Confined-space synthesis of hierarchical SnO₂ nanorods assembled by ultrasmall nanocrystals for energy storage

Abstract

A genetically modified bacterial surface not only serves as a confined environment for controlling the monodispersity of particle size, but also provides a carbon source in situ. The carbon coated SnO₂ composite exhibits good lithium storage properties as an anode electrode, which are due to the ultrasmall size of the particles, the carbon coating and mesoporous structure.

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Introduction

Living organisms are able to exquisitely synthesize biominerals under mild environments.¹ The natural process of biomineralization has evolved over billions of years and is inspiring scientists to synthesize new materials. Taking lessons from natural structure-forming processes of biominerals, bioprocess-inspired synthesis techniques have been well developed for fabricating advanced materials.^2,3 A bioprocess is temporally and spatially regulated under the control of biomolecules in confined environments.⁴ Species-specific proteins play dominant roles during the process of mineral crystallization, orientation as well as morphology.⁵ By taking advantages of properties of these mineral peptides or proteins, such as enzymatically catalyzed activity⁶ and supramolecular assembly characteristics,⁷ various artificial materials have been synthesized in vitro.^8,9 Sometimes uniformity and monodispersity of obtained inorganic materials is poor in solution. Efficient approaches were required to anchor desired proteins on particular matrix, which provided two-dimensional confined environments for growing uniform nanoparticles.^10–12 However, the complicated purification process of desired proteins and the tedious pre-treatment of matrixes are always the major obstacles for synthesizing a broad range of materials.

Bacterial cell surface display technique is a good solution to above problems.^13,14 It allows the direct expression of desired peptides or proteins on cell surface, which not only avoids the purification process, but also supplies a three-dimensionally confined environment. Although bacteria such as Erwinia herbicola and Bacillus subtilis without genetic manipulation can be used as biotemplates to direct the synthesis of materials,^15,16 the chemical compositions and physical environments are relatively stable and lack variety on the native surface of these living organisms. It limits the synthesis of a broad range of materials. Genetically modified cell surface provides more protein options as biotemplates for synthesizing materials and are more liable for accurately controlling the size of materials through protein modification.¹⁷ Therefore, one can take advantages of cell surface display technology to synthesize materials with exquisite structures.

Herein, a glutamate tripeptide (EEE) was displayed on the surface of Escherichia coli for synthesizing tin dioxides, which attracted considerable attention due to its various applications in energy and environment fields.^18–20 As a promising candidate anode materials for lithium-ion batteries (LIBs), SnO₂ has been widely studied because of the high theoretical specific capacity, low cost, and environmental benignity. However, SnO₂-based anodes suffer from a large volume expansion during charge–discharge process. It leads to active materials continuous pulverization, aggregation, and subsequent severe capacity fading.²¹ To solve these problems, effective strategies have been proposed to improve the structural stability. One strategy is to reduce particle size to nanoscale.^22,23 Nanostructured SnO₂ will shorten the diffusion pathway of Li⁺ ions. Another strategy is to add conductive carbon, which enhances the electron conductivity and reliefs the stress during charge–discharge cycling.^24–27 A facile and efficient approach to achieve both strategies is always been pursued. To the best of our knowledge, there is no reports of using living organisms to direct one-pot synthesis of carbon coated nanostructured SnO₂.

The present study aims to explore the cell surface display technology in synthesizing expectant nanostructured SnO₂ and address some key challenges in LIBs. The genetically modified E. coli cells was used to interact with tin source through electrostatic interaction and induce the deposition of tin dioxides precursor on the cell surface. Bacterial organics served as a carbon source in situ to avoid joining second phase during annealing in inert atmosphere. Effects of carbon coating on nanostructured SnO₂ were explored. The morphology and physical properties of acquired SnO₂ were examined. Electrochemical performance of as-prepared SnO₂ was investigated. The present study proposed a cell display based approach for synthesizing carbon coated hierarchical nanostructured SnO₂.

