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Single-Crystalline NiFe-Hydroxide Nanosheets for Catalyzing Oxygen Evolution

29 Jan 2018

A group of scientists led by Prof. Shizhang Qiao has synthesized an oxygen evolution reaction (OER) catalyst combining the merit of low cost, excellent catalytic activity and long lifetime. This OER catalyst is composed of single-crystalline NiFe-hydroxide nanoflakes directly grown on nickel foams. The work has been published recently in ChemComm.

OER, the reaction of producing oxygen gas from water, is an indispensable component of electricity-generation devices using sustainable energy (e.g. fuel cells and photoelectrochemical water splitting cells). OER is usually the bottleneck limiting the overall energy conversion efficiency due to its sluggish kinetics and complex reaction pathways. As such, OER catalysts are needed to accelerate the OER reaction rate. Among the various OER catalysts, noble metal oxides stand out owing to their ultrahigh catalytic activity. However, the “shining” performance is dimmed by their high cost and short lifetime. Thus, obtaining alternatives with comparable OER catalytic activity as well as long-term stability is required to advance the utilisation of sustainable energy.

To address this challenge, the authors turned their attention to a low-cost transition metal, nickel. They developed a hydrothermal method using nickel foams to grow highly crystalline and near-vertically aligned NiFe-hydroxide nanosheets as OER catalysts (Figure 1a). The seamless integration between the hydroxide nanosheets and the nickel substrates reduces the contact resistance and facilitates interfacial electron transfer. The near-vertical orientation (Figure 1b) allows water molecules to fully contact the catalysts. Both of the characteristics render excellent OER catalytic activity. Additionally, the high crystallinity (Figure 1c) ensures the catalysts are robust enough to withstand extensive use without degradation in performance.

Figure 1. (a) The schematic illustration of the synthetic procedures of the NiFe-hydroxide [Fe-Ni(OH)2] nanosheets supported on nickel foams (NF). (b) The scanning electron microscopy image shows the near-vertically aligned nanosheets on a piece of nickel foam. (c) The transmission electron microscopy image reveals the crystallinity of the synthesized catalyst.

The NiFe-hydroxide nanosheets outperform most of the state-of-the-art OER catalysts, including those containing noble metal elements. Specifically, the nanosheets exhibit an onset potential of 1.497 V (Figure 2). The onset potential is a measure of the catalytic activity that equals the magnitude of potential required to yield a current density of 10 mA/cm² (when appreciable amount of oxygen gas is evolved). Outstandingly, the onset potential of the NiFe-hydroxide is the smallest among the catalysts selected for comparison.

Figure 2. The polarisation curves of different OER catalysts. The onset potential is marked by the dotted line in the inset.

The catalytic activity is also highly stable, with no loss in performance after at least 100 h of measurement. Interestingly, the onset potential further shifts to a lower value of 1.465 V after 100 h. The authors attributed this observation to a “self-activation” process that involves the formation and accumulation of nickel oxyhydroxide (NiOOH) on the surface of the nanosheets.

The hydrothermal method demonstrated here could be used to synthesize other cost-effective crystalline catalysts to develop catalysts for reactions beyond OER, such as hydrogen evolution and carbon dioxide reduction.

To find out more please read:

Free-Standing Single-Crystalline NiFe-Hydroxide Nanoflake Arrays: A Self-Activated and Robust Electrocatalyst for Oxygen Evolution

Jinlong Liu, Yao Zheng, Zhenyu Wang, Zhouguang Lu, Anthony Vasileff and Shi-Zhang Qiao

Chem. Commun. 2017, DOI: 10.1039/c7cc08843d

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Tuning the Size of Metal-Organic Framework Crystals by Decoupling Nucleation and Growth Processes

17 Jan 2018

A group of scientists from Tsinghua University in China have made a breakthrough in enhancing the controllability of the metal-organic framework (MOF) crystal size.

MOF represents a family of microporous crystals consisting of metal node-organic ligand coordination networks. They have shown potential in versatile applications including hydrogen storage, catalysis and electrochemical energy storage. Since their performance strongly correlates to the crystal size, synthesizing MOF crystals with tunable sizes and high yields is necessary to allow fundamental studies on the size-performance relationship. Unfortunately, the conventional size-controlling methods either require complex operations or exhibit low yields.

