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[Hiroaki Maeda](https://orcid.org/0000-0001-9552-7478), [Kenji Takada](https://orcid.org/0000-0002-7531-6865), Naoya Fukui, Hiroyasu Masunaga, Sono Sasaki, [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692), Hiroshi Nishihara

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[Face-on-oriented formation of bis(diimino)metal coordination nanosheets on gold electrodes by electrochemical oxidation](https://mdr.nims.go.jp/datasets/cfe9f6b5-ddf7-432d-8985-b22530b09186)

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Face-on-oriented formation of bis(diimino)metal coordination nanosheets on gold electrodes by electrochemical oxidationrsc.li/njcNJCNew Journal of Chemistry  A journal for new directions in chemistry PAPER  Hiroaki Maeda, Hiroshi Nishihara  et al .  Face-on-oriented formation of bis(diimino)metal coordination nanosheets on gold electrodes by electrochemical oxidation ISSN 1144-0546Volume 48Number 1414 April 2024Pages 6043–6516This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024 New J. Chem., 2024, 48, 6081–6087 |  6081Cite this: New J. Chem., 2024,48, 6081Face-on-oriented formation of bis(diimino)metalcoordination nanosheets on gold electrodes byelectrochemical oxidation†Hiroaki Maeda, *a Kenji Takada, a Naoya Fukui,a Hiroyasu Masunaga,bSono Sasaki,cd Kazuhito Tsukagoshi e and Hiroshi Nishihara*afBis(diimino)metal complex nanosheets composed of metal ions and hexaaminobenzene ligands (MHABs)are fascinating two-dimensional materials which have been gaining significant attention as electrodematerials and electrocatalysts owing to their electrical conductivities, redox properties, and porousstructures obtained by the p-conjugated system and regular metal complex arrangement. Althoughelectrochemical oxidation enables the simple and direct formation of MHABs on electrodes within acouple of minutes only, the obtained MHABs are relatively rough and of low crystallinity. This studyinvestigated the effects of the applied potential during the synthesis, the type of base, and theconcentrations of bases and building blocks on the synthesis of NiHAB nanosheets. Optical microscopy(OM), atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS),and grazing-incidence X-ray scattering (GIXS) results revealed that a low oxidation potential, highconcentration of the ammonia solution, and low concentration of the building blocks in the reactionsolution provided a nanometre-thick NiHAB layer. Under the optimal reaction conditions, NiHABnanosheets were successfully synthesized in a face-on orientation manner on gold electrodes usingelectrochemical oxidation methods. Furthermore, the anisotropic electrochemical synthesis of CuHABwas achieved under the optimised conditions. The mechanism of the MHAB growth process with ananisotropic orientation and nanometre thickness was proposed and attributed to the flat adsorption ofp-conjugated oligomers composed of bis(diamino)metal moieties in a solution onto an electrodesurface and the electrochemical oxidation and deprotonation of the adsorbates on the electrode.IntroductionOrganic two-dimensional materials have been recently gainingsignificant attention owing to their designable and tailorablechemical structure.1–3 In particular, electrically conductive coordi-nation nanosheets (CONASHs),4,5 also known as conductivemetal–organic frameworks,6–10 have been employed in variousapplications such as electronics, energy storage devices, sensingdevices, and electrocatalysts owing to their fascinating electricalconductivities derived from their p-conjugated structures, redoxactivities, large surface areas, and porous structures. Since thesynthesis of crystalline CONASH production is essential forunveiling their inherent conductive properties and