Experimental section

Display of the glutamate tripeptide on E. coli surface

The coding sequence of ice nucleation protein (INP)²⁸ and the glutamate tripeptide (EEE or 3E) was synthesized and cloned into the pET28a(+) vector (Novagen, Germany). The resultant plasmid is pET(INP-3E) that encode the gene of protein INP-3E (Fig. 1a). All constructs were confirmed by DNA sequencing. The expression procedure was based on pET System Manual (Novagen, Germany). Briefly, a single colony of E. coli BL21(DE3) harboring pET(INP-3E) was inoculated into Luria–Bertani (LB) medium containing 30 μg mL⁻¹ kanamycin with continuous shaking at 37 °C overnight. The cell suspension was inoculated into LB medium, followed by shaking at 37 °C until an optical density (OD₆₀₀) of 0.5–0.6 was reached. Protein expression was initiated by supplying with 1.0 mM of isopropyl-p-D-thiogalactoside (IPTG) and shaking at 30 °C for 3 hours. Cells were harvested by centrifugation at 8000g and 4 °C, re-suspended into buffer A (50 mM Tris–HCl, pH 8.5; 100 mM NaCl), and then disrupted by using a French press style cell disrupter JG 1A (Xinzhi Biotech Co., China). The total proteins of induced cells were analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

Fig. 1 Construction and expression of protein INP-3E. (a) Scheme of the plasmid vector of pET(INP-3E). (b) The amino acid sequences of INP-3E, and the sequences of 3E are highlighted with red. (c) Expression of INP and INP-3E, analyzed with 10% SDS-PAGE. Lane 1, molecular weight marker; lane 2 and 3, lysate of un-induced and isopropyl-p-D-thiogalactoside (IPTG) induced INP cells, respectively; lane 4 and 5, lysate of un-induced and IPTG induced INP-3E cells, respectively. Arrows point to the expressed INP or INP-3E.

Preparation of SnO₂ nanorods

The mineralization procedure is illustrated in Fig. S1.† Bacterial cell pellets were re-suspended in SnCl₄·5H₂O solution (0.1 g mL⁻¹). The mixture was incubated at 37 °C with gentle shaking for 24 hours. The products were harvested by centrifugation and washed three times with deionized water, then dried in lyophilizer. Calcination of the products was completed by annealing in a muffle furnace at a heating rate of 10 °C min⁻¹ and keeping at 600 °C for 4 hours in air. The carbon coated product was obtained by annealing in tubular furnace at a heating rate of 4 °C min⁻¹ and keeping at 500 °C for 30 minutes. After heating the product to 440 °C in air, Ar was pumped into tubular furnace.

Characterization and electrochemical measurements of SnO₂ nanorods

X-ray diffraction (XRD) patterns were acquired by using a Bruker D8 Advance with Cu Kα radiation (V = 40 kV, I = 40 mA) in the range of 20–80°. Surface morphology information was revealed by field emission scanning electron microscopy (FESEM) in a Hitachi S-4800 at 5 kV. High resolution transmission electron microscopy (HRTEM) examination was carried out with a JEOL JEM 2100F at 200 kV. Specific surface area was determined in an ASAP 2020M adsorption apparatus by using the Brunauer–Emmett–Teller (BET) method. The Raman spectrum was obtained in a Renishaw InVia Raman spectrometer with excitation by Nd:YAG laser operating at 785 nm. Thermogravimetric (TG) analysis was performed in a Netzsch STA449F3 at a heating rate of 10 °C min⁻¹ from 40 °C to 900 °C.

