomes monodentate mode with 2 positioned carboxylic acid. more evident after 40 min of sonication, though large chunks of crystal Thus, in complex 1, the ligand 2,5pydc exhibits two kinds of still remain noticeable. It is only after 60 min of sonication that most of bonding modes. One type of bonding involves O, N atoms and another the macroscopic material disappears and the delamination yield in- is metallo-carboxylate monodentate bonding. As we can see in Fig. 4d, creases (see Fig. S6 in Supporting information). two O atoms from two monodentate coordination site form µ-oxo Finally, the few remaining non-exfoliated material was eliminated bridges that link the two adjacent Cu(II) ions. This leads to a rigid one- from the sample at 60 min after centrifuge. The resulting colloidal dimensional long straight chain of octahedral Cu(II) ions. Top view of a dispersion shows a characteristic Tyndall effect (Fig. 5a) and turned out single layer can be found on Fig. 4d. Fig. 4c shows the packing of the 2D to be stable at least for a week, as confirmed by DLS measurement layers from a side view which extends the structure to the final crys- (Fig. 5b). SEM images of an evaporated sample exhibit quite a mono- talline lattice according to the SXRD measurements. The layers pack dispersed distribution of rectangular NSs of 1 with micrometer-long along the (0–11) direction with an interlayer distance of 7.002 Å. Lat- lateral distances and morphologies similar to those found for the bulk tice water molecule and coordinated water molecules are involved in crystals (see Fig. 5c). Analysis of the images obtained by tilting the hydrogen bonding, packing the layers into a three dimensional (3D) sample holder 70° showcase quite uniform thicknesses (Fig. 5d and Fig. supramolecular network structure. Information on the different hy- S8) of 126 ± 35 nm (N = 45) indicating a high homogeneity on the drogen bonds found in complex 1 is summarized in Table S4. The lattice NSs suspension. water molecules allow the interconnection of the 2D layers displaying FT-IR of drop-casted flakes (Fig. S9) corroborated the chemical strong H-bonds (with short distances of 1.8–1.9 Å) in a zig-zag con- stability of 1 during its nanostructuration through the exfoliation pro- figuration (Fig. 4b). cess as stretching vibrations corresponding to C-N and C-O are seen in the same positions of complex 1. Further presence of Cu in the NSs was proved by EDX analysis (Fig. S9). Drop-casted flakes were analyzed as 2.3. Ultrasound-assisted delamination of complex 1 well by PXRD. As can be observed there, only two peaks appear in the diffraction pattern of the flakes, matching with two peaks of the dif- Crystal delamination was attempted in five different solvents fraction pattern of 1. Remarkably, the first of these two peaks corre- (acetone, DMF, ethanol, methanol and water) for 60 min while aliquots spond to the plane (0,1,−1) which is the plane inner to the 2D layers in at 10, 20, 40 and 60 min were taken and analyzed by Scanning Electron the crystallographic structure of 1 (see Fig. S9). It should be noticed Microscopy (SEM) to track the evolution with time (see Figs. S6 and S7, 4 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 Fig. 5. As-centrifuged NSs after LPE process of complex 1 in water sonicating 60 min: (a) Image of an aqueous colloidal suspension after centrifuge showing Tyndall effect; (b) DLS measurement of a colloidal suspension of flakes taken as-exfoliated and centrifuged, after 1 day and after 7 days of exfoliations. The measurement remains basically unaltered over the whole period. (c) SEM image of the aqueous colloidal suspension showing only exfoliated NSs. Bulk materials were removed from the suspension by centrifuge (d) Tilted SEM image of NSs of 1 showing uniform thicknesses. that intensity of all the other peaks from complex 1 is