enhancingtheir performance by minimising the effect of amorphous phases,boundaries, and defects, studies have focused on the synthesis ofhighly crystalline or single-crystalline samples. Several studieshave reported the successful synthesis of highly crystalline CON-ASHs and their conductive properties were also investigated.11–14Metal-hexaaminobenzene (MHAB) nanosheets are attractiveconductive CONASHs that can be employed as cathodea Research Institute for Science and Technology, Tokyo University of Science, 2641Yamazaki, Noda, Chiba, 278-8510, Japan. E-mail: h-maeda@rs.tus.ac.jp,nisihara@rs.tus.ac.jpb Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho,Sayo-gun, Hyogo 679-5198, Japanc Faculty of Fibre Science and Engineering, Kyoto Institute of Technology,Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japand RIKEN SPring-8 Centre, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5148, Japane Research Centre for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japanf Graduate School of Science and Technology, Tokyo University of Science, 2641Yamazaki, Noda, Chiba, 278-8510, Japan† Electronic supplementary information (ESI) available: Experimental details;GIXS of bare Au/glass substrate; photographs of prepared electrodes; CV ofelectrolyte solution for NiHAB synthesis; Raman and XP spectra of NiHAB-Aand NiHAB-B; CV of NiHAB-B; CVs of electrolyte solutions for NiHAB-C, NiHAB-D,and NiHAB-E synthesis; optical microscope images and Raman and XP spectra ofNiHAB-C, NiHAB-D, and NiHAB-E; Roughness analyses, Raman, and XP spectra ofNiHAB-F and NiHAB-G; two-dimensional scattering image and diffraction patternof NiHAB-F. CV of electrolyte solution for CoHAB synthesis; Raman spectra andtwo-dimensional scattering patterns of CoHAB-A and CoHAB-B; cyclic voltammo-gram of electrolyte solution for CuHAB synthesis; characterization of CuHAB-Band CuHAB-F; CVs of CoHAB-B and CuHAB-F. See DOI: https://doi.org/10.1039/d3nj05650cReceived 8th December 2023,Accepted 28th February 2024DOI: 10.1039/d3nj05650crsc.li/njcNJCPAPEROpen Access Article. Published on 12 March 2024. Downloaded on 4/2/2024 11:40:31 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0000-0001-9552-7478https://orcid.org/0000-0002-7531-6865https://orcid.org/0000-0001-9710-2692http://crossmark.crossref.org/dialog/?doi=10.1039/d3nj05650c&domain=pdf&date_stamp=2024-03-12https://doi.org/10.1039/d3nj05650chttps://doi.org/10.1039/d3nj05650chttps://rsc.li/njchttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3nj05650chttps://pubs.rsc.org/en/journals/journal/NJhttps://pubs.rsc.org/en/journals/journal/NJ?issueid=NJ0480146082 |  New J. Chem., 2024, 48, 6081–6087 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024materials in secondary ion batteries,15–17 electrode materialsfor supercapacitors,18–20 and chemiresistive sensors.21,22 Theycan be also employed as electrocatalysts for the hydrogenevolution reaction (HER)23 and for the oxygen evolution24 andoxygen reduction reactions.25 Furthermore, they can be quicklyand easily synthesised by the electrochemical oxidationprocesses23,26 (Fig. 1). Our group has previously reported thesynthesis of MHABs (M = Co, Ni, Cu) by using an electroche-mical oxidation method by applying an oxidation potential for10–180 s on a working electrode in an electrolyte solution of ametal ion and HAB.23,26The ability to directly employ the MHAB-modified electrodesas electrocatalysts for a HER was also investigated. Neverthe-less, the MHABs formed on the electrodes were rough and lowcrystalline since controlling the uniformity of the film wasdifficult under these conditions. It should be noted that wesucceeded in controlling the crystallinity of HAB-based CON-ASHs for copper by manipulating the reaction conditions of theliquid–liquid interfacial synthesis.27This