Synthesis of Heterostructure Materials Using Coordination Polymers and Metal-Organic Frameworks

Abstract

A coordination polymer, {[NiL4pyCo(SO4)2∙4H2O]∙2H2O} ([NiL4py](ClO4)2 = [Ni(C22N8H36)](ClO4)2) was synthesized by self-assembly of nickel-based macrocyclic compound, NiL4py(ClO4)2 in MeCN/H2O (MeCN = acetonitrile) mixture solution and cobalt sulfate hexahydrate in DEF (N,N-diethylformamide) solution. Resultant crystal was identified by single crystal X-ray diffraction analysis. The data revealed that the crystal composed of nickel macrocycle compounds, NiL4py, cobalt ions, sulfate anions and water molecules. This coordination polymer is a convenient precursor to synthesize complicated functional material, (Ni,Co)9S8/(Ni,Co)0@NSC (NSC is N,S-doped carbon) through thermal conversion of coordination polymer in solid state. This material was identified by diverse analysis such as X-ray powder diffraction (XRPD) pattern, X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and energy-dispersive spectrometer using transmission electron microscope (TEM-EDS) analysis. The results of analysis confirmed that (Ni,Co)9S8/(Ni,Co)0@NSC has both bimetallic sulfide nanoparticles and bimetallic phase nanoparticles embedded in N,S-doped carbon matrix. Interestingly, there were often ‘eyeball’ nanoparticles which have two phases in one particle. Black part of ‘eyeball’ was bimetallic phase and white part of ‘eyeball’ was bimetallic sulfide phase. In addition, overall synthetic mechanism of (Ni,Co)9S8/(Ni,Co)0@NSC was investigated. Firstly, nickel ions were reduced and produced nickel nanoparticles with small portion of cobalt because of reduction potential difference between two metallic species. Then, sulfidation occurred and produced sulfide phase which have same XRPD pattern with Co9S8. Furthermore, etching process to (Ni,Co)9S8/(Ni,Co)0@NSC offered pores in carbon matrix. Moreover, bimetallic property was expanded from nickel-cobalt system to nickel-copper system. Two kinds of products by conversion of nickel-cobalt coordination polymer and nickel-copper coordination polymer were estimated as electrocatalysts for hydrogen evolution reaction.

In addition, three isostructural metal-organic frameworks NicyclamBPDC, NiLethylBPDC and NiLpropylBPDC, ([Nicyclam](ClO4)2 = Ni(C10H24N4)](ClO4)2, [NiLethyl](ClO4)2 = [Ni(C12H30N6)](ClO4)2 , [NiLpropyl](ClO4)2 = [Ni(C12H30N6)](ClO4)2 , H2BPDC = 4,4`-Biphenyldicarboxylic acid) were synthesized by self-assembly of each macrocyclic compound and Na2BPDC in MeCN/H2O solvent system. Crystal structures of three MOFs were characterized by single crystal X-ray diffraction analysis. Even though they have same structure, pore aperture sizes of their one-dimensional pore channel were different. NicyclamBPDC has the largest and NiLpropylBPDC has the smallest pore aperture size. These pore sizes are suitable to be used as hydrogen isotope separation. Fortunately, NiLpropylBPDC had a chance to be measured by thermal desorption spectroscopy (TDS) in Max-Planck institute by using H2/D2 mixture gas. TDS spectra of NiLpropylBPDC showed that desorption maxima peak at around 130 K. This temperature range above liquid nitrogen is meaningful as a case which overcome cryogenic temperature limitation for hydrogen isotope separation. On the other hand, poor selectivity compared to cryogenic condition and low uptake of isotope are remained as challenge. Hence, one of the solutions for low uptake of isotope could be core-shell type heterostructure. NicyclamBPDC are selected as core due to the largest pore among the three MOFs. The others are selected as shell due to the proper pore aperture sizes. NicyclamBPDC was put into diluted solution of shell MOF to prevent homogeneous nucleation. As a result, ‘cotton swab’-like heterostructure crystal similar to core-shell was found during evaporation of shell MOF solution unexpectedly. Even though it was not homogeneous reaction with unreacted core MOF crystal, this result suggests a possibility to synthesize core-shell material for hydrogen isotope separation.