Electrochemical performance of the electrode was carried out in a CR2025-type coin cells with lithium metal foil as the counter and reference electrodes. To fabricate the working electrode, a N-methyl-2-pyrrolidone (NMP, Aladdin, China) slurry of active materials was mixed with Super P carbon black and polyvinylidene fluoride in a weight ratio of 7 : 1.5 : 1.5. After intensive grinding, the resultant slurry was pasted on Cu foil and vacuum dried at 120 °C for 12 hours. The mass loading of active materials for each coin cell is about 1.0 mg. Coin cells were assembled in an Ar-filled glove box, and 1 M lithium hexafluorophosphate in ethylene carbonate (EC)/diethyl carbonate (DEC) (1 : 1 v/v) was used as the electrolyte solution. Celgard polypropylene was used as the separator. The charge–discharge experiments were carried on using a LAND battery tester CT2001A with a voltage window of 0.01–3 V (vs. Li⁺/Li) at various current densities. Cyclic voltammetric (CV) test was performed in an electrochemical workstation with 0.01–3 V (vs. Li⁺/Li) at a scanning rates of 0.5 mV s⁻¹.

Results and discussion

To display tripeptide EEE on the surface of E. coli, the coding sequence of a glutamate tripeptide EEE was fused to the downstream of ice nucleation protein (INP) (Fig. 1a and b). INP is a stable carrier protein, which avoids the fused protein to be attacked during transport.²⁹ The tripeptide EEE was co-expressed with INP and anchored on E. coli surface. The expression of INP and INP-3E was demonstrated and visualized through gel electrophoresis analysis (Fig. 1c).

Since the glutamate tripeptide is negatively charged, it facilitates interactions between cell with INP-3E modified surface and positively charged tin dioxides precursors. After 24 hours of interactions between tin dioxides precursors and E. coli with INP-3E modified surface, SEM revealed a change in the roughness of the cell surface from smooth to rough, suggesting tin dioxide precursors deposition on cell surface (Fig. S2a†). High magnification SEM image showed that precursors deposited as intertwined ridges covering on the surface (Fig. S2b†). The mineralized E. coli cell did not lose the rod shape of the cell body. The size of bacteria is about 2 μm in length and 600 nm in width. The thickness of ridge is an average size of 30 nm. The weak and broad diffraction peaks reveal the amorphous nature of mineralized bacteria (Fig. S2c†). Thermogravimetric analysis demonstrates that the total weight loss is 53 wt%, attributed to the decomposition of precursors and organic compositions of cells (Fig. S2d†).

After annealing in air at 600 °C for 4 hours, there was a shrinkage of the rod-shaped body (Fig. 2a). The diameter reduced from 600 nm to 400 nm. The high magnification SEM image revealed that nanosheets vertically stand on the body of nanorod (Fig. 2b). Both the sheets and rod are assembled of nanoparticles. The hierarchical structure was verified by TEM image. It showed that the rod is surrounded with nanosheets (Fig. 2c). The nanoparticles have been clearly identified through the rim of a nanosheet (Fig. 2d). The nanosheet is comprised of uniform and ultrasmall nanoparticles with an average diameter of 3 nm. The XRD pattern indicated that all diffraction peaks are indexed to SnO₂ (JCPDS no. 41-1445) (Fig. 2e). The porous size distribution is mainly around 5 nm in diameter as revealed by N₂ adsorption–desorption isotherm using the BJH method (Fig. 2f inset). The pore volume and BET surface area were determined to be 0.22 cm³ g⁻¹ and 114.8 m² g⁻¹, respectively (Fig. 2f).

Fig. 2 Characterization of nanostructured SnO₂ annealed in air. (a) Low and (b) high magnification SEM images of SnO₂. (c) TEM image of representative rod-shaped structure. The scale bar in inset is 2 nm⁻¹. (d) HRTEM image of the margin of rod-shaped structure. (e) XRD pattern of nanostructured SnO₂. (f) Nitrogen adsorption and desorption isotherms and pore size distribution (inset) of SnO₂ annealed in air.