strongly sup- some of the peaks. Hence, bottom-up strategies involving sonication did pressed in the pattern from the flakes because of the reduced size of not allow the direct and large yielded synthesis of NSs with a mono- these. Finally, elemental analysis results were in excellent agreement dispersed distribution of sizes as the delamination process. with those expected for the theoretical formula [Cu2,5pydc]·5.4 H2O confirming that the NSs retain the composition of 1 (Theor. % C 25.78, 2.4. Electrochemical analysis % H 4.27; % N 4.29; Exptal. % C 25.84, % H 2.76, % N 3.89; Fit Error: 1.57%). The electrocatalytic performance of as-prepared samples toward Just for comparison purposes, attempts to obtain similar rectangular HER was estimated in a three-electrode cell in N2-sataurated 1 M KOH flakes following a bottom-up synthesis were done using the same ex- electrolyte, wherein the carbon paper (CP) loading with catalyst is perimental conditions than those used for the bulk material but assisted applied as the working electrode. As shown in Fig. 7a, the NS of 1 with ultrasounds, a bottom-up approach already used for the synthesis demonstrate a higher HER performance of more positive onset potential of nanoscale coordination polymers (for additional details see and dramatically larger current density (j) compared to the bulk Experimental Section) [28–30]. Remarkably, FT-IR spectra of the re- counterpart sample, which is likely due to the increased density of sulting crystals proved the coordination of the 2,5pydc with a metallic exposed catalytic sites and improved electron transfer kinetics after the center as the same shifts previously seen in crystals of 1 are observed exfoliation. The overpotential (η) of the flake sample is 340 mV to (Fig. 6c). Moreover, PXRD pattern of the nanocrystals matches with the deliver a current density of 10 mA cm−2, far exceeding that of the bulk simulated one from SXRD data (Fig. 6d). The resulting crystals present sample (530 mV), which is even comparable with the typical Ni/Co micrometer scale lateral sizes but thicker thicknesses and specially a based MOF electrocatalyst for HER (Table 1). It is worth noting that the much broad distribution with thicknesses ranging from 50 to 200 nm CP shows negligible HER current density even at large overpotential, (Fig. 6a and b). This is corroborated by the presence of all peaks in the revealing that CP acts as the current collector rather than contributing PXRD as nanostructuration of all crystals would lead to suppression of to the catalytic activity of the complex 1 NS electrode. 5 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 Fig. 6. (a) SEM image of nanocrystals of complex 1; (b) Higher magnification SEM image of nanocrystals of 1 showing a wide distribution of sizes; (c) FT-IR spectra of 2,5pydc before the reaction (black) and of nanocrystals of 1 (red); (d) Comparison of the PXRD spectra of crystals of 1 and nanocrystals of 1 with the SXRD of 1. The Tafel slope (Fig. 7b) of the NSs catalyst was calculated to be as Exfoliation of complex 1 was achieved in water applying local and low as 70 mV dec−1, which is lower than that of bulk counterpart large power ultrasounds for 60 min. The resulting suspension show- (86 mV dec−1), indicating the faster kinetics and the Volmer Heyrovsky cased NSs of 1 with rectangular shape and micrometer-long lateral mechanism at the NSs electrocatalyst during the HER process (Tafel sizes. LPE process provided quite monodispersed distribution of thick- slope is between 40 and 120 mV dec−1). In term of Volmer-Heyrovsky nesses, with values close to 100 nm. Chemical and structural char- mechanism, HER is proceeded through transferring one electron to the acterization proved the high stability of the NSs during the delamina- absorbed hydrogen and thus coupling of one proton at the surface of