Co 3 O 4 Nanocrystals on Single-Walled Carbon Nanotubes as a Highly Efficient Oxygen-Evolving Catalyst

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1 Nano Res 95 Electronic Supplementary Material Co 3 O 4 Nanocrystals on Single-Walled Carbon Nanotubes as a Highly Efficient Oxygen-Evolving Catalyst Jian Wu 1, Yan Xue 1, Xin Yan 1, Wensheng Yan 2, Qingmei Cheng 1, and Yi Xie 1 ( ) 1 Hefei National Laboratory for Physical Sciences at Microscale, University of Science & Technology of China, Hefei, Anhui , China 2 National Synchrotron Radiation Laboratory, University of Science & Technology of China, Hefei, Anhui , China Supporting information to DOI /s y 1. Experimental section Synthesis of Co 3 O 4 nanocrystals. The suspension of monodisperse Co 3 O 4 nanocrystals was prepared according to a reported process with some modifications. Briefly, 30 ml of ethanol was mixed with Co(NO 3 ) 2 6H 2 O (1 mmol, g) in a two-necked round-bottom flask to form a red solution. Subsequently octadecenylamine (2 ml) and ethanol (10 ml) was added under magnetic stirring for at least 30 min, which resulted in a green solution. The resulting solution was transferred into 40 ml autoclave with a Teflon liner and heated at 180 C for 12 h. The resulting product was filtered and washed several times with cyclohexane and ethanol, and dried under vacuum at 60 C for 2 h, and finally calcined in air at 350 C for 3 h [1]. Synthesis of Co 3 O 4 nanoparticles without surfactant. Co 3 O 4 nanoparticles without surfactant were prepared following the previously reported method [5, 12]. In a typical procedure, 0.50 g of Co(CH 3 COO) 2 4H 2 O was dissolved in mixed solvent of 2 ml of water and 23 ml of ethanol and 2.5 ml of 25% ammonia solution was added under vigorous stirring. The mixture was stirred in air for about 10 min to form a homogeneous fuscous slurry. The resulting suspension was transferred into a 48 ml autoclave with a Teflon liner and heated at 150 C for 3 h. The resulting black solid product was filtered and washed several times with water and ethanol, and dried under vacuum at 60 C for 5 h, and collected for characterization. The SWNTs and MWNTs as received from Aldrich were purified according to a published procedure [2]. Briefly, CNTs (SWNTs and MWNTs) were initially mixed with concentrated hydrochloric acid (HCl) (100 ml) and ultrasonicated for 1.5 h, followed by refluxing at 120 C overnight to remove metallic and organic impurities. Then, the resulting mixture was washed with deionized water several times and then dried in an oven. Subsequently, CNTs were treated ultrasonically in toluene solution for 5 h to break up large CNT aggregates and disperse the CNTs. The weight percentage of Co 3 O 4 in Co 3 O 4 /CNTs was estimated by thermogravimetric analysis (TGA). TGA analysis of the final products gave the weight percentages of Co 3 O 4 in Co 3 O 4 /MWNTs (~50%) and Co 3 O 4 /SWNTs (~50%) (Fig. S-9). Address correspondence to yxie@ustc.edu.cn

2 96 Nano Res Figure S-1 Co 3 O 4 /MWNTs hybrid materials: (a) SEM image of the Co 3 O 4 /MWNTs hybrid deposited on an ITO substrate from a suspension in solution; (b) low magnification and high magnification TEM images of the Co 3 O 4 /MWNTs hybrid; (c) XRD pattern of Co 3 O 4 and the Co 3 O 4 /MWNTs hybrid; (d) XPS spectrum of the Co 3 O 4 /MWNTs hybrid Figure S-2 High magnification TEM image of the Co 3 O 4 /SWNTs hybrid