study investigated the optimum reaction conditions(applied oxidation potential and concentrations of the chemicalreagents) for the synthesis of crystalline NiHABs on goldelectrodes by using an electrochemical oxidation method.Optical microscopy (OM), atomic force microscopy (AFM),Raman spectroscopy, X-ray photoelectron spectroscopy (XPS),and 2D grading-incidence X-ray scattering (GIXS) revealed theanisotropic formation of NiHABs in a face-on orientationmanner on the electrodes. Furthermore, the anisotropic synth-esis of CuHAB on a gold electrode was also assessed under theoptimised conditions. Finally, a possible mechanism of theMHAB growth process using the electrochemical oxidationmethod was also proposed.Results and discussionOxygen in air works as an oxidizing agent and advances MHABformation before the oxidation potential is applied. To preventthis oxygen-assisted NiHAB formation in air, the electrochemicalsynthesis of NiHABs on a gold electrode was carried out in an Ar-purged glovebox with solvents degassed with nitrogen. Ag/AgCl,Pt, and Au/glass employed as the reference, counter, and workingelectrodes, respectively, were immersed in an aqueous ammonia(conc. NH3 aq./H2O = 1 : 5, ca. 2.3 M) solution in the presence ofNaBF4, Ni(OAc)2, and HAB�3HCl (Fig. 1b). Initially, we investigatedthe optimum applied potential during NiHAB formation since ourgroup has recently reported that the crystallinity of CuHABsynthesised at a liquid–liquid interface in the presence of a weakoxidising agent was enhanced.27 It was thus hypothesised thatapplying a low oxidation potential could induce the formation ofmore crystalline NiHABs. To investigate the effect of appliedoxidation potential precisely, we employed the chronoamperome-try technique to only apply the targeted potential to the workingelectrode instead of the cyclic voltammetry technique which hasbeen often used in electrochemical polymerization.28–30 Althoughan oxidation potential of +0.56 B +0.58 V vs. Ag/AgCl was appliedto the working electrode in previous studies,23,26 we hypothesisedFig. 1 (a) Schematic representation of the synthesis of MHAB CONASHs.(b) Schematic illustration of the electrochemical synthesis of NiHAB ongold electrodes.Table 1 Synthetic conditions for MHAB-X (M = Ni, Co, Cu, X = A–G) electrochemical synthesisCondition Applied potential (V vs. Ag/AgCl) BaseBase concentration(mol L�1)Metal ion concentration(mmol in 5 mL)HAB concentration(mmol in 5 mL)A High NH3 2.3 6 4NiHAB: +0.58CoHAB: +0.21B Low NH3 2.3 6 4NiHAB: �0.10CoHAB: �0.10CuHAB: �0.20C Low NH3 0.1 6 4D Low TEAa 0.1 6 4E Low EDAb 0.1 6 4F Low NH3 2.3 0.6 0.4G Low NH3 0.1 0.6 0.4a Triethylamine. b Ethylenediamine.Paper NJCOpen Access Article. Published on 12 March 2024. Downloaded on 4/2/2024 11:40:31 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3nj05650cThis journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024 New J. Chem., 2024, 48, 6081–6087 |  6083that a potential of �0.10 V vs. Ag/AgCl, which corresponds to thepotential at the end of the first oxidation peak at �0.27 V vs. Ag/AgCl, is enough to advance the oxidation process in the NiHABformation (Fig. S2, ESI†). NiHAB-A and NiHAB-B were prepared byapplying oxidation potentials at +0.58 V and �0.10 V vs. Ag/AgClfor 180 s, respectively (Table 1 see also the ESI† for furtherexperimental details). In the case of NiHAB-B, the colour of thelower area (ca. 20 mm � 5 mm) of the Au electrode changed dueto the NiHAB formation while NiHAB-A did not show an obviouscolour change (Fig. S3, ESI†). The OM images of NiHAB-A andNiHAB-B revealed clear differences between the two modifiedsurfaces (Fig. 2a and b), where black domains were sparselypresent on the NiHAB-A surface, while the NiHAB-B surface wascovered with a grey film and exhibited smaller black