Now in ChemComm, Tiefeng Wang and coworkers demonstrate a method that can easily tune the size of MOF crystals. The mechanism is based on decoupling nucleation and growth processes. Unlike traditional strategies that mingle all metal precursors and organic ligands together in a solvent, this newly developed protocol initially mixes only a small portion of metal precursors with organic ligands. The metal precursors quickly coordinate with surrounding ligands to form small MOF clusters (the “nucleation” stage). Due to the limited supply of the metal precursors, the growth of these clusters into large crystals is unfavorable. Subsequently, the remaining metal precursors are introduced into the cluster-containing solution. The clusters then continue to grow into MOF crystals (the “growth” stage). Because the crystals develop directly from the small clusters (i.e. the seeds), the number of the seeds and the total concentration of the added metal precursors control the resulting MOF crystal size (Figure 1).

Figure 1. A schematic illustration of the growth of MOF crystals via a typical conventional method (top) and the reported decoupling method (down).

Using this method, the authors prepared a series of Pt@ZIF-8 MOF crystals (with sizes ranging from 45 nm to 440 nm) and investigated their ability to catalyze the reaction of 1-hexene hydrogenation. The catalytic activity of different sized crystals was quantified, with a linear correlation observed between the size and the activity (Figure 2).

Figure 2. The linear relationship between the Pt@ZIF-8 MOF size (r) and the hydrogenation reaction rate (D). r0 and D0 represent the size and the reaction rate of the smallest MOF (45 nm).

This reported approach is expected to be applicable for synthesizing MOF crystals other than Pt@ZIF-8. The availability of size-tunable MOFs will facilitate mechanistic studies in determining the optimal crystal size for different applications.

To find out more please read:

A General and Facile Strategy for Precisely Controlling the Crystal Size of Monodispersed Metal-Organic Frameworks via Separating the Nucleation and Growth

Xiaocheng Lan, Ning Huang, Jinfu Wang and Tiefeng Wang

Chem. Commun. 2017, DOI: 10.1039/c7cc08244d

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is an online blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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In-Vivo Visualization of Glucose Metabolism with a Two-Color Imaging Technique

20 Dec 2017

A group of scientists from Columbia University in United States have developed a state-of-the-art probing technique that can simultaneously map glucose uptake and incorporation activities in living cells.

Glucose is a ubiquitous “fuel” for most living organisms. Its metabolism, including uptake and incorporation, is vital to sustain the energy consumption of living organisms. Visualization of glucose metabolism is of critical importance for clinical diagnostics and fundamental biological researches. However, current imaging techniques are destructive to living cells, poorly resolved or incapable of probing uptake and incorporation at the same time.

Now in ChemComm, Prof. Min Wei’s research team demonstrates a breakthrough based on a vibrational imaging technique coupled with stimulated Raman scattering microscopy. This technique utilizes two glucose analogues to present the glucose metabolism, the ¹³C-labelled 3-O-propargyl-D-glucose (3-OPG-¹³C₃) for glucose uptake and the D7-glucose for glucose incorporation. Conventional Raman spectroscopy is unable to distinguish the aforementioned two species due to their overlapping Raman peaks. The authors addressed this challenge by labelling 3-OPG with ¹³C that exhibits a blue shifted Raman peak, thus separating it from the peak of D7-glucose. Decoupling of the two peaks allows in-vivo imaging and simultaneous observation of glucose uptake and incorporation in cells with sub-cellular resolution.

Figure 1 shows the two-color mapping images collected for human cancer cells, PC-3. The blue (panel a) and red (panel b) areas display the regions where glucose incorporation and uptake are taking place, respectively. The two images can be easily obtained by tuning the wavenumber of the incident light to match with corresponding Raman peak positions. Use of light with other wavenumbers results in the black image (panel c) containing virtually no colored regions, showing the excellent selectivity of the technique. Additionally, this approach differentiates between cancer cells and healthy cells by comparing the blue to red color intensity ratio.

This novel and versatile imaging technique is expected to serve as a useful tool in advanced bio-imaging and future cancer diagnostics.

Figure 1. Two-color mapping images of PC-3 cells highlighting the (a) glucose-incorporation regions (Raman peak: 2133 cm^-1) and (b) glucose-uptake regions (Raman peak: 2053 cm^-1). (c) An image collected with a wavenumber (2000 cm^-1) that does not match with either of the Raman peaks. Scale bar: 20 µm.

