Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Improvement of the Heat Resistance of Prussian Blue Nanoparticles in a Clay Film Composed of Smectite Clay and ε‑Caprolactam Kenta Ono,† Takashi Nakamura,*,† Takeo Ebina,† Manabu Ishizaki,‡ and Masato Kurihara‡ †
National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake Miyagino-ku, Sendai, Miyagi 983-8551, Japan Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
‡
S Supporting Information *
CO poisoning is improved, and electrochemical reaction can be promoted.19,20 To address these issues, we focused on the use of clay, which has desirable properties such as a strongly layered molecular structure,21,22 the availability of intercalation sites between nanocrystals,23 thermal stability,24 and impermeability to gas.25,26 Recent studies have demonstrated that composite films consisting of a smectite clay and an organic polymer offer attractive degrees of flexibility and impermeability.27,28 In this Communication, we report the fabrication of large, relatively robust PB/clay composite films by mixing PB NPs, smectite clay, and an aqueous solution of the organic compound εcaprolactam. We investigated the structure and thermal properties of these composite films, which dramatically raise the oxidation temperature of the PB NPs. PB has a formula of Fe4[Fe(CN)6]3·xH2O (x = 10−15) and a molecular structure consisting of a three-dimensional framework of FeIICNFeIII. In bulk, PB is an insoluble solid consisting of aggregated NPs of approximately 10 nm diameter. We have previously reported that these NPs can be dispersed in water by surface modification of the [Fe(CN)6]4− ion via the FeIIIOH2 sites (Figure 1a).13,29 This chemical modification has improved the compatibility of PB NPs with the smectite clay phase for the preparation of PB/clay composite films. The resulting flexible, self-standing film contained 21 wt % PB NPs and could be fabricated at the size of an A4 paper (Figure 1b).
ABSTRACT: Prussian blue (PB) is limited in its application by its breakdown at elevated temperatures. To improve the heat resistance of PB, we prepared a composite film comprising PB nanoparticles (NPs), smectite clay, and an organic compound. The composite film had a microstructure in which PB NPs were intercalated between smectite/organic compound layers. The predominant oxidation temperature of the PB NPs in the composite film was around 500 °C in air, higher than the oxidation temperature of bulk PB in air (250 °C). This improvement in the oxidation temperature may be due to the composite film acting as a barrier to oxygen gas. These results indicate the effectiveness of clay materials for the improvement of heat resistance for low-temperature decomposition compounds, not only PB but also other porous coordination polymers.
F
unctional nanoparticles (NPs) used in technological applications are commonly fabricated, in combination with base materials, as films that can be shaped into appropriate forms. Composite films combining NPs and polymers are widely used to develop membranes that retain the function of the NPs.1−4 Research into NPs consisting of porous coordination polymers has attracted attention because their tailor-made structures offer advantages for control of their properties.5−7 Prussian blue (PB) is a classic porous coordination polymer, used in multifunctional materials for industrial applications based on electrochromism, ionic conduction, adsorption, and catalysis.8−12 It is a promising candidate for a component of composite film materials because its NPs can be transformed into dispersions by chemical modification of its surface coordination sites.13 In recent years, functional composite films incorporating PB NPs have been fabricated in combination with graphene14 and organosilica.15 Nevertheless, two issues have hampered applications: First, composite films including PB have been limited to dimensions no greater than 5 cm. Second, PB undergoes thermal oxidation in an air atmosphere at 250 °C.16 Recent research has underlined the need for improved high-temperature performance of materials. For example, fuel cells employing high-temperature proton-exchange membranes (HT-PEMs) are expected to be used at an operating temperature of 100−300 °C.17,18 With improvement in the operating temperature of a protonexchange-membrane fuel cell, electrode catalyst tolerance to © XXXX American Chemical Society
Figure 1. (a) Schematic drawing of the experimental procedures. (b) Photograph of the PB/clay composite film showing its flexibility. Received: March 16, 2018
A
DOI: 10.1021/acs.inorgchem.8b00707 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 2. (a) XRD patterns of clay and PB/clay composite films. PB data (PDF card no. 00-052-1907) are also shown for comparison in the bottom plot. (b) Schematic cross section of the PB/clay composite structure and detail of the smectite/organic compound layer.