domains. Thehigh-resolution AFM images at the boundary of the unmodifiedand NiHAB-modified surfaces revealed topographical differences(Fig. 2c and d). NiHAB-A showed many spherical structuresexpected to be NiHAB domains with heights of approximately25 nm, while NiHAB-B showed a uniformly modified area with athickness of B6 nm, corresponding to B20 layers of NiHAB.These results confirmed the suitability of a low oxidation potentialfor the uniform formation of NiHAB on an electrode. The Ramanspectra collected at the black domains on NiHAB-A and the greyfilm area on NiHAB-B exhibited peaks at 440, 615, and 1510 cm�1which are in accordance with the reported spectra of NiHAB, thusindicating the formation of NiHAB at both oxidation potentials(Fig. S4a, ESI†).16 However, the Raman spectra of the areasbetween the black domains did not show any significant peaks,thus indicating the absence of NiHAB and exposure of the goldsurface (Fig. S4b, ESI†). The results show that electrochemicaloxidation at a high oxidation potential caused the spot formationof NiHAB on the electrode surface, thus inducing a rough surfacemorphology, which is in accordance with the results of a previousstudy.26 XPS revealed peaks at 854.7, 872.3, and 397.8 eV attrib-uted to those of Ni 2p3/2, Ni 2p1/2, and N 1s, respectively, whichwere in accordance with previously reported values suggestingthat the nickel centre is divalent (Fig. S5, ESI†).23,26 The calculatedNi : N atomic ratios of NiHAB-A and NiHAB-B were 1 : 4.9 and1 : 5.4, respectively. These values suggest the presence of slightlyexcess nitrogen atoms compared with the ideal atomic ratio forthe bis(diamino)nickel complex, Ni : N = 1 : 4. The absence of Na,Cl, and F, the components of HAB�3HCl and NaBF4, indicatedthat NiHAB-A and NiHAB-B were in the neutral (zero-valence)states, and that these elements were present neither in thecationic nor in the anionic form (Fig. S5, ESI†).The crystallinities of NiHAB-A and NiHAB-B were evaluatedby GIXS (Fig. 3a and b). The two-dimensional scattering imageand XRD patterns obtained by integrating the scatteringpattern of NiHAB-A did not exhibit any significant signal whichcan be attributed either to its amorphous nature or very lowcrystallinity (Fig. 3a). NiHAB-B produced two spots in the in-plane direction and one spot in the out-of-plane direction inthe scattering image which can be attributed to the XRD peaksat 51 and 17.51, respectively (Fig. 3b). Upon their comparisonwith the simulated XRD pattern of NiHAB in the eclipsed (AA)stacking structure with the unit cell parameters P6/mmm, a =13.01 Å and c = 3.25 Å, these peaks were attributed to thediffraction from the (1 0 0) and (0 0 1) planes of NiHAB,respectively (Fig. 3c).26 Furthermore, the two-dimensionalscattering image reflected the crystal anisotropy of NiHAB-B.The XRD pattern of NiHAB-B extracted from the in-planeFig. 2 OM images of (a) NiHAB-A and (b) NiHAB-B, and AFM topographyimages and height profiles at the corresponding white lines of (c) NiHAB-Aand (d) NiHAB-B.Fig. 3 Two-dimensional scattering images obtained by GIXS measure-ments of (a) NiHAB-A and (b) NiHAB-B. (c) Diffraction patterns convertedfrom the scattering images. (d) In-plane (solid line) and out-of-plane(dashed line) diffraction profiles of NiHAB-B. (e) Schematic representationof NiHAB in a face-on orientation manner formed on an Au electrode.Gray blocks represent face-on-oriented crystalline NiHAB domains andthe green part represents NiHAB films formed on the electrode.NJC PaperOpen Access Article. Published on 12 March 2024. Downloaded on 4/2/2024 11:40:31 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3nj05650c6084 |  New J. Chem., 2024, 48, 6081–6087 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024direction of the scattering image showed only one peak