To find out more please read:

Two-color Vibrational Imaging of Glucose Metabolism Using Stimulated Raman Scattering

Rong Long, Luyuan Zhang, Lingyan Shi, Yihui Shen, Fanghao Hu, Chen Zeng and Wei Min

Chem. Commun. 2018, DOI: 10.1039/C7CC08217G

About the blogger:

Tianyu Liu obtained his Ph.D. in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a web blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Binder-free Integration of Bismuth Nanoflakes onto Nickel Foams for Sodium-ion Batteries

13 Dec 2017

A new type of bismuth-based electrode material for sodium-ion batteries has been synthesized. This electrode consists of bismuth metal nanoflakes seamlessly integrated onto nickel foams. The electrode contains no polymer binders, a crucial component required to retain the structural integrity of most battery electrodes. This binder-free feature improves the amount of charge being stored (i.e. capacity) at fast charging rates.

Sodium-ion batteries are attracting worldwide research efforts as electric energy storage devices, in addition to the prevalent lithium-ion batteries, due to the abundance of sodium. Similar to the preparation of other battery electrodes, fabricating sodium-ion battery electrodes generally requires binders, e.g. polyvinylidene fluoride (PVDF), to hold powdered electrode materials together and glue them to metal supporting substrates. However, the electrically insulating nature of the binders impedes fast electron transport between electrode materials and supporting substrates, consequently degrading the capacity of the batteries at fast charging rates.

Now in ChemComm, researchers from Nankai University & the Collaborative Innovation Center of Chemical Science and Engineering in China demonstrate a bismuth-based electrode material that does not involve a binder. This characteristic is realized by the in-situ growth of bismuth nanoflakes onto nickel foams through a solution-based replacement reaction (Figure 1). Because the nanoflakes grow directly from the nickel foam surface and firmly anchor onto nickel (Figure 2a), the resultant Bi/Ni composite can be directly used as an electrode. Specifically, the bismuth nanoflakes and nickel foam serve as the active material and supporting substrate, respectively.

The Bi/Ni composite exhibited excellent electrochemical performance. It achieved a high capacity of 377.1 mAh/g at a current density of 20 mA/g. Significantly, when the current density increased 100-fold, its capacity could still retain 206.4 mAh/g, which is more than half of the capacity obtained at 20 mA/g (Figure 2b). This outstanding capacity retention is a benefit of the binder-free characteristic that reduces the resistance of electron transport.

The authors then elucidated the working mechanism of the bismuth nanoflakes by in-situ Raman spectroscopy. They concluded that a two-step alloying process was responsible for the charge storage activity.

Figure 1. A schematic illustration showing the synthetic process of the binder-free Bi/Ni electrode. By inserting a piece of nickel foam into an ethylene glycol (EG) solution containing bismuth(III) nitrate, Bi3+ can replace Ni metal, be reduced to Bi metal and deposit on the Ni metal surface.

Figure 2. (a) A scanning electron microscopy image of the bismuth nanoflakes. (b) A plot showing the capacity of the Bi/Ni electrode at different current densities.

The successful synthesis of the binder-free electrode is expected to encourage future works on the design and synthesis of integrated electrode materials to advance the performance of sodium-ion batteries.

To find out more please read:

In situ Synthesis of Bi Nanoflakes on Ni Foam for Sodium-ion Batteries

Liubin Wang, Chenchen Wang, Fujun Li, Fangyi Cheng and Jun Chen

Chem. Commun. 2017, DOI: 10.1039/c7cc08341f

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Creating Defects to Enhance Oxygen Evolution Activity: A Case Study using CoFe Layered Double Hydroxides

23 Nov 2017

A group of scientists recently made a breakthrough in promoting the oxygen evolution activity of metal hydroxides. They developed a simple yet efficient strategy of immersing the metal hydroxides in diluted acid solutions.

The oxygen evolution reaction (OER) is a critical component for solar-driven water splitting that can sustainably acquire a clean fuel — hydrogen gas by solar energy. Certain noble metal oxides, such as iridium dioxide (IrO₂) and ruthenium dioxide (RuO₂), work extremely well for catalyzing OERs. However, their scarcity restricts their potential for large-scale applications. To address the cost bottleneck, inexpensive alternatives such as metal hydroxides are being investigated worldwide. Unfortunately, their performance cannot compete with IrO₂ or RuO₂, partly due to their limited active sites for oxygen evolution. As such, there is a current need to develop strategies to promote the oxygen evolution activity of these metal hydroxides.

