Processing of α-Phase Metal–Phthalocyanine Powders by Interface Neutralization Method

Abstract

Metal phthalocyanines (MPcs; M = Co, Ni, Cu, Zn) with α-phase powders were obtained at the interface between hydrophilic and hydrophobic solutions starting from β-phase powders. This method was also applied to the deposition of a highly dispersed α-phase MPc on a carbon substrate, which showed high electrochemical activity for oxygen reduction. Metal phthalocyanines (MPcs) with a divalent transitionmetal ion at the center of a phthalocyanine ring are twodimensional 18π-electron aromatic porphyrin analogs, and have been extensively studied due to a specific catalytic property, and electrical and/or magnetic properties for application as electrocatalysts 1 and semiconductors in various electronic devices. 2Their properties are due not only to the types of the central metals 3 but also to the polymorphic structures of MPcs. 48 MPc forms various stacking constructions by ππ interaction between two MPc molecules. The distance between the central metals and the stacking angle of the molecular plane determined the electronic conductivities and the catalytic properties of MPc.It is well known that MPcs have two major phases, i.e., α-and β-phase. Although monomolecular MPc preferentially forms an α-phase by a large ππ interaction with overlapping areas of macrocyclic ligands, 8 α-MPc powders have not been applied to catalytic and electrochemical devices despite their low resistances 5 and high chemical activities, 9 because α-MPc powders are difficult to prepare by organic synthesis methods and the formed α-MPcs are easy to transform to a more stable β-MPc structure that is densely packed with large van der Waals' force. 5,10Thin films of α-MPcs have mostly been prepared with vacuum deposition systems because of their poor solubility in solvents. They have been used in electrical devices like solar cells 11 and chemical sensors. 12,13 However, an α-MPc thin film should be less than 40 nm in thickness for preventing transformation to β-phase.14 It seems that α-MPc is not stable under the condition of a dense molecular assemblage, in which the ππ interaction is dominant. If stable α-MPc powders are prepared without a dense molecular assemblage, they could provide high performance for electrocatalysts and materials. So far, α-MPcs have been prepared only under a vacuum condition; however, these methods are not appropriable for powder preparation. It was reported that α-MPc thin films could be prepared from an organic solution containing trifluoroacetic acid (TFA). 1519In this study, we tried to establish a new process for the preparation of α-MPc by a solution route, in which protonated MPcs are neutralized and deposited as powders at the interface between organic and aqueous solutions. We could also apply this method to deposit the MPc on carbon black for a highperformance oxygen reduction electrode.Commercially available MPc (M = Co, Ni, Cu, Zn) powders were used without further purification. CoPc and CuPc were purchased from Tokyo Chemical Industry, and NiPc and ZnPc were purchased from Kishida Chemical and Kanto Chemic...

Carbon Nanotube‐Based Non‐Precious Metal Electrode Catalysts for Fuel Cells, Water Splitting and Zinc‐Air Batteries

“…Shimizu et al. prepared metal phthalocyanines (MPcs; M=Co, Ni, Cu, Zn) with stable α‐phase powders from its β‐phase powders, and deposited them onto carbon supports . The fabricated catalysts showed a high ORR performance.…”