corres-ponding to the (1 0 0) plane, while only the peak of the (0 0 1)plane was observed in the out-of-plane direction, suggesting ananisotropic NiHAB formation in a face-on fashion on a goldsurface at the oxidation potential of �0.10 V vs. Ag/AgCl (Fig. 3dand e). These results revealed that electrochemical oxidation atthe low potential allows an anisotropic formation of NiHABwith a thickness of several nanometres. To the best of ourknowledge, this is the first success of the direct orientationobservation of electrochemically synthesized HAB-based CON-ASHs on electrodes. The type of base and concentrations of thebase, nickel ion, and HAB were then optimised to obtain moreuniform and thinner NiHABs using the electrochemical oxida-tion method.The electrochemical activity of NiHAB-B was also investi-gated in a 1 M nBu4NPF6/CH3CN electrolyte solution (Fig. S6,ESI†). A redox couple of around 0.2 V vs. ferrocenium/ferrocene(Fc+/Fc) derives from the redox reaction of bis(diimino)nickelunits in NiHAB. A similar redox behaviour was also observed inNiHABs synthesized by the gas–liquid interfacial synthesis andthe electrochemical oxidation method at the high oxidationpotential.26To investigate the effect of the base type and concentrationon NiHAB formation, the NiHABs were synthesised with aqu-eous solutions of three different bases: NH3 (0.1 M), triethyla-mine (0.1 M, TEA), ethylenediamine (0.1 M, EDA) instead of the2.3 M NH3 solution (Table 1). The cyclic voltammograms ofthese reaction mixtures were measured to confirm the suit-ability of the oxidation at �0.10 V for the synthesis of NiHABunder these conditions (Fig. S7, ESI†). The appearance of thefirst oxidation peak at �0.265 V vs. Ag/AgCl in the NH3 solution(0.1 M) and two oxidation peaks at approximately �0.28 V and�0.18 V vs. Ag/AgCl in the TEA and EDA solutions, respectively,indicated that an oxidation potential of �0.10 V can sufficientlypromote the oxidation reaction for NiHAB formation. Hence,the electrochemical formation of NiHAB in the NH3 (0.1 M),TEA, and EDA solutions at �0.10 V for 180 s to prepare NiHAB-C, NiHAB-D, and NiHAB-E, respectively (Fig. S3, ESI†). NiHAB-Cexhibited a brown-coloured modified area of approximately20 mm � 5 mm while NiHAB-D and NiHAB-E did not showobvious colour changes in the electrode surfaces. The OMimages revealed that the NiHAB-C surface was covered with agrey film and small black domains, similar to the NiHAB-Bprepared under the higher base concentration conditions(Fig. S8a, ESI†). NiHAB-D exhibited black film domains with alateral size 410 mm, and NiHAB-E showed small film domainswith a lateral size of a few micrometres. The presence of thereported peaks in the microscopic Raman spectra measure-ments revealed NiHAB formation on NiHAB-C (Fig. S8b, ESI†).The black domains in NiHAB-D were characterised as NiHABwhereas the gold surface was exposed without film formation inthe remaining area (Fig. S8c, ESI†). The film observed onNiHAB-E showed a peak at approximately 2860 cm�1 attributedto the C–H stretching vibration, thus implying the formation ofan organic compound including EDA instead of NiHAB, whileother significant peaks were not observed in the remaining area(Fig. S8d, ESI†). It was thus concluded that TEA and EDA areunsuitable for the electrochemically assisted synthesis ofNiHAB because of their strong coordination abilities with metalions which compete for HAB, and thus prevent the complexa-tion between nickel ions and HAB.31 XP spectra of NiHAB-Cshowed Ni 2p3/2, Ni 2p1/2, and N 1s peaks at 855.2 eV, 872.2 eV,and 398.2 eV, respectively, with the Ni:N atomic ratio of 1 : 5.8,suggesting the formation of NiHAB with the slightly excessamount of nitrogen (Fig. S9a, ESI†). On the other hand, NiHAB-D and NiHAB-E gave weak peaks derived from Ni 2p and N 1s inthe XP