3 Nano Res 97 Figure S-3 EDX spectra of Co 3 O 4 /SWCNTs (a) and Co 3 O 4 /MWCNTs (b) on ITO electrodes Figure S-4 CV curves of ITO anodes loaded with Co 3 O 4 /MWNTs and MWNTs in aqueous PBS at ph 7. Scan rate is 20 mv/s

4 98 Nano Res Figure S-5 Electrolysis of the Co 3 O 4 /SWNTs electrode at 1.2 V vs. Ag/AgCl in PBS electrolyte ph 7 Figure S-6 Wide-angle powder XRD patterns of a Co 3 O 4 /SWNTs catalyst-loaded ITO anode before and after 1000 cycles of accelerated stability tests at ph 7 2. Tafel plots The current density against overpotential data were collected in electrolyses at different applied potentials ranging from 0.8 V to 1.58 V vs. Ag/AgCl under the same conditions as described for bulk electrolysis. At each applied potential, electrolysis was run for 5 min and a sustained current was obtained. The solution resistance was measured to be 40 Ω and ir compensation was used to correct for the overpotential [3, 4]. 3. Determination of Faradaic efficiency In a typical electrolysis experiment, the O 2 evolution was simultaneously monitored by s fluorescence sensor. Electrolysis was performed for 6 hours at 1.2 V vs. Ag/AgCl in PBS electrolyte ph 7. The theoretical amount of oxygen produced can be calculated according to the recorded passed charge using Faraday s law. Faradaic efficiency was finally determined by comparing the measured oxygen amount with the theoretical oxygen amount [3, 4].

5 Nano Res 99 Figure S-7 O 2 production measured by fluorescence sensor (black line) and the theoretical amount of O 2 produced (red line) assuming a Faradaic efficiency of 95% 4. Calculation of turnover frequency (TOF) TOFs were calculated according to the formula TOF = n O2 /n Co = (Q/4F)/n Co (Faraday constant F = C/mol) by converting the measured charges, resulting from electrolysis carried out for 15 min at a specific potential, to μ moles of oxygen using Faraday s Law [4, 6]. The number of surface Co atoms in the Co 3 O 4 nanocrystals was calculated by assuming that the 100 crystal face is exposed in all cases. On this face, in total four Co atoms are exposed in the Co 3 O 4 unit cell length (8.084 Å), and thus the density of surface Co atoms is surface Co atoms/m 2. Brunauer Emmett Teller (BET) measurements of surface area gave a value of 170 m 2 /g for Co 3 O 4 nanoparticles. Using mg ( %) of the 6 nm Co 3 O 4 nanocrystals (surface area ~170 m 2 /g) gives m 2 (0.025 mg m 2 /g) of total surface area, and thus surface Co atoms total in Co 3 O 4 /SWNTs [5]. Overpotential (η) was calculated according to the equation: η = E appl E i s R, where E appl is the potential applied to the catalyst (substrate), E is the ph-dependent standard potential for H 2 O oxidation, E (V) vs. Ag/AgCl (3 mol/l KCl) = ph. The solution resistance was measured to be 40 Ω and ir compensation was used to correct the overpotential. Table S-1 Turnover frequencies (TOF) for oxygen evolution at Co oxide materials reported in the literature Catalyst material TOF (s 1 ) Overpotential (mv) ph Reference Co 3 O 4 /SWNTs ~ This work Co-Pi film [8] Ru 4 -POM/MWNTs [11] Co 3 O [9] Co 3 O 4 nanocubes 0.09 (0.12) [5] Co 3 O 4 nanoparticles [6] Co 3 O 4 (~0.4ML)/Au substrate [10]