Figure 1. A schematic illustration showing the structural change of CoFe layered double hydroxide after being immersed in diluted nitric acid. Acid soaking creates Fe, Co and O defects (represented by V_Fe, V_Co, and V_O in the illustration, respectively) as well as separating the hydroxide layers.

Recently, Zhou et al. from Hunan University and Shenzhen University in China, demonstrated an easy acid-etching method that is capable of significantly improving the oxygen evolution activity of CoFe layered double hydroxide. When the hydroxide comes into contact with the nitric acid, protons remove some Co, Fe and O atoms and leave behind vacancies. These vacancies are named defects (Figure 2). Oxygen gas prefers to evolve at these defects and thus the defective hydroxide exhibits improved oxygen evolution activity. In addition, the nitrate anions can intercalate in between the metal hydroxide layers and break adjacent layers apart, exposing a large number of defect-containing surfaces and thus further boosting the oxygen evolution activity (Figure 1).

Figure 2. Transmission electron microscopy images of untreated CoFe layered double hydroxide (a, b) and acid-etched CoFe layered double hydroxide (c, d). After etching, the hydroxide nanoplates crack (due to layer separation) and surfaces become rough (due to creation of defects).

This method is expected to be applicable for a wide range of other metal hydroxides. The simplicity and efficiency of this method could make oxygen evolution catalysts cost-effective for commercialization.

To find out more please read:

Acid-etched Layered Double Hydroxides with Rich Defects for Enhancing the Oxygen Evolution Reaction

Peng Zhou, Yanyong Wang, Chao Xie, Chen Chen, Hanwen Liu, Ru Chen, Jia Huo and Shuangyin Wang

Chem. Commun. 2017, 53, 11778-11781

About the blogger:

Tianyu Liu obtained his Ph.D. in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is an online blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Desalinating Seawater with Carbon “Sandwiches”

01 Nov 2017

A joint group of scientists from China, Japan and Australia recently made a breakthrough in water desalination. They designed and synthesized a multilayered electrode consisting of a graphene nanosheet sandwiched between two porous carbon particle layers. This “sandwich” electrode can be used for capacitive desalination to produce fresh water from seawater, and exhibited the highest desalination capacity among the reported graphene sheet-based electrodes.

Capacitive desalination is an emerging water desalination technique. It removes water-soluble salts, mostly sodium chloride, by applying an electric field to move the salts to the surface of electrodes. Because the amount of ions being removed is directly proportional to the surface area of the electrodes, using electrodes with abundant surface to electro-adsorb ions is critical for excellent desalination performance.

The researchers utilized graphene oxide (GO) and zeolitic imidazolate framework-8 (ZIF-8, a metal organic framework) as the two components (Figure 1a). When dissolved in water, ZIF-8 nanocrystals became attached to the surface of GO and completely covered both sides of the GO nanosheets. This process was driven by the coordination interaction between the two species. The formed ZIF-8/GO/ZIF-8 “sandwiches” were then annealed at near 1000 ^oC in nitrogen gas. The annealing step converted GO nanosheets and ZIF-8 nanocrystals into graphene nanosheets and porous carbon particle layers, respectively. Owing to the presence of pores on the surface of the yielded carbon particles, the carbon “sandwiches” had a high surface area of 1360 m²/g, much higher than that of the graphene sheets alone (150 m²/g).

Figure 1. (a) A schematic diagram displaying the key steps for the synthesis of the carbon “sandwiched” electrodes. 2-MeIM = 2-methylimidazole, a building block for ZIF-8. (b) The change of NaCl concentration collected for a “sandwiched” electrode (NC/rGO) and a graphene sheet electrode (rGO). When an electric field is applied, the concentration of NaCl starts to drop and reaches a plateau; When the electric field dissipates, the concentration of NaCl returns to its initial level. The salt concentration decreased to a much lower level with NC/rGO (red curve) than rGO (black curve).

The desalination capacity of the carbon “sandwich” reaches 17.52 mg/g, meaning 1 gram of the electrode can remove 17.52 mg of sodium chloride. Consistent with the enhanced surface area, the capacity of the “sandwich” is much higher than that of the graphene alone (Figure 1b). More significantly, the “sandwich” electrode outperforms all other previously reported graphene sheet-based electrodes in terms of the desalination capacity.

This work has greatly advanced the development of capacitive desalination, a promising and affordable technique to mass produce fresh water by desalting seawater.

