Author’s Accepted Manuscript One-pot hydrothermal synthesis of graphene-RuO2TiO2 nanocomposites Xian Leng, Jianpeng Zou, Xiang Xiong, Hanwei He www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)31003-X http://dx.doi.org/10.1016/j.matlet.2015.12.050 MLBLUE20010
To appear in: Materials Letters Received date: 9 September 2015 Revised date: 29 November 2015 Accepted date: 10 December 2015 Cite this article as: Xian Leng, Jianpeng Zou, Xiang Xiong and Hanwei He, Onepot hydrothermal synthesis of graphene-RuO2-TiO2 nanocomposites, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.12.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot hydrothermal synthesis of graphene-RuO2-TiO2 nanocomposites Xian Leng, Jianpeng Zou*, Xiang Xiong, Hanwei He State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Abstract: A porous ternary nanocomposite based on graphene, RuO2 and TiO2 has been prepared via a facile in-situ co-assembly without any surfactants. A unique morphology of highly-curled graphene sheets loaded with sporadic spots or coatings comprised of mixed RuO2 and TiO2 nanodots was achieved for the first time. This uniform decoration indicates strong interaction between graphene and the as-grown nanoparticles favoring the full separation of graphene sheets. The formation mechanism of such a porous but tightly bonded hetero-nanostructure benefiting from linking effect of TiO2 is proposed. Electrochemical tests showed a characteristic capacitive behavior and the specific capacitance values of 216.7-396.5 F/g depending on the ratios of Ru/Ti, which has potential for energy storage. Keywords: Graphene-RuO2-TiO2 composite, Nanoparticles, Functional, Energy storage and conversion
Since the discovery of graphene by Geim in 2004, extensive investigations on the preparation and application of this ultra-thin sp2-bonded carbon nanostructure have been reported. Among the numerous applications, flexible and porous graphene is widely used as a conductive matrix for accommodating electroactive nanoparticles in supercapacitors and lithium ion batteries, where novel structures of bimetallic oxide-carbon or binary metal oxide-carbon composites were adopted to introduce synergistic effects of individual components[2, 3]. RuO2 has eletroactive redox couples, metallic conductivity, excellent chemical and thermal stability, which combined with TiO2 or SnO2 can be used as effective dimensional stable anodes (DSAs). In addition, mesoporous RuO2-TiO2 nanocomposites were investigated to enhance the power performance of lithium batteries by forming an efficient hierarchical mixed conducting network. Other _______________ *Corresponding author. Tel.: +86 138 7495 8418 E-mail address: [email protected]
elaborately designed RuO2-TiO2 based architectures such as vertically aligned nanorods, aerogels and core-shell structures were synthesized through metal-organic chemical vapor deposition, sol-gel chemistry, and multistep ultrasonic spray. However, as far as we know, the well-connected nanostructures based on RuO2, TiO2 and graphene sheets have seldom been reported. The nanoparticles that firmly bound to graphene matrix with an elaborate design of uniform distribution probably possess unusual properties compared with individual nanoparticles or their random aggregates. Therefore, it is desirable to construct a novel nanostructure that combines RuO2-TiO2 with graphene sheets intimately for various advanced technological purposes. Herein, a ternary graphene-RuO2-TiO2 (GRT) nanocomposite with a unique morphology of sporadic spots or coatings composed of mixed RuO2-TiO2 decorating on graphene matrix was formed via a facile hydrothermal process for the first time. The coverage of nanoparticles on the graphene surface can be tuned by altering the ratio of Ru/Ti. Electrochemical tests of GRTs performed here showed a typical capacitive behavior.
Synthesis of GRT nanocomposites: 0.4 g mixed precursors (RuCl3 and TiCl3 with mole ratio of Ru/Ti=3/1) were dissolved in water, then added slowly into 50 ml GO dispersion (2.25 mg/ml) while stirring and the resulting sample was labeled as GRT 1. Samples prepared under similar procedures but different mole ratios of Ru/Ti=1:1 and 1:6 were labeled as GRT 2 and GRT 3 respectively. These samples were hydrothermal treated at 200℃ for 15 h, washed by centrifugation (8000 rpm, 5 min), lyophilized followed by annealing at 190℃ for 3 h. Under similar procedures, reduced graphene oxide (RGO), graphene-TiO2 (GT) and graphene-RuO2 (GR) nanocomposites were prepared as the contrast group. Characterization: samples were analyzed by X-ray diffraction (XRD, D/max 2550PC, Rigaku Ltd.), Fourier transform infrared spectroscopy (FTIR, Nicolet 6700), scanning electron microscopy (SEM, FEI NOVA Nano 230) equipped with an energy dispersive x-ray spectroscopy (EDS), and transmission electron microscopy (TEM, JEOL 2010Ⅱ). [在此处键入]
The fabrication of working electrodes and configuration for electrochemical tests on a CHI 660B workstation (Chenhua Corp. China) were similar to a previous study. Strategies of cyclic voltammetry (CV) and chronopotentiometry were carried out.