tion and ultrasound applying indicating the retention of high density of catalysts. Furthermore, electrochemical impedance spectroscopy (EIS) metallic center. Enhancement on the density of exposed active sites of 1 was performed to investigate the electron transfer kinetics. The results after exfoliation was confirmed using HER electrochemical analysis. A (Fig. 7c) verify the faster electron transfer kinetics of the HER process at sufficient electron transfer between the catalytic sites and the current the flakes electrode compared to the bulk catalyst and CP. As another collector is thus allowed, which is accounting for the enhanced catalytic important parameter for the practical application of HER electro- activity of 2D NSs compared to the bulk counterparts. Thus, our results catalyst, the stability measurement was also carried out at complex 1 evidence a higher catalytic activity using NSs of 1 when compared to NS electrode. As displayed in Fig. 7d, only trace change (Δη = 22 mV) bulk crystals of 1 as more positive overpotentials and larger currents of the overpotential at 10 mA cm−2 is observed for the NSs sample after are obtained. Further, faster electron kinetics are obtained using NS of 1 1000 potential cycles, highlighting its good stability for HER. There- as expected for large exposed surface areas. Moreover, NSs of 1 show a fore, the exfoliation of the bulk materials into 2D NSs is indeed an ef- high stability over many HER measurements. fective strategy to create more exposed catalytic sites, expedite the electron transfer between the catalytic sites and electrode, and thus 4. Materials and methods enhance the electrocatalytic activity. 4.1. Chemicals and materials 3. Conclusions Nickel nitrate hexahydrate salt, pyridine-2,5-dicarboxylic acid (2,5- pydc) and copper acetate monohydrate were purchased in Sigma A copper-based 2D coordination polymer has been synthesized and Aldrich. characterized. The structure of [Cu(2,5pydc)(H2O)]n·2H2O (1) showed Cu(II) ions coordinated in two modes to the carboxylic and pyridine groups forming a linear high dense packing of metallic centers along the 4.2. Synthesis of {[Cu(2,5-pydc)(H2O)].2H2O}n (1) 2D layers. Occluded water solvent molecules stabilized the structure through hydrogen bonds interactions, turning 1 into a suitable candi- 360 mg of 2,5-pydc were dissolved in 150 mL of water under reflux date for LPE exfoliation. at 50 °C. After dissolving, 0.4 mmol of trimethylamine and a solution of 6 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 Fig. 7. Electrocatalytic performance of complex 1 bulk on CP (carbon paper), complex 1 NS on CP and CP in 1 M KOH. (a) HER polarization plots with IR compensation (i: current, R: resistant); (b) The corresponding Tafel plot; (c) EIS; (d) HER polarization curves of complex 1 NSs on CP before and after 1000 potential cycles. All HER polarization curves are IR-compensation. Table 1 4.3. Synthesis of complex [C14H12NiO10N2]n·2H2O (2) Comparison of HER catalytic activity of NS of 1 and other MOF based HER electrocatalysts. A solution of Ni(NO3)·6H2O (30 mg, 0.1 mmol) in 5 mL water was Materials Active η mV (at Tafel slope Ref. added to a mixture of 2,5pydc acid (36 mg, 0.2 mmol) and triethyla- centre 10 mA cm−2) (mV dec−1) mine (0.4 mmol) in 15 mL water. The resulting solution was filtered and yellow crystals of complex 2 for X-ray crystallography were ob- NS of 1 Cu-N 340 70 This tained by slow evaporation after 3 days. FT-IR (KBr pellet, cm−1): 3316 work 2D CTGU-5 Co(II)-O 388 125 [31] (s), 1730 (m), 1642 (s), 1593 (m), 1486 (w), 1413 (w), 1384 (s), 1290 AB &CTGU-5 Co(II)-O 44 45 (w), 1120 (w), 1036 (w), 855 (m), 793 (m), 763 (m), 750 (m), 688 (w). NiFe-MOF Ni-O 134 34 [9] Elemental analysis calculated for [C14H12NiO10N2]n·2H2O: C, 36.32%, THTA-Co S-Co-N 283 71 [32] H, 3.48%, N, 6.05%. Found: C, 35.58%, H, 4.01%, N, 6.25%. (Fit error: THTNi 2DSP Ni-S 333 80.5 [18] CoBHT Co-S 185 88 [33] 0.93%). ICP %Ni: Theor. 