spectra (Fig. S9b and c, ESI†). In addition, the respectiveNi : N atomic ratios calculated to be 1 : 32 and 1 : 13 were farfrom identical values. The Raman and XP spectra resultsindicate the NiHAB formation in NiHAB-C and the insufficientnanosheet formation in NiHAB-D and NiHAB-E. Hence, weconducted further topography and crystallinity evaluations onlyfor NiHAB-C.The AFM images at the boundary of the modified area andbare gold exhibited a film with a thickness of 10 nm in additionto the presence of spherical particles with heights exceeding10 nm on the film (Fig. 4a). These particles were estimated tohave overreacted with the NiHAB grown on the NiHAB film. Theappearance of in-plane scattering spots for the (1 0 0) and(2 0 0) planes and an out-of-plane scattering spot of the (0 0 1)plane in the 2D GIXS scattering image in addition to the XRDdiffraction peaks at 51, 101, and 17.51 confirmed the face-onanisotropic formation of crystalline NiHAB on the electrode(Fig. 4b and c).Although the optimisation of the reaction conditionsdescribed above revealed that the application of a low oxidationpotential in NH3 aq. with an electrolyte is suitable for theanisotropic growth of NiHABs on an electrode, black domains,considered as spotty-grown NiHABs, were still present onNiHAB-B and NiHAB-C. To prevent overgrowth after film for-mation, the electrochemical synthesis was performed by dilut-ing the nickel ion and HAB ligand concentrations ten times.NiHAB-F and NiHAB-G were thus prepared using ammoniaFig. 4 (a) AFM topography image and height analysis at the corres-ponding white line of NiHAB-C. (b) Two-dimensional scattering imageobtained by GIXS and (c) diffraction pattern converted from the scatteringimage of NiHAB-C.Paper NJCOpen Access Article. Published on 12 March 2024. Downloaded on 4/2/2024 11:40:31 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3nj05650cThis journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024 New J. Chem., 2024, 48, 6081–6087 |  6085solutions of concentrations 2.3 and 0.1 M, respectively (Table 1and Fig. S3, ESI†). The OM images revealed a uniform grey filmon NiHAB-F. Simultaneously, NiHAB-G still exhibited smallblack domains on its surface, thus indicating that the over-reaction of NiHAB was more effectively suppressed at high NH3concentrations and low metal ion and ligand concentrations(Fig. 5a and b). The AFM images showed that both samples hadmodified areas with thicknesses of 5 and 2 nm for NiHAB-F andNiHAB-G, respectively, thus corresponding to approximately 15and 6 layers of NiHAB, respectively (Fig. 5c and d). Domainsapproximately twice the height of the modified area were alsoobserved, implying the partial overgrowth of NiHAB. Further-more, the root means square values of the modified region andgold surface for NiHAB-F were 1.1 and 1.0 nm, respectively, and0.7 and 0.6 nm for NiHAB-G, thus indicating a uniform NiHABformation on the electrodes while maintaining the surfaceroughness (Fig. S10, ESI†). The Raman spectra of NiHAB-F andNiHAB-G exhibited peaks derived from the NiHAB framework(Fig. 1a and Fig. S11a, ESI†). Furthermore, the XPS resultsexhibited the respective peaks of Ni 2p3/2, Ni 2p1/2, and N 1swith the calculated atomic ratios of Ni : N of 1 : 3.8 for NiHAB-Fand 1 : 4.2 for NiHAB-G, thus corresponding to the expected ratioof the NiHAB structure (Ni : N = 1 : 4) (Fig. S11b and c, ESI†).These spectroscopic results confirmed the formation of theNiHAB structure. The 2D scattering image of NiHAB-F showeda scattering spot derived from the (0 0 1) plane of NiHAB in theout-of-plane direction (Fig. S12, ESI†). Although the scatteringfrom the (1 0 0) plane of NiHAB was not clearly displayedbecause of the thinness of the sample and the overlap with astrong background signal from the incident X-rays, the