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Cite this: DOI: 10.1039/c4ta03789h
Ionothermal synthesis of cobalt iron layered double hydroxides (LDHs) with expanded interlayer spacing as advanced electrochemical materials† X. Ge, C. D. Gu,* X. L. Wang and J. P. Tu Traditional methods to synthesize layered double hydroxides (LDHs) with large interplanar spacing require the synthesis of precursor and subsequent ion exchange process. In this work, a one-step ionothermal strategy involving a “two-stage water injection” method is developed for the first time to synthesize cobalt iron layered double hydroxide (CoFe LDH) with 11.3 A ˚ interplanar spacing from an easily available ionic liquid analog, deep eutectic solvent (DES) system. The expansion of interplanar spacing is accompanied with the occurrence of tetrahedrally coordinated cobalt, and the insertion of molecules possibly being acetaldehyde, ethanol and biuret. In addition, more CO32 and fewer Cl and cyanate/ isocyanate anions were intercalated. The nominal formula of a CoFe LDH sample with exclusively 11.3 A ˚ (003) interplanar spacing was determined to be Co0.901Fe0.099C1.398O6.857N0.925H3.375Cl0.018. As a representative demonstration for its improved electrochemical activity, supercapacitor application and
Received 23rd July 2014 Accepted 21st August 2014
OER catalytic activity of the LDHs with different interplanar spacings are displayed, which indicates that
DOI: 10.1039/c4ta03789h
the increasing interplanar spacing is a powerful way to improve the electrochemical activity of LDH. The innovative methodology developed in this work should be widely applicable based on the mechanism
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we investigated, thus providing a fast, simple and cheap way towards LDH with large interplanar spacings.
Introduction Two-dimensional materials have attracted enormous attention since the development of graphene.1 Layered double hydroxides (LDHs) are a class of ionic lamellar compounds made up of positively charged brucite-like layers with compensating anions occupying the interlayer region.2 LDHs have a general formula of [M1x2+M3+x(OH)2][An]$zH2O, where M2+, M3+ represents divalent and trivalent cations, respectively. An is a nonframework charge compensating species.2 Specically, LDHs containing only divalent cations have been reported, including a-Ni(OH)2 or a-Co(OH)2 (also expressed as Ni LDH or Co LDH in the following part).3 The unique interlayer chemistry and structure-determined performance of LDHs in various applications (catalysis, ion-exchange, pharmaceutics, photochemistry, electrochemistry, additives, etc.) have attracted intensive research in recent years.4 Most previous studies have focused on the synthesis of LDHs with desired chemical composition, crystallinity, size, local structure, etc.5–13 However, the inuence of interlayer space has been rarely discussed. Recently, Wang
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. E-mail: cdgu@zju. edu.cn;
[email protected]; Fax: +86 571 87952573; Tel: +86 571 87952573 † Electronic supplementary 10.1039/c4ta03789h
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Lei and coworkers used Co LDH nanocones with different interlayer distances via the intercalation of various anions through ion exchange as models to investigate space-effect on electrochemical activities.14 Their results indicate LDHs with higher interlayer space show better performance in pseudocapacitor application. It is also suggested that the enhancement of pseudocapacitive performance is due to a larger quantity of accessible ions from electrolyte.14 However, developing a simple synthetic strategy towards LDHs with high interlayer space is challenging. Traditionally, synthesis of LDHs with high interlayer space can be achieved by preparing a LDH precursor followed by an ion-exchange route. However, this method is not only time consuming (hours, days or even weeks) due to the limit of ion diffusion,7,15 but is also complicated because inert atmosphere is required to avoid the interference of CO32, which has a very strong affinity to LDH layers.16 Furthermore, the preparation of precursor oen requires nitrate metal salts and HMT (hexamethylenetetramine) such that highly exchangeable NO3 intercalated precursor can be obtained.7,9 Gardner and coworkers reported a direct protocol to synthesize LDH with expanded interlayer space based on alcohol intercalation.17 The ˚ when obtained Mg, Al based LDH has a (003) distance of 9.2 A intercalated with methanol. Unfortunately, when intercalated with alcohols with longer chains (ethanol, propanol or butanol), LDH with larger (003) distance cannot be obtained because the alkoxy groups derived from larger alcohol molecules might be
