Spent Tea Leaf Templating of Cobalt-Based Mixed ... - ACS Publications

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Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation Xiaohui Deng,† Candace K. Chan,‡ and Harun Tüysüz*,† †

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States

‡

S Supporting Information *

ABSTRACT: The facile synthesis of nanostructured cobalt oxides using spent tea leaves as a hard template is reported. Following an impregnation−calcination and template removal pathway, sheetlike structures containing nanosized crystallites of Co3O4 are obtained. Co3O4 incorporated with Cu, Ni, Fe, and Mn (M/Co = 1/8 atomic ratio) are also prepared, and the materials are thoroughly characterized using X-ray diffraction, electron microscopy, and N2 sorption. The method is applicable to several commercial tea leaves and is successfully scaled up to prepare over 7 g of Co3O4 with the same nanostructure. The oxides are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co3O4, achieving performance comparable to levels from benchmark electrocatalysts. These data suggest that tea leaf templating can be utilized as a facile and promising approach to prepare nanostructured functional catalyst. KEYWORDS: spent tea leaves, oxygen evolution, hard templating, cobalt oxide, nanocrystal

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NixFe1−xOOH have drawn considerable attention in this field since comparable or better specific activity per electrochemically active surface area (ECSA) has been demonstrated in alkaline electrolytes compared with noble metal counterparts that are tested under same conditions.12,13,17−21 With advantages such as great abundance and decent activity/ stability under alkaline conditions, first row transition metal oxides/hydroxides have the potential to be applied in large scale applications. Among all the candidates, cobalt-based materials have been intensively investigated as benchmark electrocatalysts.10,22−26 Apart from the well-known Co−Pi system, a vast amount of work has been focused on Co3O4, which has a spinel structure with two Co atoms occupying the octahedral center and one Co atom occupying the tetrahedral center.27,28 Recent research has shown that by substituting the Co sites with other transition metal cations the catalytic activity of cobalt oxide can be significantly enhanced.29−31 Since a large amount of surface-active sites is favorable for catalysis, numerous efforts have been devoted to the development of nanosized or nanostructured Co3O4. For instance, various sizes of Co3O4 nanoparticles can be obtained by adjusting the solvent and precursor concentration used in hydrothermal synthesis.32,33 The composition of nanoparticles can be further altered by varying the metal precursors. Another common approach for fabrication of mixed cobalt spinel oxides

INTRODUCTION The world’s pursuit for renewable and clean energy sources has been increasing due to the exhaustion of traditional fossil fuels and the presence of environmental issues.1,2 Since the amount of energy available from the sun is larger compared with the current energy consumption of human beings, numerous efforts have been made to achieve the efficient conversion of solar power to electricity or chemical energy, of which the former has been realized with silicon-based and newly emerging perovskite solar cells.3−7 Regarding chemical energy, energy carriers such as hydrocarbons and hydrogen can be both generated by artificial photosynthesis pathways, namely carbon dioxide reduction and water splitting.8,9 In either approach, the overall scheme has to be complemented by the oxidation of water since this is the most economical way to provide a sufficient number of electrons.10,11 However, because the oxygen evolution reaction (OER) involves transfer of four electrons and formation of the oxygen−oxygen double bond, it has always been hindered by high overpotentials and slow kinetics, which eventually affects the efficiency of solar energy utilization. Thus, development of robust water oxidation catalysts is considered to be the key for high energy conversion efficiency. Metal oxides have been intensively studied as water oxidation catalysts. Among those catalysts, Ru- and Ir-based noble metal/ metal oxides have shown the highest catalytic activity for OER.12−16 However, their practical application is still limited by the scarcity of noble metals and the high costs of corresponding metal species. On the other hand, first row transition metal oxides/hydroxides such as MnOx, NiOx, Ni(OH)2, CoOx, and © XXXX American Chemical Society