12.68%; Expt. 12.43%. NiBHT Ni-S 331 67 MOS1 Co-S 340 149 [4] NiAT N-Ni-S 370 128 [34] 4.4. Ultrasound-assisted synthesis of {[Cu(2,5-pydc)(H2O)].2H2O}n (1) NENU-500 Mo-O 237 96 [35] A mixture of 2,5pydc (0.4 mmol, 72 mg) and triethylamine (0.8 mmol) in 30 mL distilled water was stirred for 15 min to solve 300 mg Ni(NO3)2·6H2O in 50 mL of water were sequentially added. A them. Then, 10 mL aqueous solution of Ni(NO3)3·6H2O (0.2 mmol, solution of 200 mg of Cu(CH3COO)2·H2O in 20 mL of water was added 60 mg) was added. Then, a 40 mL aqueous solution of Cu to above-mentioned mixture. The overall is kept close at room tem- (CH3COO)2·4H2O (0.2 mmol, 40 mg) was added dropwise to the above- perature. After one day, blue crystals for X-ray crystallography were mentioned solution, while this last was being sonicated in ultrasound obtained. Crystals were washed with water and ethanol and dried under bath equipment (Elma S40H Elmasonic) with effective ultrasonic power vacuum. FT-IR (KBr pellet, cm−1): 3357 (w), 3194 (w), 1626 (s), 1409 of 140 W. After addition, the overall solution was sonicated for 15 min. (m), 1386 (m), 1360 (s), 1288 (m), 1182 (w), 1148 (w), 1130 (w), 1044 After sonication, the obtained crystals were centrifuged for 2 min at (m), 902 (w), 840 (m), 773 (m), 743 (w), 692 (w). Elemental analysis 2000 rpm, washed with water and then dried under vacuum. calculated for [C7H3CuO4N]n·3.17H2O: C, 29.94%, H, 3.35%, N, 4.99%. Found: C, 29.81%, H, 2.84%, N, 4.93%. 4.5. Liquid phase exfoliation Delamination was performed with the LPE method assisted by 7 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 ultrasound. Exfoliation parameters like sonication time and used sol- Standard reference material (PerkinElmer) with known values of nickel vent were optimized. First, in order to optimize the sonication time and was analysed with each batch of samples. All the samples were mea- used solvent, 1 mg of crystals were dispersed in 4 mL of five solvents: sured per quadruplicate. DMF, ethanol, acetone, methanol and water. Ultrasounds were applied with a Branson Digital Sonifier SFX 550 (Emerson) sonicator equipped 4.8. Physico-chemical characterization of flakes with a double step 1/8 in. microtip. This equipment has an effective ultrasonic power of 550 W. The microtip was immersed in the disper- Flakes were characterized by Scanning Electron Microscopy (SEM) sions to produce the assistant ultrasound. The dispersions also were with SEM FEI Quanta 650F microscope. The samples were prepared by stirred to avoid sedimentation of the crystals and were kept at 20–25 °C drop-casting of the aliquots onto metallic sample holders. After eva- by immersing it in a water-ice bath replacing it every 30 min. in order poration of the solvent under ambient conditions, the previously sus- to avoid thermal decomposition. The temperature is measured peri- pended flakes remained on the holders. A 5 nm Pt layer was sputtered odically during the sonication. Ultrasounds were applied for 1 h in with Leica EM ACE600 on top of the samples, reproducing their topo- cycles of 2 min spaced by 30 s at 25% amplitude of the tip. Aliquots graphy. Afterwards, the particles were imaged with high acceleration from every suspension were taken after 10 min, 20 min, 40 min and voltages, between 10 and 20 kV at a working distance of 10 mm. Cross- 60 min of sonication. section images were taken tilting the sample 70°. EDX was performed with SEM FEI Quanta 650F model. The sample was prepared by drop- 4.6. Isolation of 2D flakes casting of the aliquot suspension onto metallic sample holders. FT-IR of the flakes was performed with Hyperion 2000 FT-IR microscope Non-exfoliated bulk crystals in suspension were removed by cen- (Bruker) working in reflexion