Taking E. coli cells with INP modified surface as the control, deposition of tin dioxide precursors on cell surface was possible since the outer membrane of E. coli contains phospholipid that may interact with tin ions. However, it lost rod-shaped structure after calcination (Fig. S3†). This may be due to that the amount of deposited precursor is not sufficient to maintain the rod shape. Simultaneously, the nanoparticles are not confined by the glutamate tripeptide, and are prone to aggregate and coarsen after calcination.

Since organisms are rich of carbon element, it may serve as carbon source in situ when using bacterial cells as the template for synthesizing of hierarchical structure. In order to finely control the structure and composition, the mineralized products firstly heat to 440 °C in air and then anneal at 500 °C for 30 minutes in an argon atmosphere to avoid carbothermal reduction during carbonization (Fig. S4†). The hierarchical structure and ultrasmall particles size of as prepared products (SnO₂/C) are almost same to those annealed in air (Fig. 3a and b). The nanorod/nanosheets structure was further confirmed by TEM imaging (Fig. 3c). The BET surface area of SnO₂/C is 106 m² g⁻¹ (Fig. S5†). The characteristic interplanar spacing of SnO₂ nanoparticles indicates good crystallinity and nanoparticles are surrounded by carbon (Fig. 3d). The Raman spectrum of carbon coated SnO₂ consists of two peaks at 1370 cm⁻¹ and 1589 cm⁻¹, which is assigned to the disorder D-band and graphitic G-band, respectively (Fig. 3e). No characteristic carbon peaks are observed for SnO₂ annealed in air. Based on thermogravimetric (TG) analysis, the exothermic peaks at 140 °C and 446 °C are ascribed to the evaporation of absorbed water and the oxidation of carbon to carbon dioxide, respectively. The carbon content is estimated to be 10 wt% (Fig. 3f).

Fig. 3 Characterization of nanostructured SnO₂/C annealed in Ar. (a) Low and (b) high magnification SEM images of carbon coated SnO₂. (c) TEM image of representative rod-shaped structure. The scale bar in inset is 2 nm⁻¹. (d) HRTEM image of the margin of rod-shaped structure. The carbon layer is pointed by black arrows. (e) Raman spectra of nanostructured SnO₂ annealed in different atmosphere. (f) Thermogravimetric analysis of carbon coated SnO₂.

Electrochemical performance of as prepared nanostructured SnO₂ was evaluated as LIB anodes. Fig. 4a presents the cyclic voltammetry curves of the first five cycles of SnO₂/C electrode at a scanning rate of 0.5 mV s⁻¹ in the potential range from 3.0 to 0.01 V vs. Li⁺/Li. Two large peaks at 1.2 V and 0.6 V in the first lithiation process are assigned to the formation of solid electrolyte interphase (SEI) layer and alloying process to form Li_xSn. The two broad peaks at 0.8 V and 1.4 V in the first delithiation process are attributed the de-alloying process from Li_xSn to SnO_x.³⁰ The following cycles of CV curves are almost overlapping, implying the well stability of SnO₂/C electrode.

Fig. 4 Electrochemical performance of nanostructured SnO₂ and SnO₂/C electrodes. (a) Cyclic voltammetry curves of SnO₂/C electrode at a scan rate of 0.5 mV s⁻¹. (b) Voltage–capacity curves of SnO₂/C electrode at various current rates. (c) Cycling performance of both electrodes at a current rate of 0.2 A g⁻¹. (d) Rate capability of both electrodes at various current rates.

