High Surface Proton Conduction in Nanostructured ZIF-8.

Muñoz-Gil Muñoz-Gil,Figueiredo Figueiredo

doi:10.3390/nano9101369

Abstract

The zeolitic imidazolate framework-8 (ZIF-8) combines a significantly high microporosity with an excellent thermal, chemical, and hydrothermal stability. Here, we demonstrated that ZIF-8 can display significant levels of protonic conductivity through a water-mediated surface transport mechanism associated to the presence of di-coordinated Zn ions revealed by X-ray photoelectron spectroscopy. A set of powders with particle sizes from 2.8 µm down to 80 nm studied by dynamic water vapour sorption analysis was used to demonstrate that water adsorbs predominantly in the micropore cavities of microcrystalline ZIF-8, whereas adsorption on the external surface becomes the dominant contribution for the nanostructured material. Impedance spectroscopy in turn revealed that the protonic conductivity of the nanocrystalline ZIF-8 was two orders of magnitude higher than that of the micron-sized powders, reaching approximately 0.5 mS·cm−1 at 94 °C and 98% relative humidity. Simple relations were derived in order to estimate the potential gains in water uptake and conductivity as a function of the particle size. This new strategy combining particle nanostructuring with surface defects, demonstrated here for one of the most know metal organic framework, is of general application to potentially boost the conductivity of other materials avoiding chemical functionalization strategies that in most if not all cases compromise their chemical stability, particularly under high humidity and high temperature conditions.

Highlights

Over the past two decades, metal organic frameworks (MOFs) have undergone rapid development due to the fact of their designable and tuneable structures–properties, which may present a wide range of important applications like gas storage, catalysis, and energy applications. [1,2,3,4,5,6]
The three compounds are single phase and the patterns can be indexed using a cubic system with space group I-43m demonstrating the typical sodalite (SOD) zeolite-type structure (Figure S1), in agreement with others [12,25] and confirming that zeolitic imidazolate framework of type 8 (ZIF-8) was successfully synthesized
We prepared a set of samples of ZIF-8 with average particle sizes of 80 nm, 1.1 μm and 2.8 μm to investigate the influence of particle size on ionic conductivity

Summary

Introduction

Over the past two decades, metal organic frameworks (MOFs) have undergone rapid development due to the fact of their designable and tuneable structures–properties, which may present a wide range of important applications like gas storage, catalysis, and energy applications. [1,2,3,4,5,6]. The zeolitic imidazolate framework of type 8 (ZIF-8) is an organic–inorganic hybrid material with the sodalite-type structure, where Zn (or Co) metal centres are tetra-coordinated by imidazolate linkers forming a three-dimensional network of micropores with an equivalent spherical diameter of 11.6 Å connected by narrow 3.4 Å apertures [12,13,14] This particular structure allied to the chemical composition are the reason for the exceptional thermal and hydrothermal stability, and the extraordinarily high specific surface area (typically in excess of 1500 m2·g−1) of these materials, which make them attractive in applications of storage [15,16], separation [17,18], and catalysis [19] of various chemicals. The design of MOFs with a hierarchical micro- and mesopore structure has recently been proposed to achieve proton transport using only changes in water vapour pressure, the demonstrated conductivity levels are rather low (less than 0.1 mS·cm−1) [10]

Methods

Results

Conclusion