Phase evolution and electronic properties of cryptomelane nanorods Pawel Stelmachowski * , Piotr Legutko, Tomasz Jakubek, Andrzej Kotarba Faculty of Chemistry, Jagiellonian University in Krakow, ul. Gronostajowa 2, 30-387 Krak ow, Poland article info Article history: Received 23 April 2018 Received in revised form 28 June 2018 Accepted 12 July 2018 Keywords: Cryptomelane Structure Stability Potassium Work function abstract Potassium functionalized cryptomelane-type manganese oxide (K-OMS-2) is gaining more and more attention due to its exceptional catalytic, electronic, magnetic and ion transport properties. It is therefore of great importance to have a general picture of the changes this material undergoes with the increase of the temperature in reducing and oxidizing atmospheres. Potassium cryptomelane was synthesized by the reflux method in the reaction of manganese(II) acetate and potassium permanganate. It was char- acterized by means powder X-ray diffraction and Raman spectroscopy at characteristic stages of high- temperature treatment, determined by means of thermogravimetric analysis. Moreover, the mobility of potassium and thus induced changes of electronic properties were determined by using temperature programmed potassium desorption experiments and by following work function changes at elevated temperatures. The presented results can be used as a guideline for potential application limits, where the operation temperature and atmosphere play a crucial role. © 2018 Elsevier B.V. All rights reserved. 1. Introduction Cryptomelane-type manganese oxide (OMS-2) materials are of particular interest due to their exceptional catalytic properties [1 ,2], and their potential use as gas sensors, electrodes, magnetic, and battery materials [3]. OMS-2 is a class of manganese oxides having a tunnel size of 0.46 nm, constructed from edge-shared double MnO 6 octahedral chains, part of which are corner-shared forming one- dimensional tunnels. Potassium is one of the preferred cations for maintaining the tunnel structure of OMS-2 (K-OMS-2). The potas- sium ions inside the tunnel sites or manganese ions within the polyhedral structure can be substituted with other inorganic cat- ions, such as Fe 2þ , Cu 2þ ,V 5þ , and Mo 6þ , with suitable sizes, to further modify the physicochemical properties of the material to better suit the specific application, Fig. 1 . Other modifications can be achieved by combination with different phases to form com- posite materials [4,5]. The preparative routes for K-OMS-2 include reflux, hydrothermal, solvent-free, high-temperature calcination, microwave-assisted and sol-gel with reaction time varying from hours to days [3]. The synthesis of cryptomelane nanocrystallites described as rods, needles or fibers were recently successfully conducted and tested for their catalytic properties [6e8]. Due to the abundance of a variety of manganese oxides, both containing potassium and simple Mn x O y type (Fig. 2), particular attention should be given to the thermal stability of the potassium cryptomelane-type manganese oxide, especially from the catalytic point of view. Some of the investigated catalytic reactions are being carried out in the low-temperature range, below 300  C[6], but other studies require much higher temperatures, up to 800  C[2,9]. Thus, the phase stability both in an inert and oxidative atmospheres is important to relate the observed surface effects to the specific manganese oxide structure. Specifically, total oxidation reactions are often carried out with the O 2 concentration in the range 5e10 vol%, with maximum temperatures up to 800  C[2, 10, 11]. On the other hand, reducing atmosphere is applied in preferential oxidation of CO (CO-PROX), with up to 30 vol% of H 2 , and in a se- lective hydrogenation with up to 60 vol% of H 2 [12, 13]. This report addresses the evolution of structural properties of the cryptome- lane type oxide. Thermal stability in various atmospheres, such as air, argon, carbon monoxide, and vacuum is investigated, and mobility of potassium ions at high temperatures is followed by work function changes and thermal desorption experiments. 2. Experimental 2.1. Synthesis The reflux method was used to obtain the cryptomelane phase according to [2, 14]. Briefly, 11 g of Mn(CH 3 COO) 2 was dissolved in 40 ml of distilled water and the pH was adjusted to 5 by adding * Corresponding author. E-mail address: pawel.stelmachowski@uj.edu.pl (P. Stelmachowski). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2018.07.147 0925-8388/© 2018 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 767 (2018) 592e599