face-onarrangement of NiHAB on the electrode was maintained whenthe synthesis was performed at low-concentrations.The electrochemical synthesis of CoHAB and CuHAB underthe optimised conditions was also investigated. CoHAB hasbeen previously synthesized at 0.21 V vs. Ag/AgCl, and anapplication potential of �0.10 V vs. Ag/AgCl was chosen sincethe first oxidation peak of the electrolyte solution ended at thispotential in the cyclic voltammogram (Fig. S13, ESI†). Althoughthe Raman spectra of both CoHAB-A and CoHAB-B prepared onAu electrodes at 0.21 and �0.10 V vs. Ag/AgCl, respectively, werein accordance with those of a previous study,16 they did notshow any significant scattering patterns in the GIXS measure-ments, thus indicating their amorphous nature (Fig. S14, ESI†).A possible reason for the amorphous nature of CoHAB even inthe synthesis at the lower oxidation potential is that Co2+ is a d7transition metal ion that can assume not only square planar butalso other coordination geometries such as tetrahedral andoctahedral, which prevents the formation of two-dimensionalstructures. Hence, it is more difficult for CoHAB to assume ahigh crystallinity than NiHAB.The first oxidation peak of the reaction mixture of CuHABended at �0.20 V vs. Ag/AgCl (Fig. S15, ESI†) and the CuHABprepared at the corresponding potential (CuHAB-B) showed adark purple modified area and the GIXS measurementsrevealed an anisotropic scattering pattern suggesting a unitcell size for CuHAB (eclipsed stacking structure, P6/mmm, a =13 Å, c = 3.2 Å) (Fig. S3 and S16, ESI†). Simultaneously, a roughsurface morphology consisting of small grains of B60 nm inheight was observed in the AFM measurements (Fig. S16, ESI†).Performing the synthesis in an electrolyte solution with copperions and HAB concentrations diluted ten times resulted in theuniform formation of CuHAB-F (Fig. S17a, ESI†). The heightanalysis at the edge of CuHAB-F by AFM revealed a thickness ofapproximately 2 nm (Fig. S17b, ESI†). The Raman and XPspectra, in addition to the Cu and N atomic ratio valuecalculated from XPS (Cu : N = 1 : 4) and were in accordance withthe previously reported data (Fig. S17c and d, ESI†).23,27Furthermore, the scattering pattern of CuHAB-F revealed theanisotropic formation of CuHAB on the Au electrode in a face-on orientation manner (Fig. S17e, ESI†).The electrochemical activity of CoHAB-B and CuHAB-F werealso investigated (Fig. S18, ESI†). CoHAB-B showed two broadoxidation peaks at ca. 0.47 V and 0.11 V vs. Fc+/Fc and a broadreduction peak at �0.52 V vs. Fc+/Fc, suggesting the slow redoxreaction. The voltammogram of CuHAB-F exhibited severalbroad redox waves derived from a slow multistep redoxreaction.The MHAB (M = Ni, Cu) formation process in the face-onorientation manner on gold electrodes can be proposed asfollows (Fig. 6a). Initially, metal ions and HAB ligands form flatbis(diamino)metal moieties in the electrolyte solution becauseNi2+ and Cu2+ ion are d8 and d9 transition metal ions which havea preference to form square planar complexes, respectively. Inthis step, NH3 is expected to work as a competitor of HABdecreasing the formation rate of bis(diamino)metal moieties,especially under the condition of high NH3 concentration. Then,the formed complexes are adsorbed on a gold electrode in a flatorientation because of the interaction between the aromatic ringFig. 