Received: September 21, 2016 Accepted: November 7, 2016

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DOI: 10.1021/acsami.6b12005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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with nanoscale dimensions is through ball-milling of coprecipitation precursors. In the case of Co2FeO4, layered cobalt−iron hydroxide carbonates were first formed by coprecipitation, and then high-energy ball-milling was employed in order to obtain nanocrystalline spinel oxides.34,35 Pulsed-laser ablation was also applied to prepare Co3O4 nanoparticles, whereby an intense laser pulse was focused onto a Co metal target that was placed in water.36 However, the aforementioned methodologies generally require large energy input and dedicated instruments (e.g., autoclave, ball-milling, laser source, etc.). Another class of materials that can offer comparable or even higher external and internal surface area are porous materials, which are generally prepared using soft or hard templating.37−39 Soft templating often involves the selfassembly of surfactant molecules under well-defined conditions (pH, temperature, concentration, solvent, etc.), followed by the solidification of the precursors around the formed micelles. In the hard templating approach, the templates (carbon or silica in most cases) are first filled with precursors, and then the desired solids are formed following the meso/nanostructure of the template. In the end, the hard templates have to be removed by calcination in the case of carbon templates or leaching with HF or alkaline solution (NaOH) for silica templates. Although mesoporous Co3O4 with high surface area has been successfully fabricated through the hard templating pathway, it often requires a few days to accomplish the whole synthesis.40 Therefore, a facile and economical method to prepare nanostructured cobalt based oxides is still highly desirable. Herein, we report for the first time the preparation of nanostructured cobalt-based mixed oxides using a hard template derived from spent tea leaves (STL). Tea is the most widely consumed drink in the world after water, and massive amounts of STL are produced as a result of the mass production of bottled and canned tea drinks.41,42 According to the Food and Agriculture Organization of the United Nations, over 5 million tons of STL are produced annually (2013).43 Since the disposal of such waste has become an issue to be faced with the repurpose and utilization of the STL, particularly for energy and environmental applications, is very attractive. On the other hand, it is a challenging task to obtain materials with suitable physiochemical properties and performance. Several research efforts have been made on this subject. For instance, it has been shown that recycled tea leaves can be utilized as a biocarbon source to coat cathode materials for lithium ion batteries.44 In another study, STL with porous structure was employed as a type of adsorbent to effectively remove basic dye molecules from aqueous solution through a chemisorption process. The presence of hydroxyl and phenolic groups in STL could also facilitate the uptake of heavy metals and transition metal ions such as Pd(II) and Cu(II) from water.45,46 Being inspired by the aforementioned efforts, we utilize the STL as hard templates for the synthesis of nanostructured electrocatalysts. Through a simple impregnation−calcination process, crystalline Co3O4 and Cu, Ni, Fe, and Mn incorporated Co3O4 (M/Co 1/8) were obtained. Electron microscopy studies showed that the final products displayed sheetlike structures consisting of nanosized crystallites. The materials were then tested as catalysts for electrochemical water oxidation, and it was found that Cu, Fe, and Ni incorporated cobalt oxides exhibited enhanced water oxidation activity while introduction of Mn cations showed detrimental effects. Moreover, the performance and mesostructure of templated Co3O4 remained stable after long-term stability tests.