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oriented with the hydrocarbon chain parallel to the LDH layers. This method also requires 3 days of reaction and repeated washing–redispersion steps to completely exchange out NO3 anions. In 2010, Takayoshi and coworkers introduced an innovative synthetic route to fabricate a-Co(OH)2 nanocones ˚ 18 Then, a general with a larger interlayer distance of 24 A. strategy was developed to fabricate bimetallic hydroxide (Co–Ni, Co–Cu, Co–Zn) nanocones.19 However, a two-step reaction and the specic surfactant/structure directing agents should be required for the bimetallic hydroxides.19 Very recently, the same group developed a high yield synthetic strategy to fabricate Co LDH.20 However, low concentration (0.01 M) of metal ion appears to be a requirement.18–20 The high-yield preparation method was achieved by using 500 mL solvent instead of increasing the concentration of metal ion.20 Therefore, it appears that the requirement for apparatus-simplicity (conventional heating in the open air) and time-saving cannot be satised at the same time.18,20 Thus, developing a fast, simple, surfactant free, high yield and economically feasible synthetic strategy to obtain LDHs with high interlayer space should be meaningful. Recently, our group developed an “all-in-one” ionothermal strategy based on a reactive and cheap ionic liquid analog, deep eutectic solvents (DESs). In the case of synthesizing a-Ni(OH)2 and a-Co(OH)2, the reaction is essentially completed in seconds.21,22 The type of DES we used is a choline chloride–urea mixture (CU). The idea is to heat the CU solution containing metal salts to high temperature (120–210 C) followed by injecting water into the solution. The hot-injection step can not only provide OH to precipitate the metal ions in seconds, but also quench the solution to obtain products, which are difficult to synthesize (e.g., a-Ni(OH)2 nanoower as small as 100 nm) in a traditional manner.21 This ionothermal protocol is attractive because the solution not only acts as reaction media, but also as a reagent.23,24 In this work, we proposed a facile and time-saving method to synthesize cobalt iron based LDH with very a large (003) ˚ in the hot CU based solutions via an elabdistance of 11.3 A orate two-step water injection process. The underlying mechanism for the formation of LDH with expanded interlayer space is investigated by analyzing its chemical composition, functional group of guest species and coordination condition of the layers. Signicantly, controlling the water injection process leads to various LDHs with two interlayer distances of ˚ As a representative demonstration of the 7.4 and 11.3 A. ˚ (003) enhanced electrochemical activity of LDH with 11.3 A space, we provide their electrochemical performance when used as a pseudocapacitor material and oxygen evolution reaction (OER) catalyst.
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interplanar spacing.25 Second, a modied two-stage hot injection step is developed to optimize the property of the solution (see Fig. 1 and Experimental section for details). The water content has been known to alter the property of deep eutectic solvent dramatically.23 When a small amount of water is injected, it has a profound inuence on the physiochemical property of the CU system, including coordination environment, decomposition rate, polarity, etc. However, most previous works used a complicated recrystallization process to reduce water content instead of employing it. In our case, 100 mL water was divided into different volumes and injected twice at an interval of 5 min into the CU system (see the schematic synthesis route in Fig. 1). Depending on the volume of water injected in each stage, four samples were prepared and denoted as 0–100 CoFe LDH, 5–95 CoFe LDH, 20–80 CoFe LDH and 50–50 CoFe LDH accordingly. The elaborate operation is important to controllably repeat the experiments. The design of the two-stage water injection protocol is based on the following considerations. First, rapid injection of water is key to avoid continuous nucleation, which would result in the defocus of size distribution of products.26 This aspect has been employed in our previous reports.21,22 Second, the introduction of a small amount of water could help control the decomposition process of urea and choline chloride portion in CU.23,27 Meanwhile, the molecular dynamic simulation indicates that CU forms a supermolecular structure via hydrogen bond.28,29 In addition, NMR study shows that enough water content (CU concentration in water