To find out more please read:

High Performance Capacitive Deionization Electrodes Based on Ultrathin Nitrogen-doped Carbon/graphene Nano-Sandwiches

Miao Wang, Xingtao Xu, Jing Tang, Shujin Hou, Md. Shahriar A. Hossain, Likun Pan and Yusuke Yamauchi

Chem. Commun. 2017, 53, 10784-10787

About the blogger:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web blogger for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Synthesis of Tin Dioxide Nanotubes for Lithium-ion Batteries with “A Grain of Oxalate Salt”

26 Sep 2017

Preparation of tube-shaped electrode materials for lithium-ion batteries is a trending topic. Tubes with hollow cylindrical bodies allow exposure of the electrodes’ interior surface and can accommodate the large volumetric expansion commonly observed when lithium ions diffuse (either via intercalation or alloying) into the electrodes. The aforementioned two characteristics improve the specific capacity (a measure of how much electric energy one electrode can hold) and lifetime of electrodes.

Recently, the Mai research group from Wuhan University of Technology, China demonstrated a straightforward method for the synthesis of tin dioxide nanotubes as high-performance anodes for lithium-ion batteries. They adopted manganese(III) oxyhydroxide (MnOOH) nanowires as the sacrificial templates and immersed them in a batch of aqueous solutions containing tin(II) cations and oxalate anions (C₂O₄^2-). Afterwards, they warmed the mixture at 60 ^oC under constant magnetic stirring for 4 h and collected a white precipitate consisting of tin dioxide nanotubes. These nanotubes were then washed and coated with carbon thin films to improve their electrical conductivity and structural stability before being subjected to performance evaluations.

The presence of oxalate anions was crucial for producing the nanotubes with a well-defined shape. The function of these anions was revealed through a series of experiments. Oxalate anions first reduced MnOOH to manganese(II) cations and consumed protons in the vicinity of the MnOOH surface. The consumption of local protons increased the local pH and triggered precipitation and oxidation (by dissolved oxygen) of Sn²⁺ to tin dioxide. The two reactions proceeded, and eventually the MnOOH nanowires disappeared but tubes of tin dioxide formed around their surfaces (Figure 1). Samples obtained without oxalate salts were irregularly shaped.

Figure 1. (a) The schematic illustration of the synthesis steps of the tin dioxide nanotubes. (b) Scanning electron microscopy and (c) transmission electron microscopy images of the as-prepared tin dioxide nanotubes.

The carbon-coated tin dioxide nanotubes showed superior stability performance to bare tin dioxide nanotubes, as shown from the slower capacity-fading rate depicted in Figure 2a. In addition, carbon coating did not significantly sacrifice nanotubes’ charge-storage performance as both electrodes with and without a coating exhibited comparable capacity at all tested current densities (Figure 2b).

Figure 2. Performance comparison between carbon-coated tin dioxide nanotubes (SnO2@C NTs) and bare tin dioxide nanotubes (SnO2 NTs): (a) long-term stability and (b) capacity achieved at different current densities and charge-discharge cycle numbers.

To find out more please read:

Oxalate-assisted Formation of Uniform Carbon-confined SnO2 Nanotubes with Enhanced Lithium Storage

Chunhua Han, Baoxuan Zhang, Kangning Zhao, Jiashen Meng, Qiu He, Pan He, Wei Yang, Qi Li and Liqiang Mai

DOI: 10.1039/c7cc05406h

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Janus Particle Chains that Can Rotate, Dissipate and Recombine

23 Aug 2017

Janus is a god in ancient Roman mythology with two opposing faces. Its name has been brought to materials science to label particles with two or more distinct faces as “Janus particles”. Integrating multiple functions into one physical entity, Janus particles with various properties are extensively adopted as catalysts, electronic components and other applications.

Reporting in Chemical Communications, Bart Jan Ravoo and co-workers from Westfälische Wilhelms-Universität Münster in Germany developed a Janus particle colloidal assembly using a sandwich micro-contact printing method, a strategy reported previously by the same group. The Janus particle assembly consists of Janus particle chains, with the structure of one chain illustrated in Figure 1b. The authors first capped a batch of silica micro-beads with tri-block co-polymers on opposing ends (green parts shown in Figure 1). These copolymers serve as arms that extend and attach to functionalized magnetite (Fe₃O₄) nanoparticles. Two Janus particles will become magnetically glued together if they connect to the same nanoparticle at the two caps. This connection propagates and eventually forms Janus particle chains mainly consisting of two to four particles.