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EXPERIMENTAL SECTION

All of the chemicals and reagents except tea leaves were purchased from Sigma-Aldrich and used without further purification. Synthesis of Tea Leaf Templated Co3O4 and Mixed Metal Oxides. The leaves from Ceylon pure leaf tee (Goran Mevlana) were first treated in a Soxhlet extractor with boiled water for 48 h and then dried at 90 °C before being used as templates. In a typical templating process, 15 mL of aqueous solution of Co(NO3)2·6H2O (1 g) was added to the treated tea leaves (2 g), and the mixing was conducted at room temperature for 2 h. A 2:1 weight ratio of tea to metal precursors was used throughout this study. Afterward, the mixture was dried at 60 °C, and the obtained solid was calcined at 550 °C for 4 h with a ramping rate of 2 °C/min. Finally, the product was obtained after being washed with 0.1 M HCl solution and cleaned with deionized water. In the preparation of mixed metal oxides, Cu(NO3)2·3H2O, Fe(NO3)2·9H2O, Ni(NO3)2·6H2O, and Mn(NO3)2·4H2O were used as precursors and mixed with Co(NO3)2·6H2O with a molar ratio of 1 to 8 (metal/Co). In the large scale synthesis of Co3O4, the tea leaves were first cleaned using hot water until no color was visible in the tea water. After drying, 60 g of dried tea leaves was used as the templates. To make the cobalt precursor solution, 30 g of cobalt nitrate hexahydrate was dissolved in 750 mL of deionized water. Then the solution was added to the tea leaves, and the mixing was conducted using gentle stirring for 2 h. Afterward, the mixture was heated at 70 °C until the water was completely evaporated. In the final step, the cobalt loaded tea leaves were calcined, and the obtained solids (ca. 7 g) were cleaned following the same procedures described above. The same synthesis protocol was also applied to the following commercial tea leaves without variation of the experimental conditions: Chinese green tea, Westcliff Pfefferminze (peppermint tea), Westcliff Salbei (herbal tea), Westcliff Earl Gray (black tea), and Westcliff Melisse (herbal tea). Synthesis of Bulk Co3O4. Bulk Co3O4 was prepared from the direct thermal decomposition of Co(NO3)2·6H2O. The calcination was conducted at 500 °C for 4 h under air. Electrochemical Measurements. Electrochemical water oxidation measurements were carried out in a three-electrode configuration using a rotating disc electrode (Model: AFMSRCE, PINE Research Instrumentation) with a hydrogen reference electrode (HydroFlex, Gaskatel) and Pt wire as counter electrode. 1 M KOH was used as the electrolyte, and argon was purged through the cell to remove oxygen before each experiment. The temperature of the cell was kept at 298 K using a water circulation system. Working electrodes were fabricated by depositing target materials onto glassy carbon (GC) electrodes (5 mm in diameter, 0.196 cm2 surface area). The surface of the GC electrodes was polished with Al2O3 suspension (5 and 0.25 μm, Allied High Tech Products, Inc.) before use. 4.8 mg of catalyst was dispersed in a mixed solution of 0.75 mL of H2O, 0.25 mL of isopropanol, and 50 μL of Nafion (5% in a mixture of water and alcohol) as the binding agent. Then the suspension was sonicated for 30 min to form a homogeneous ink. After that, 5.25 μL of catalyst ink was dropped onto the GC electrode and then dried under incandescent light irradiation. The catalyst loading was calculated to be 0.12 mg/cm2 in all cases. All linear scans were collected in a rotating disc electrode configuration by sweeping the potential from 0.7 to 1.7 V vs RHE with a rate of 10 mV/ s and rotation of 2000 rpm. Cyclic voltammetry measurements were carried out in the potential range between 0.7 and 1.6 V vs RHE with a scan rate of 50 mV/s. In all measurements, the Ohmic drop was compensated at 85%. Stability tests were carried out using controlled current electrolysis in 1 M KOH electrolyte where the potential was recorded at 10 mA/cm2 over a time period of 6 h. The reproducibility of the electrochemical data was checked on multiple electrodes. Material Characterization. Wide-angle X-ray diffraction (XRD) patterns collected at room temperature were recorded on a Stoe theta/ theta diffractometer in Bragg−Brentano geometry (Cu Kα 1/2 radiation). The measured patterns were evaluated qualitatively by comparison with entries from the ICDD-PDF-2 powder pattern database or with calculated patterns using literature structure data. B


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ACS Applied Materials & Interfaces Transmission electron microscopy (TEM) images of samples were obtained with an H-7100 electron microscope (100 kV) from Hitachi. Energy dispersive X-ray spectroscopy (EDX) was conducted using a Hitachi S-3500N. The microscope is equipped with a Si(Li) Pentafet Plus-Detector from Texas Instruments. High-resolution TEM (HRTEM) and scanning electron microscopy (SEM) images were taken on HF-2000 and Hitachi S-5500 microscopes, respectively. Samples for cross section images were prepared on 400 mesh Au grids in the following way: (1) two-step embedding of the sample in Spurr resin (hard mixture); (2) trimming with a milling system (Leica EM TRIM); (3) sectioning using a microtome with a 35° diamond knife (Reichert Ultra-Cut); (4) dispersion into water and transfer from the water surface to a lacey-film/400 mesh Au grid. N2-sorption isotherms were measured with an ASAP 2010 adsorption analyzer (Micrometrics) at 77 K. Prior to the measurements, the samples were degassed at 150 °C for 10 h. Total pore volumes were determined using the adsorbed volume at a relative pressure of 0.97. Brunauer− Emmett−Teller (BET) surface areas were determined from the relative pressure range between 0.06 and 0.2. Pore size distribution curves were calculated by the Barrett−Joyner−Halenda (BJH) method from the desorption branch.