The relative plateau regions can be verified by the charge–discharge profiles. In the first cycle, the charge and discharge capacity were 1041 and 1918 mA h g⁻¹, respectively, with a coulombic efficiency (CE) of 54% (Fig. S6†). The large 46% irreversible capacity loss is mainly ascribed to the formation of SEI layer.³¹ The CE of the second cycle exceeds 94%, and maintains 98% for the following cycles (Fig. 4b). The cycling stability of SnO₂/C and SnO₂ electrodes is investigated at a current of 0.2 A (Fig. 4c). A stable specific capacity of SnO₂/C has been achieved to 520 mA h g⁻¹ at a current of 0.2 A g⁻¹ after 50 cycles. At high current rates of 0.5 A g⁻¹ and 2 A g⁻¹, the specific capacities of SnO₂/C are 440 mA h g⁻¹ and 310 mA h g⁻¹ after 50 cycles, respectively (Fig. S7†). The specific capacity value of carbon coating SnO₂ electrode was calculated by SnO₂/C composite. The specific capacity of pure carbon is stable at 150 mA h g⁻¹ at a current of 0.2 A g⁻¹ (Fig. S8†). The SnO₂ exhibits continuous capacity decay within 50 cycles and the specific capacity is 194 mA h g⁻¹ after 50 cycles. The mass ratio of carbon in SnO₂/C is about 10 wt%, the sum of capacity contributed by separate carbon and SnO₂ is 189.6 mA h g⁻¹, which is lower than that of SnO₂/C. It indicates that the synergistic effect between carbon and SnO₂ in SnO₂/C composite is beneficial for improving the lithium storage capacity. The specific capacity of commercial 8 nm SnO₂ powder is 187 mA h g⁻¹ after 50 cycles at a current of 0.5 A g⁻¹. It is lower than that of SnO₂/C electrode (Fig. S9 and S10†). This may due to that the commercial nanoparticles are prone to aggregation and hinder the Li⁺ and electron transport.

The rate capability of SnO₂/C and SnO₂ electrodes was tested in the current range of 0.1 to 1 A g⁻¹ (Fig. 4d). The SnO₂/C electrode delivers higher capacities (varying from 756, 653, 574, 491, 435, 378 mA h g⁻¹) than those of SnO₂ (varying from 498, 348, 264, 206, 166, 133 mA h g⁻¹) at each testing rate. After testing at high rate of 1 A g⁻¹, SnO₂/C electrodes can recover to 540 mA h g⁻¹ when the current density finally returns to 0.1 A g⁻¹. The electrochemical performance of various SnO₂ based materials was presented in Table S1.† The SnO₂/C electrode shows a relatively good capacity.

The electrochemical performance of SnO₂/C electrode is benefit from its unique structure. First, the nanorod/nanosheets hierarchical structure insures the sufficient contacting area between active particles and liquid electrolyte, and prohibits the aggregation of nanoparticles during lithiation/delithiation process. Second, the ultrasmall SnO₂ nanoparticles shorten the lithium ions diffusion path and enhance the lithium storage capacity compared with the bulk electrode. Third, the mesoporous structure maintains more channels for lithium ion transport. The large pore volume also accommodates the volume change of active nanoparticles during alloying and de-alloying process. Last, the conductive carbon coating improves the electron transport properties and serves as a buffer layer to tolerate the stress of volume expansion.

Conclusions

In the present study, we developed a bioprocess-inspired synthesis technique to fabricate the hierarchical SnO₂ nanorods assembled by ultrasmall nanoparticles. Briefly, the bacterial was genetically modified to have its surface displayed with a glutamate tripeptide EEE that can induce the deposition of tin presursors on cell surface. After calcination, the bacterial surface not only provided a confined environment for controlling the monodisperse of particle size, but also acted as the framework for maintaining the hierarchical structure of SnO₂. Carbon coating was achieved by annealing mineralized bacteria in argon atmosphere. The carbon coated SnO₂ composite exhibited good lithium storage properties as an anode electrode, which are benefited from ultrasmall particles, carbon coating and mesoporous structure.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51521001), the Ministry of Science and Technology of China (2015DFR50650), and the Fundamental Research Funds for the Central University (WUT 2016IB006). We acknowledge Miss Tingting Luo (center for materials research and analysis of Wuhan University of Technology) for the help in HRTEM analysis. We are grateful to Bi-Chao Xu of the Core Facility and Technical Support, Wuhan Institute of Virology for her technical support in sample embedding.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18039f

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