5 OM of (a) NiHAB-F and (b) NiHAB-G, and AFM topography imagesand height profiles at the corresponding white lines of (c) NiHAB-F and (d)NiHAB-G.NJC PaperOpen Access Article. Published on 12 March 2024. Downloaded on 4/2/2024 11:40:31 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3nj05650c6086 |  New J. Chem., 2024, 48, 6081–6087 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024and gold surface.32–34 The adsorbed bis(diamino)metal moietiesare converted to the bis(diimino)metal form by electrochemicaloxidation and deprotonation by a base, thus giving the firstlayer of two-dimensional MHAB. Further adsorption of bis-(diamino)metal moieties on the formed MHAB layer occursbecause of p–p interactions and the electrochemical oxidationof the attached layer can progress by the through-space electrontransfer via the overlapped p-conjugated orbitals between thestacked layers.9 These adsorption and oxidation processes arerepeated until the bis(diamino)metal moieties existing near theelectrode surface are consumed, which can explain the thinnerfilm formation observed at the low metal ion and HAB ligandconcentrations. The applied potential affects the growth rate ofMHAB. At a high oxidation potential, a large overpotential for theoxidation reaction of bis(diamino)metal moieties gives a largeelectron transfer rate. In this situation, the bis(diamino)metalmoieties adsorbed on the electrode and the MHAB domains areimmediately oxidized (Fig. 6b). Hence, the MHAB formation willproceed kinetically and rapidly at the highly reactive points suchas the edges of Au domains rather than the flat planes of Audomains, giving the rough topographies observed in Fig. 2a and c.Furthermore, a rapid supply of bis(diamino)metal moieties to theelectrode surface under the condition of a high concentration ofbuilding blocks also induces spot growth of MHAB. In contrast, asmall electron transfer rate at a low oxidation potential and slowsupply of bis(diamino)metal moieties under the condition of a lowconcentration of building blocks are preferred for a uniformedcrystalline MHAB formation because the film growth is expectedto be more thermodynamically controlled (Fig. 6c). As a result,under the optimal conditions (applying a low oxidation potential,high concentration of ammonia solution, and low concentrationof building blocks), a slow MHAB growth results in a face-onorientation manner on the gold electrode.ConclusionsThis study reported a facile and rapid electrochemical oxida-tion method for the synthesis of MHAB (M = Ni, Cu) with face-on anisotropy and nanometre thickness on an electrode. Theoptimal reaction conditions were investigated and the resultsrevealed that applying a low oxidation potential, high concen-tration of ammonia solution, and low concentration of buildingblocks were suitable for an oriented and uniform MHABformation owing to the effective depression of local growth.Furthermore, a mechanism for the anisotropic growth of MHABon the electrode was proposed. The results of this study can beapplied to other CONASHs composed of p-conjugated ligandswith amino ligating groups which can thus contribute tofurther unveiling the CONASH growth mechanism at liquid–solid interfaces. Furthermore, this simple anisotropic synthesismethod proposed for the synthesis of crystalline CONASHs cancontribute to the development of high-performance devices.Conflicts of interestThere are no conflicts to declare.AcknowledgementsThis work was financially supported by JSPS KAKENHI (GrantNumber: JP19H05460, 22K14569, and 22K05055) and the WhiteRock Foundation. Synchrotron radiation experiments wereperformed at BL05XU in SPring-8 (Hyogo, Japan). XPS measure-ments were supported by the Advanced Research Infrastructurefor Materials and Nanotechnology in Japan (ARIM) of theMinistry of Education, Culture, Sports, Science and Technology(MEXT) (JPMXP12-A-22-UT-0007 & JPMXP12-A-23-UT-0025).References1 W. Wang, W. Zhao, H. Xu, S. Liu, W. Huang and Q. Zhao,Coord. Chem. 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This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttps://doi.org/10.1007/s10904-023-02920-5https://doi.org/10.1007/s10904-023-02920-5http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3nj05650c