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RESULTS AND DISCUSSION Herein, the utilization of spent tea leaves (STL) as hard templates to prepare cobalt oxide and mixed oxide nanocrystal is presented. First, the STL were impregnated with aqueous solution of metal nitrates; after the drying and thermal treatment that causes the combustion of the tea leaves, corresponding metal oxides were obtained. The morphology of the as-prepared STL-templated oxides after calcination and template removal was first characterized using electron microscopy. As seen from the low-magnification transmission electron microscopy (TEM) images (Figure 1), all samples exhibit a unique nanostructure which consists of nanosized crystallites. The obtained nanoparticles of cobalt oxide and mixed metal oxides are sintered, which results in particles with sheetlike nanostructured morphology. This was further supported by scanning electron microscopy (SEM) investigation of the morphology of one of the selected mixed oxides, namely Co3O4 incorporated with Ni (Ni−Co3O4). As shown in Figure 2a,b, one can clearly see well-packed nanoparticles in the range of 10−15 nm that are connected to form a sheetlike nanostructure with a domain size of few hundred nanometers. The sintering of particles was also observed in the cross-section SEM analysis (Figure 2c). Moreover, the high-resolution TEM image of selected Ni−Co3O4 (Figure 2d) displays distinct atomic planes in various directions, indicating a high degree of poly crystallinity. The crystal structure of the as-prepared Co3O4 and mixed oxides was then examined using wide-angle XRD, and the patterns are shown in Figure 3. The STL-templated cobalt oxide showed distinct reflections at 31.2°, 36.7°, 38.4°, 44.7°, 55.6°, 59.2°, and 65.2° 2 theta values that can be assigned to the spinel structure of Co3O4 with cobalt atoms located at both tetrahedral and octahedral centers.31 Once the second transition metal species were introduced into the oxides, the XRD patterns displayed characteristic reflections at the same positions as pure cobalt oxide, indicating that the cobalt atoms in the spinel structure were successfully substituted by the incorporated metal cations without forming additional phases.31 However, the substituted cobalt sites varied depending on the incorporated metal species. According to the literature, in Ni and Cu−Co3O4, the tetrahedrally coordinated Co2+ is substituted, while in Fe and Mn incorporated Co3O4, the octahedrally coordinated Co3+ is substituted.48,49 In order to

Figure 1. TEM images of STL-templated Co3O4 and Cu, Ni, Fe, and Mn incorporated mixed oxides.

Figure 2. SEM images (a, b), cross-section SEM image (c), and HRTEM image (d) of selected mixed oxide, Ni−Co3O4, prepared by STL templating.

check if the second metal cations are incorporated in the spinel structure, the XRD patterns of Co3O4, Fe−Co3O4, and Mn− Co3O4 were calculated from Rietveld refinement (Figure S1), and the calculated microstructure parameters are shown in Table S1. It can be read that the mixed oxides show higher C


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is due to the formation of a small amount of CuO phase during calcination (Figure S2). This result indicates that after the impregnation step, the distribution of cobalt and copper precursors is not homogeneous on the tea leave. Since HCl can dissolve CuO in the cleaning step, the copper content in the sample was significantly lower than expected. The textural parameters of the templated metal oxides were further determined using N2 sorption measurements, and the isotherms are depicted in Figure S3a. All materials showed type IV isotherms which are characteristic for mesoporous materials. The calculated BET surface area of Co3O4 and the mixed oxides shows a clear correlation with the crystal size calculated from the XRD patterns. Mn−Co3O4 showed the highest BET surface area of 63 m2/g, nearly double that of pure cobalt oxide (34 m2/g) and the Cu-doped counterpart (35 m2/ g). Ni and Fe incorporated cobalt oxide had BET surface areas of 40 and 53 m2/g, respectively. The pore size distribution as determined from the desorption branches of the isotherms is plotted in Figure S3b. As shown, all samples possessed pores with size between 3 and 4 nm. This can be attributed to the space between neighboring nanocrystals. This preparation method was found to be easily scaled up, and Co3O4 with the same morphology (Figure S4) and textural parameters was acquired when 60 g of tea leaves was used as the templates. More than 8 g of Co3O4 with the BET surface area of ∼40 m2/g was obtained as the final product. In order to investigate the applicability of the synthesis protocol, five other commercially available tea species (refer to Experimental Section for details) were selected and used as hard templates. As can be seen from Figure S5, the Co3O4 final product in all cases showed similar nanostructure with distinguishable nanocrystals, which indicates the flexibility and wide applicability of this novel synthetic approach. The measured BET surface areas for these samples were in the range of 60−90 m2/ g, depending on the tea species. The data presented above demonstrate the successful replication of mixed transition metal oxides using spent tea leaves as the hard template. The formation of such nanostructures is illustrated in Scheme 1. The tea leaves were first intensively treated in boiled water. Afterward, the transition metal precursors were impregnated on treated tea leaves (SEM image shown in Scheme 1) using water as the solvent. Upon immersion into the water, the leaves tend to swell and accommodate the metal precursors. Besides, due to the pretreatment process, additional porosity is likely to be created that is beneficial for the absorption of metal cations due to the release of organic compounds.51 Once the water is evaporated, calcination is applied to obtain crystalline oxides and meanwhile