3.Results and discussion
The XRD patterns of GRTs with different mole ratios of Ru/Ti are shown in Fig. 1a. Broad reflection peaks corresponding to rutile RuO2 (JCPDS 21-1172) and anatase TiO2 (JCPDS 04-0477) in nanocrystalline nature are indexed. The phenomenon of phase separation in Ru-Ti oxides was observed by Chang before. GRT 1 shows anatase peaks with lowest intensity, which indicates smallest TiO2 dimensions as a result of retarded crystal growth rate originating from the adsorption of ruthenium species on quickly-formed titanium hydroxide surface. As the Ti content increases, diffraction peaks associated with anatase TiO2 become sharper owing to faster crystal growth. No appreciable characteristic peaks for graphite (002) or GO (001) in GRTs are seen, which suggests a significant reduction of GO and the as-grown nanoparticles act as spacers separating graphene layers effectively. The FTIR spectra (Fig. 1b) show that GRTs contain sp2 hybrid C from graphene (C=C, 1621 cm-1) and minor oxygen-containing groups (C=O at 1715 cm-1 and C-O at ~1000-1260 cm-1). Compared to GRT 1, stronger hydroxyl vibrations (the O-H stretching band at ~3425 cm-1) are seen in GRT 2 and GRT 3, which is in agreement with the graphene-TiO2 hybrids suggesting more abundant hydroxyl groups on TiO2 surface in comparison to RuO2. Broad peaks in the region of 400-900 cm-1 come from an integrated influence of vibration modes of hydrous RuO2 (466 cm-1) and Ti-O-Ti vibration of the TiO2 phase (607 cm-1). Notably, as seen in Figs. 2a-d, the highly-curved veil-like morphology of graphene (Fig. 2e) is well retained in ternary porous GRTs. Interestingly, a comparison of Figs. 2b-d shows that the coverage of mixed RuO2-TiO2 nanodots on graphene surface gradually increases with the Ti content in precursor solutions, forming a peculiar graphene-based surface with varying coating of RuO2-TiO2 nanodots. GRT 3 (Fig. 2d) shows a rather coarse surface densely packed with nanodots, which is distinctly different from the smooth surface of RGO (Fig. 2e), GT (Fig. 2f) and GR (Fig. [在此处键入]
2g) with discrete nanoparticles near the surface (marked by arrows). High-resolution TEM images of GRT 1 (Fig. 2h) and GRT 2 (Fig. 2i) clearly show that spherical nanodots with slight aggregation are ~ 2-4 nm. GRT 3 (Fig. 2j) exhibits lager irregular nanodots of ~ 7-12 nm, and the gradually increased particle size is consistent with progressively intensified diffraction peaks shown in Fig. 1a. The TEM image of GRT 3 (Fig. 2k) corroborates well with the morphology of mixed oxides nanodots decorating on the veil-like graphene surface in Fig. 2d and agrees with size distribution shown in Fig. 2j. The EDS results (Fig. S1) confirm the co-existence of RuO2, TiO2 and graphene, and the atomic ratios of Ru/Ti in GRT 1-3 are 9.62/2.57, 4.79/5.01, and 1.39/11.94 respectively, which has strong correlation to the input mole ratio of Ru/Ti. The C content in GRT 1-3 from RGO component is 26.09 ~ 31.72 wt%. Figure 2l schematically depicts a novel in-situ route to form a well-connected three dimensional ternary GRT nanostructure. Titanium ions, electrostatically attracted to the negative graphene oxide surface, are easily hydrolyzed to titanium hydroxide, which may attract ruthenium ions electrostatically and act as seeds offering active sites for nucleation and growth of RuO2 in wet chemistry. Furthermore, the abundant hydroxyl groups on titanium hydroxide provide additional H bond interacting with both oxygen groups from graphene oxide surface and hydroxyl groups from hydrolyzed Ru species. Titanium hydroxide could also be employed to link metal ions through inorganic grafting, therefore it probably serves as linkers between graphene and RuO2, resulting in a porous but tightly bonded hetero-nanostructure. This mechanism is confirmed by the increasing amount of nanodots covering graphene matrix by lowering the ratios of Ru/Ti and can be extended for homogeneous loading of metal/metal oxide on a hydroxyl-group-rich surface. As shown in Fig. 3a, similar to the CV curve of GR, GRTs reveal a typical capacitive E-i response with slight redox peaks, suggesting both the electrochemical double layer capacitance from graphene and apparent pseudocapacitance mainly from RuO2 nanocrystals. The CV curves of GRT 1 and GRT 2 show a close-to-ideal behavior with larger enveloped area, indicating better reversibility, smooth electron conduction and mass transport within the ternary electrodes. The deviation of CV curve for GRT 3 from a rectangle shape signifies larger equivalent series resistance as a result of higher content of TiO2, but it still has larger enveloped area than RGO and GT. As shown in Fig. 3b, the corresponding specific capacitance values based on discharge curves [在此处键入]
at 0.1 A/g of GR, GRT 1-3, GT and RGO are 405.8, 396.5, 282.4, 216.7, 172.2 and 148.5 F/g respectively, which is in good accordance with the decreasing current density responses in Fig. 3a. Fig. 3c shows the curves of discharge capacitance retention versus cycle number at 0.1 A/g. GR and GRT 1, 2 maintain high capacitance retention of ~95% after 1000 cycles benefiting from the negligible internal resistance. The electrochemical performance of GRTs are comparatively good compared with previous literature as concisely shown in Table 1. This ternary hetero-nanostructure may also enhance the functionality of graphene-based nanocomposites in catalytic processes and lithium batteries.