Figure 1. The schematic illustration depicting the structure of a Janus particle chain.

The artificial chains are responsive to an external magnetic field and photons with different wavelengths. Owing to the magnetic nanoparticles, the chains tend to arrange themselves according to the direction of the applied magnetic field. As shown in Figure 2a, the authors successfully rotated a chain by moving around a magnet.

Moreover, radiating the chains using UV light and green visible light will alter the chain configuration. The light sensitivity is rooted in a light-induced isomerization reaction of the co-polymer linkers: green light yields adhesive trans-isomers, whereas UV light produces cis-isomers that detach from magnetite. Hence, dissipation of the chains into individual Janus particles and then rejoining the particles together can be readily accomplished (Figure 2b).

Figure 2. Optical microscopy images showing (a) the magnetic and (b) the photo-switching properties of one Janus particle chain. All scale bars are 10 µm.

The demonstrated assembly is just the tip of the iceberg for Janus particle assemblies. As claimed by the authors, any acrylate in principal can be used to build the co-polymer linkers, resulting in colloidal assemblies with versatile features.

To find out more please read:

Self-assembly of Colloidal Molecules that Respond to Light and a Magnetic Field

Sven Sagebiel, Lucas Stricker, Sabrina Engel and Bart Jan Ravoo

DOI: 10.1039/c7cc04594h

About the blogger:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found here.

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Elucidating the Stability of Two Metal-Organic Frameworks toward Carbon Dioxide Sorption: A Comparative Study

14 Aug 2017

Metal-organic frameworks (MOFs) are coordination networks consisting of organic ligands and metal cores. They possess crystalline structures with metal complexes as the basic building blocks. These complexes assemble together and extend periodically to form the MOF structures. MOFs represent a family of highly porous materials with ultrahigh surface area (typically >1000 m² g^-1). Other attractive characteristics for MOFs are abundant active metal cores and unique porous structures with tunable pore width, useful for gas storage applications.

Capturing carbon dioxide has evolved into an intriguing research area, mainly due to environmental concerns triggered by high levels of greenhouse gas emissions. Some MOFs have already been explored as carbon dioxide storage materials and exhibited storage capability exceeding that of conventional absorbents (e.g. amines). Aside from the absorption capacity of carbon dioxide, the performance stability over prolonged operation periods is another figure of merit for MOF-based absorbents. However, there are limited studies in this area. Now for the first time, research groups led by Zeng and Zhao from National University of Singapore compared the performance stability of two representative MOFs, HKUST-1 and UiO-66(Zr). The unit cell of the two MOFs are shown in the inset of Figure a.

The two aforementioned MOFs were subjected to 500 carbon dioxide absorbing and desorbing cycles (Figure a). The carbon dioxide uptake amount of the two MOFs was gauged at specific cycle numbers (Figure b). Whilst HKUST-1 displayed a consistent decreasing storage capacity with increasing cycle number, the capacity of UiO-66(Zr) fluctuated but remained relatively constant. The results clearly indicate that HKUST-1 is more vulnerable and instable than UiO-66(Zr) during long-term working cycles.

The authors then investigated the mechanisms associated with the different stability performances. They first observed that the surface area of HKUST-1 decreased 24% to 1270 m² g^-1 after the stability test, whereas that of UiO-66(Zr) remained relatively intact. Moisture-induced structural collapse was excluded as a possible reason by carrying out a control experiment with ultra-pure and dry hydrogen gas. The authors then exploited multi-frequency atomic force microscopy and concluded that the difference in elastic modulus of the two MOF crystals played an important role in determining the corresponding MOF durability. UiO-66(Zr) has an elastic modulus (ca. 28 GPa) much higher than that of HKUST-1 (ca. 19 GPa), meaning that the former is more elastic than the latter. The high elasticity of UiO-66(Zr) can efficiently buffer the volumetric deformation caused by carbon dioxide absorption and desorption, preventing UiO-66(Zr) crystals from structural failure.

Figure. (a) Illustration of one cycle of the carbon dioxide absorption-desorption test. The inset shows where one carbon dioxide molecule resides in the corresponding MOFs. (b) The evolution of carbon dioxide uptake capacity (blue) and surface area (black) of HKUST-1 and UiO-66(Zr).

This work is expected to provide general guidelines on studying the structural stability of other MOFs with applications associated with gas storage and separation.