oxide (Figure S4). The conversion of PB NPs to iron oxide is attributed to air oxidation.16 In our TG−DTA evaluation of the composite film, exothermic reactions at 340 and 400 °C were attributed to the polymerization and combustion of εcaprolactam, respectively (Figure 3a), and an exothermic peak
Figure 2a shows the X-ray diffraction (XRD) patterns for the composite film and a clay film fabricated without NPs. For the clay film, peaks at 2θ = 6.01°, 17.8°, 24.0°, and 30.0° were attributed to the basal spacing of the smectite/organic compound layers. For the PB/clay composite film, peaks were observed at 2θ = 6.02°, 17.9°, 23.8°, 29.9°, and 35.4°. Most of those peaks were common to both samples. The smectite/ organic compound layers in the PB/clay composite film had a basal spacing of 1.47 nm, as derived from the (001) plane at 2θ = 6.02°.27 The smectite/organic compound layer thickness, corresponding to approximately 10 repeating units of smectite platelets and ε-caprolactam, was estimated at 15.4 nm from the full width at half-maximum (fwhm) of the observed peaks using Sherrer’s equation (Figure S1). The XRD peak at 2θ = 35.4° in the PB/clay composite film, attributed to the (400) plane of PB,30 indicates that PB NPs were incorporated in the clay film. Scanning electron microscopy (SEM) images of the composite film intercalated or the absence of PB in the cross section (Figure S2) showed that PB NPs were only intercalated between smectite/organic compound layers. Referring to previous work on the smectite clay orientation mechanism,21,24 we propose the following sequence of interactions between smectite/organic compound layers and PB NPs during formation of the composite. Both the partially stacked smectite platelets and PB NPs, which have negatively charged surfaces in water, are dispersed during the initial stage of fabrication because of their mutual electrostatic repulsion. The PB NPs then become intercalated between the layers of smectite platelets during the drying process with the evaporation of water molecules and the integration of smectite/organic compound layers (Figure 2b). We compared the thermal properties of the PB NP powder and PB/clay composite film by using a thermogravimetric and differential thermal analysis (TG−DTA) technique. The PB NP powder exhibited weight loss, with an exothermic peak at 250 °C, due to the decomposition and oxidation of PB (Figure S3). In Fourier transform infrared (FT-IR) spectroscopy measurements, the absorption band at 2055 cm−1 attributed to the cyano group in PB NPs disappeared above 250 °C; at the same time, an absorption band emerged at around 500 cm−1, derived from iron
Figure 3. TG−DTA results for heating of (a) clay and (b) PB/clay composite films from room temperature to 800 °C in air (heating rate 10 °C/min). The gray area signifies an exothermic peak with 5.6% weight loss by the decomposition and oxidation of PB NPs.
with 5.6% weight loss at around 500 °C (Figure 3b) was attributed to the decomposition and oxidation of PB NPs. The TG−DTA profile of the PB NP powder showed a 29.4% weight loss at 250 °C due to the decomposition and oxidation of PB NPs (Figure S3), whereas in the composite film, a 26.7% weight loss at around 500 °C corresponded to the loss of 21 wt % PB NPs contained in the film. The crystal structures of the clay and PB/clay composite films before and after heating for 1 h in air at different temperatures from 150 to 500 °C were evaluated by XRD (Figure 4a). The PB NPs contained in the composite film were not decomposed and oxidized below 350 °C, as shown by the persistence of the reflection at 2θ = 35.4°. When the film was heated above 500 °C, the 2θ = 35.4° reflection disappeared and a new reflection at 2θ = B
DOI: 10.1021/acs.inorgchem.8b00707 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 4. (a) XRD patterns (including details of the 2θ = 35.4° peak for PB) and (b) FT-IR spectra of PB/clay composite films heated in air at four temperatures.
self-standing, flexibility, and high gas-barrier properties. Our results may contribute to applications for the HT-PEM fuel cell of composite films including PB, and perhaps other porous coordination polymers, for which low thermal stability and small area have limited its practical use.
35.8° emerged that it is attributed to the (311) plane of a typical γ-Fe2O3.16 The NPs in the PB/clay composite films before and after heating at four temperatures in an air atmosphere had average crystallite sizes of 8.8, 8.7, 9.2, 5.6, and 8.3 nm, respectively. The values were estimated from the fwhm of the observed peaks by using Sherrer’s equation. No crystal growth of those NPs in the PB/clay composite films caused by heating was observed (Figures 2a and 4a). The intense absorption peak at around 2050 cm−1, attributed to the C−N bond with iron atoms in the PB crystals in the films, was observed up to 350 °C from the FT-IR measurements (Figure 4b). The decomposition and oxidation of PB was also apparent in the color of the composite film, which was blue up to 350 °C and then turned brown, signifying the presence of iron oxide, at 500 °C. The blue color of the heated PB/clay composite films persisted to temperatures higher than 350 °C in air, much higher than the decomposition temperature of bulk PB in air (250 °C) and comparable to its decomposition temperature in an inert atmosphere (370 °C).31,32 Nevertheless, the edges of the specimens began to change from blue to brown above 300 °C (Figure S5). We infer that PB NPs at the edge and surface of the film were exposed to oxygen and oxidized after the decomposition of PB. This visual evidence corroborates the XRD, FT-IR, and TG−DTA evidence of the partial decomposition and oxidation of PB NPs (Figures 3 and 4). PB NPs are transformed into iron oxides by thermal oxidation at 250 °C in air. In this study, PB NPs in the composite film underwent thermal oxidation at around 500 °C in air, indicating that the incorporation of PB NPs into the clay film delays their oxidation. The XRD patterns of the basal spacing of the smectite/organic compound layers (Figures S6 and S7) suggest that the interlayer spacing of the smectite platelets shrinks at 200−250 °C, thus increasing its efficacy as a gas barrier. To explore this possibility, we measured the gas permeability of the PB/clay composite film at room temperature and 250 °C in oxygen gas and found that it decreased from 34.3 cm3·μm/m2· day·atm at room temperature to below the measurable limit (