6 100 Nano Res Figure S-8 XANES data at the Co L-edge of Co 3 O 4 and Co 3 O 4 /SWNTs Figure S-9 Thermogravimetric analysis (TGA) in air of SWNTs, MWNTs, Co 3 O and Co 3 O 5. Characterization of Co3O4 nanoparticles without surfactant The phase and purity of Co 3 O 4 nanoparticles without surfactant were determined by XRD, while the morphology was identified by TEM. TEM images show that the average size of Co 3 O 4 nanoparticles without surfactant was ~5 7 nm (Figs. S-10(a) and S-10(b)). The XRD pattern (Fig. S-10(c)) can be indexed as the spinel phase (space group Fd3m) with a lattice constant a = Å, consistent with the values in the literature (JCPDS file No ). The catalytic activity of the physical mixture was studied initially by cyclic voltammetry (CV) in a standard three-electrode cell, equipped with an Ag/AgCl reference electrode and a platinum wire counter electrode in aqueous phosphate buffer solution (PBS) at ph 7. Carbon nanotubes (0.025 mg) were first loaded onto an ITO (1 cm 2 ) electrode, followed by Co 3 O 4 nanoparticles without surfactant (0.025 mg). The electrodes are denoted by Co 3 O and Co 3 O respectively.

7 Nano Res 101 Figure S-10 (a) and (b) TEM images of Co 3 O 4. (c) XRD pattern of Co 3 O 4 nanoparticles without surfactant. (d) CV curves of ITO anodes loaded with Co 3 O 4 nanoparticles without surfactant, Co 3 O and Co 3 O Scan rate = 20 mv/s. The dotted line denotes E H2 O/O 2 References [S-1] Hu, L.; Peng, Q.; Li, Y. Selective synthesis of Co 3 O 4 nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion. J. Am. Chem. Soc. 2008, 130, [S-2] Kannan, R.; Bipinlal, U.; Kurungot, S.; Pillai, V. K. Enhanced electrocatalytic performance of functionalized carbon nanotube electrodes for oxygen reduction in proton exchange membrane fuel cells. Phys. Chem. Chem. Phys. 2011, 13, [S-3] Kanan, M. W.; Nocera, D. G. In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co 2+. Science 2008, 321, [S-4] Li, F.; Zhang, B.; Li, X.; Jiang, Y.; Chen, L.; Li, Y.; Sun, L. Highly efficient oxidation of water by a molecular catalyst immobilized on carbon nanotubes. Angew. Chem. Int. Ed. 2011, 50, [S-5] Esswein, A. J.; McMurdo, M. J.; Ross, P. N.; Bell, A. T.; Tilley, T. D. Size-dependent activity of Co 3 O 4 nanoparticle anodes for alkaline water electrolysis. J. Phys. Chem. C 2009, 113, [S-6] Chou, N. H.; Ross, P. N.; Bell, A. T.; Tilley, T. D. Comparison of cobalt-based nanoparticles as electrocatalysts for water oxidation. ChemSusChem. 2011, 4, [S-7] Jiao, F.; Frei, H. Nanostructured cobalt oxide clusters in mesoporous silica as efficient oxygen-evolving catalysts. Angew. Chem. Int. Ed. 2009, 48,

8 102 Nano Res [S-8] Kanan, M. W.; Nocera, D. G. In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co 2+. Science 2008, 321, [S-9] Singh, R. N.; Mishra, D.; Anindita; Sinha, A. S. K.; Singh, A. Novel electrocatalysts for generating oxygen from alkaline water electrolysis. Electrochem. Commun. 2007, 9, [S-10] Yeo, B. S.; Bell, A. T. Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2011, 133, [S-11] Toma, F. M.; Sartorel, A.; Iurlo, M.; Carraro, M.; Parisse, P.; Maccato, C.; Rapino, S.; Gonzalez, B. R.; Amenitsch, H.; Ros, T. D., et al. Efficient water oxidation at carbon nanotubepolyoxometalate electrocatalytic interfaces. Nat. Chem. 2010, 2, [S-12] Dong, Y.; He, K.; Yin, L.; Zhang, A. A facile route to controlled synthesis of Co 3 O 4 nanoparticles and their environmental catalytic properties. Nanotechnology 2007, 18,