mode. The instrument is equipped with a trifugation. The flakes were remaining suspended in the supernatant. room temperature detector and a mid-IR source (4000 to 600 cm−1). Aliquots from the suspensions were centrifuged for one minute at The spectra were recorded and treated with OPUS data collection 7000 rpm. After delamination and centrifugation, the colloidal sus- software. Samples were prepared by drop-casting of the colloidal sus- pensions are kept under constant agitation in IKA HS 260 basic shaker. pensions on top of gold substrates and left evaporated under ambient The complete exfoliation process was repeated with all the optimized conditions. XRD measurements (theta-2theta scan) were performed in a parameters for full characterization of the obtained flakes. Materials Research Diffractometer (MRD) from Malvern PANalytical company. This diffractometer has an horizontal omega-2theta goni- 4.7. Physico-chemical characterization of crystals ometer (320 mm radius) in a four-circle geometry and it works with a ceramic X-ray tube with Cu Kα anode (λ = 1.540598 Å). The detector Optical images were obtained with a Zeiss Primo Star microscope used is a Pixcel which is a fast X-ray detector based on Medipix2 equipped with a Zeiss Axiocam ERc 5 s camera on top. The software technology. Stability of the flakes in colloidal suspension was studied used for image acquirement was Zen Imaging Software (version ZEN by Dynamic Light Scattering (DLS) with Zetasizer Nano 3600 equip- 2012 blue edition). The crystals were dispersed on top of glassy and ment (Malvern Instruments) at room temperature (20–25 °C). The data transparent microscope slides and put underneath the microscope. was collected with the Zetasizer 7.04 software. The samples were in- Powder X-ray diffraction (PXRD) patterns of the crystals were obtained troduced in disposable plastic cuvettes and afterwards measured. with X’Pert3 Powder equipment (PANalytical), using Cu-Kα radiation. Intensity data were collected in continous-scan mode with a step size of 4.9. Electrochemical test 0,0334° and time per step of 160,02 s (speed 0,026526°/s). Data from the equipment was recorded and treated with X’Pert Data collector All the electrochemical tests were conducted in a three-electrode software (PANalytical). The obtained patterns were compared to the cell in 1 M KOH electrolyte, wherein carbon paper loading with cata- one from the resolved structure with single crystal X-ray diffraction lysts, Ag/AgCl electrode and carbon rod were serviced as working (SXRD). SXRD of 1 and 2 was carried out using a Rigaku Oxford electrode, reference electrode and counter electrode, respectively. Diffraction: KUMA KM-4 diffractometer delivering a Mo microfocus Catalysts (1 mg) were dispersed into 200 μL of ethanol and 20 μL of radiation (λMo Kα 0.71073 Å) and equipped with a CrysAlis CCD de- Nafion solution by sonication for 30 min. Then, the catalyst inks were tector. Fourier transform infrared spectroscopy (FT-IR) for both com- drop-casted on carbon paper giving a catalyst loading of ca. 1 mg cm−2. plexes was performed with Tensor 27 FT-IR spectrometer (Bruker) in Cyclic voltammetry (CV) tests were conducted in N2-saturated 1 M KOH attenuated total reflectance (ATR) mode. The instrument is equipped solution from −0.6 V to −1.6 V vs. Ag/AgCl with a scan rate of with a room temperature detector and a mid-IR source (4000 to 10 mV s−1. The accelerated durability tests were carried out with po- 400 cm−1). ATR was performed with a single window reflection of tential cycles from −0.6 to −1.6 V vs RHE with a sweep rate of diamond model MKII Golden Gate, Specac. The spectra were recorded 50 mV s−1. The reference electrode was calibrated with respect to the and treated with OPUS (Bruker) data collection software. Background reversible hydrogen electrode (RHE: ERHE = EAg/ spectrum in