Fig. 1 (a) XRD patterns: (1) RuO2 rutile and (2) TiO2 anatase; and (b) FTIR spectra of GRTs
l graphene oxide
hydroxyl-group-rich titanium hydroxide ruthenium hydroxide 3+ 3+ Ru Ti
graphene hydrothermal treatment graphene-RuO2-TiO2 nanocomposites
Fig. 2 SEM images of (a, b) GRT 1, (c) GRT 2, (d) GRT 3, (e) RGO, (f) GT, and (g) GR; TEM images of (h) GRT 1, (i) GRT 2, (j, k) GRT 3 and (l) schematic representation of the formation of GRTs
Fig. 3 (a) CV curves, (b) galvanostatic charge-discharge curves and (c) cycle performance of RGO, GT, GR and GRTs Table 1 Electrochemical performance of related composites
Specific capacitance (F/g)
Graphene-TiO2 Graphene-TiO2 RuO2·xH2O-TiO2 RuO2-TiO2 nanotubes Ru0.4Ti0.6O2 Graphene-RuO2
75-84 F/g at 10 mV/s 113-136 F/g at 2.5-10 mA/cm2 400±25 F/g at 25 mV/s 50.5-229.9 F/g at 6 mA
87.5% after 1000 cycles at 2 A/g 91 % after 5000 cycles at 100 mV/s Not mentioned High
   
192-341 F/g at 25 mV/s 365 F/g at 20 mV/s
216.7-396.5 F/g at 0.1 A/g
98% after 2000 cycles at 100 mV/s  97% and 90% after 1000 cycles and  6000 cycles at 5 mA respectively 90~95 % after 1000 cycles at 1 A/g present work
In summary, we have demonstrated a one-pot strategy for depositing mixed RuO2 and TiO2 nanodots on graphene surface featuring tunable coverage by altering the precursor ratios of Ru/Ti. The good homogeneity and intimate contact might be ascribed to the hydroxyl-group-rich surface of titanium hydroxides, which serves as effective linker between graphene and RuO2. The GRT nanocomposites demonstrate negligible internal resistance originating from a well-connected three [在此处键入]
dimensional network, and exhibit strong potential for applications in energy storage.
This work was supported by the National Natural Science Foundation of China (Project 51274284) and the International Scientific and Technological Cooperation Projects of China (Project 2013DFA31440).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
References  Gao H, Duan H. Biosens Bioelectron 2015;65:404-19.  Meyyappan M. J Vac Sci Technol A 2013;31:050803 - -14.  Lian P, Cai D, Luo K, et al. Electrochim Acta 2013;104:267–73.  Panić V, Dekanski A, Mišković-Stanković VB, et al. J Electroanal Chem 2005;579:67–76.  Guo YG, Hu YS, Sigle W, Maier J. Adv Mater 2007;19:2087-91.  Chen CA, Chen YM, Chen KY, et al. J Alloy Compd 2009;485:524-8.  Swider KE, Merzbacher CI, Hagans PL, Rolison DR. Chem Mater 1997;9:1248-55.  Stopic S, Friedrich B, Schroeder M, Weirich TE. Mater Res Bull 2013;48:3633–5.  Du G, Wang X, Zhang L, et al. Mater Lett 2013;98:168–70.  Leng X, Zou J, Xiong X, He H. Rsc Adv 2014;4:61596-603.  Chang KH, Hu CC. Electrochim Acta 2006;52:1749–57.  Ramadoss A, Sang JK. Carbon 2013;63:434-45.  Deshmukh P, Pusawale S, Bulakhe R, Lokhande C. B Mater Sci 2013;47:1546-53.  Zhitomirsky I. Mater Lett 1998;33:305–10.  Zhong LS, Hu JS, Cui ZM, et al. Chem Mater 2007;19:4557-62.  Xiang S, Ming X, Wang G, et al. J Electrochem Soc 2012;159:364-9.  Hu C, Guo H, Chang K, Huang C. Electrochem Commun 2009;11:1631-4.  Wang YG, Zhang XG. Electrochim Acta 2004;49:1957–62.  Rakhi RB, Alshareef HN, Chen W, Cha D. J Mater Chem 2011;40:16197-204. [在此处键入]
A ternary graphene-RuO2-TiO2 nanocomposite is prepared via an in-situ co-assembly.
A unique morphology of highly-curled graphene sheets loaded with sporadic spots or coatings comprised of mixed RuO2 and TiO2 nanodots is achieved.
The coverage of nanodots on graphene surface can be tuned by altering the precursor ratios of Ru/Ti.
The graphene-RuO2-TiO2 nanocomposite shows strong potential for applications in energy storage.