To find out more please read:

Structure Failure Resistance of Metal-organic Frameworks toward Multiple-cycle CO₂ Sorption

Zhigang Hu, Yao Sun, Kaiyang Zeng, and Dan Zhao

DOI: 10.1039/c7cc04313a

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Dissolving and Stabilizing the Precursor of Graphene in Organic Solvents

09 Aug 2017

Graphene, a two-dimensional single-layer graphite sheet, has aroused worldwide attention since the last decade. Its ultrahigh electrical and thermal conductivities, high mechanical stiffness and unique band structure have attracted extensive research efforts to develop graphene-based electronics, photonics, printing materials etc. Currently, among various strategies, the wet-chemical method still remains the most practical protocol for large-scale production of graphene in laboratories. This process in general involves two steps: the oxidative exfoliation of graphite, a.k.a. Hummers’ method, followed by reduction of the oxidized graphite sheets. Graphite oxide (GO), possessing a layered structure analogous to graphite but with rich oxygen functionalities (such as hydroxyl and carboxyl groups) anchored on each layer, is the product of the first step and thus serves as a precursor of graphene.

As the aforementioned wet chemical method is usually carried out in water, GO is primarily stored as aqueous-based colloidal dispersions. However, GO is reported to be chemically unstable in water since water molecules can react with electropositive carbons of GO. Though the reaction is not rapid, it partially removes the oxygen functionalities and breaks the carbon matrix, which eventually forces GO to precipitate and reduces the shelf life of the GO precursor.

Recently, Shi and coworkers from Tsinghua University have successfully prolonged the lifetime of GO by dispersing it in organic solvents. During the last purification step of the Hummers’ method, instead of using de-ionized water, anhydrous ethanol was utilized to rinse the GO product and obtain ethanol-wetted GO. X-ray diffraction revealed that ethanol molecules existed in the inter-layer space between adjacent layers. The ethanol-wetted GO could be readily dissolved in propylene carbonate, an organic solvent, for concentrations ranging from 0.1 mg mL^-1 to 40 mg mL^-1 (Figures a and b). More importantly, GO could be stored in propylene carbonate for at least a month without a colour change, whilst the colour of aqueous GO dispersion discernibly darkened (Figure c). Spectroscopic studies indicated that the colour change was attributed to the loss of oxygen functionalities. The results unambiguously prove that GO in propylene carbonate is much more stable than GO in water.

Figure. (a) Dissolution of ethanol-wetted GO in propylene carbonate. (b) GO colloidal dispersions with various concentrations. (c) Color evolution of GO dispersions (1 mg mL^-1) with water and propylene carbonate as solvents before and after storing for 28 days under ambient conditions.

Aside from propylene carbonate, dimethyl sulfoxide, ethylene glycol and N,N-dimethylformamide are solvents that can dissolve the ethanol-wetted GO. The successful stabilization of GO colloidal dispersions could ensure the steady production of graphene in laboratories, as well as reveal new opportunities to develop GO-based devices.

To find out more please read:

Organic Dispersions of Graphene Oxide with Arbitrary Concentrations and Improved Chemical Stability

Wencheng Du, Mingmao Wu, Miao Zhang, Guochuang Xu, Tiantian Gao, Liu Qian, Xiaowen Yu, Fengyao Chi, Chun Li and Gaoquan Shi

DOI: 10.1039/c7cc04584k

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web blog writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Chemical Communications Blog

Author Archive

Single-Crystalline NiFe-Hydroxide Nanosheets for Catalyzing Oxygen Evolution

Tuning the Size of Metal-Organic Framework Crystals by Decoupling Nucleation and Growth Processes

In-Vivo Visualization of Glucose Metabolism with a Two-Color Imaging Technique

Binder-free Integration of Bismuth Nanoflakes onto Nickel Foams for Sodium-ion Batteries

Creating Defects to Enhance Oxygen Evolution Activity: A Case Study using CoFe Layered Double Hydroxides

Desalinating Seawater with Carbon “Sandwiches”

Synthesis of Tin Dioxide Nanotubes for Lithium-ion Batteries with “A Grain of Oxalate Salt”

Janus Particle Chains that Can Rotate, Dissipate and Recombine

Elucidating the Stability of Two Metal-Organic Frameworks toward Carbon Dioxide Sorption: A Comparative Study

Dissolving and Stabilizing the Precursor of Graphene in Organic Solvents

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