air was performed before the measurements, since the in- AgCl + 0.0592*pH + 0.197). All HER polarization curves were cor- strument is single beam. The dried samples were placed on the window rected with IR-compensation. And the impedance was tested by elec- reflection for measurement. Energy Dispersive X-ray Spectroscopy trochemical impedance spectroscopy (EIS) measurements with an AC (EDX) technique was performed with SEM FEI Quanta 650F model voltage with 10 mV amplitude in a frequency range from 100 kHz to (Thermo Fisher Scientific). The sample was prepared by drop-casting a 0.1 Hz at open-circuit voltage. diluted aqueous dispersion of crystals onto a metallic sample holder. Thermogravimetric analysis (TGA) was performed in a NETZSCH -STA Declaration of Competing Interest 449 F1 Jupiter equipment in ambient atmosphere, applying a heating ramp of 25 to 350 °C at a constant rate of 10 °C/min. Inductively coupled plasma mass spectrometry (ICP-MS) ICP-MS was used for the The authors declare that they have no known competing financial determination of the nickel percentage in 2. For the quantification of interests or personal relationships that could have appeared to influ- metal, a dilution 1:100 was necessary for detecting the correct signals. ence the work reported in this paper. For ICP-MS analysis, atomic spectroscopic analytical standards were purchased from PerkinElmer Pure Plus. The isotope 58Ni was selected as Acknowledgements tracer. The nickel concentration of each sample was measured using a calibration curve obtained in the range of 0.01–250 ppm of metal. This work was supported by grant RTI2018-098027-B-C21 from the 8 N. Contreras-Pereda, et al. Ultrasonics - Sonochemistry 70 (2021) 105292 Spanish Government funds and by the European Regional Development [18] R. Dong, M. Pfeffermann, H. Liang, Z. Zheng, X. Zhu, J. Zhang, X. Feng, Large-area, Fund (ERDF). The ICN2 is funded by the CERCA programme/ free-standing, two-dimensional supramolecular polymer single-layer sheets for highly efficient electrocatalytic hydrogen evolution, Angew. Chem. Int. Edit. 54 Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa (2015) 12058–12063, https://doi.org/10.1002/anie.201506048. Centres of Excellence programme, funded by the Spanish Research [19] R. Dong, T. Zhang, X. Feng, Interface-assisted synthesis of 2D materials: trend and Agency (AEI, grant no. SEV-2017-0706). Noemí Contreras Pereda’s challenges, Chem. Rev. 118 (2018) 6189–6235, https://doi.org/10.1021/acs. chemrev.8b00056. project that gave rise to these results received the support of a fellow- [20] N. Contreras-Pereda, P. Hayati, S. Suárez-García, L. Esrafili, P. Retailleau, ship from “laCaixa” Foundation (ID 100010434). The fellowship code is S. Benmansour, F. Novio, A. Morsali, D. Ruiz-Molina, Delamination of 2D co- LCF/BQ/ES17/11600012. Renhao Dong acknowledgements the fi- ordination polymers: the role of solvent and ultrasound, Ultrason.-Sonochem. 55 (2019) 186–195, https://doi.org/10.1016/j.ultsonch.2019.02.014. nancial support from ERC Starting Grant (FC2DMOF, No. 852909) and [21] A. Gallego, C. Hermosa, O. Castillo, I. Berlanga, C.J. Gómez-García, E. Mateo-Martí, DFG project (SPP 1928, COORNETs). J.I. Martínez, F. Flores, C. Gómez-Navarro, J. Gómez-Herrero, S. Delgado, F. Zamora, Solvent-induced delamination of a multifunctional two dimensional coordination polymer, Adv. Mater. 25 (2013) 2141–2146, https://doi.org/10. Appendix A. Supplementary data 1002/adma.201204676. [22] S. Benmansour, A. Abhervé, P. Gómez-Claramunt, C. Vallés-García, C.J. Gómez- Supplementary data to this article can be found online at https:// García, Nanosheets of two-dimensional magnetic and conducting Fe(II)/Fe(III) doi.org/10.1016/j.ultsonch.2020.105292. mixed-valence metal-organic frameworks, ACS Appl. Mater. 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