Figure 3. Wide-angle XRD patterns of STL-templated Co3O4 and mixed metal oxides.

concentration of microstrain and larger lattice parameters compared with pristine Co3O4, indicating that they are present in the crystal lattice. Moreover, the broadness of the reflection peaks suggests the nanocrystallinity is present in all samples, although the average crystal size for obtained oxides was different. As calculated using the Scherrer equation, the average crystal size of pure Co3O4 was 13 nm and 15, 12, 9, and 8 nm for Cu, Ni, Fe, and Mn incorporated Co3O4, respectively. In the case of Ni−Co3O4, the calculated particle size was in good agreement with the electron microscopic investigation (Figures 1 and 2). In order to confirm the successful incorporation of the second metal species, elemental analysis was conducted to gain information on the composition as well as the possible residues that may have been left behind from the tea leaves. Other than carbon, tea leaves contain other elements such as Ca, Mg, Na, Al, S, P, and Mn, and their elemental composition might vary depending on the type and nature of the tea.50 After the calcination of tea/metal precursor composites, the treatment of the calcined materials with dilute HCl was necessary since a small amount of CaCO3 was present after calcination at 500 °C. Table S2 shows the elemental analysis results of the HCltreated Co3O4 and mixed oxides as obtained using EDX in a SEM. Although residual Al, S, P, Mg, and Ca were detected in the final products, the total combined atomic ratio was lower than 3%. More importantly, the relative ratio of the incorporated transition metal cations to the cobalt cations matched well with the expected value (1/8) except in the case of Cu, where a relative ratio of 1/20 was obtained instead. This

Scheme 1. Illustrated Formation Process of Metal Oxide Nanocrystals Templated from Spent Tea Leaves (STL)

D


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Figure 4. (a) Oxygen evolution linear scans, (b) Tafel plots, and (c) cyclic voltammetry curves of STL-templated Co3O4 and Cu, Ni, Fe, and Mn incorporated mixed oxides in 1 M KOH electrolyte (catalyst loading ∼0.12 mg/cm2).

resulted in Co3O4 with a particle size of 60−80 nm. In terms of water oxidation activity, although a similar onset potential was shown in both samples, the STL-templated Co3O4 exhibited higher current density and lower Tafel slopes than its bulk counterpart. This clearly demonstrates the advantage of using STL as the template. Figure 4a depicts the initial linear sweep voltammetry (LSV) curves of Co3O4 and mixed oxides measured in 1 M KOH electrolyte. As shown, the influence of transition metal cations on the OER activity of cobalt oxide was clearly present, as Mn showed detrimental effects while Cu-, Ni-, and Fe-doped samples exhibited similar enhanced activity over pristine Co3O4. To reach a current density of 10 mA/cm2, STL-templated Co3O4 requires an overpotential of 401 mV, which is comparable to the benchmarked nanoparticulate water oxidation catalyst.13 In comparison, the overpotential negatively shifted to 382 mV for Cu− and Ni− Co3O4 and 378 mV for Fe−Co3O4, indicating enhanced water oxidation activity. This matches well with our previous study on ordered mesoporous materials and other research work conducted on transition metal oxides.29,30 It is also worth mentioning that the results obtained from these cobalt oxides are comparable with the benchmarked nanoparticulate OER electrocatalyst proposed by Jaramillo.13 The OER kinetics were investigated, and the Tafel plots of the catalysts are depicted in Figure 4b. As calculated, the highest Tafel slope was 63 mV/dec in the case of Mn−Co3O4, indicating relatively sluggish OER kinetics. Pure Co3O4 and other mixed oxides showed Tafel slopes in the range of 45−53 mV/dec, being in good agreement with values obtained from cobalt-based nanoparticulate OER catalysts.13 The cyclic voltammetry curves of the catalysts in 1 M KOH were also collected. As shown in Figure 4c, all samples exhibited one redox couple with a broad anodic peak prior to the onset of the water oxidation reaction. This is correlated with the formation of oxyhydroxide species and oxidation of Co(III) to Co(IV).52 As shown, Mn-doped Co3O4 showed much lower oxidation current compared with others, indicating that the oxidation of cobalt cations to higher valence was strongly inhibited by the addition of Mn cations despite the highest BET surface area. On the contrary, the oxidation peak of Fe−Co3O4 and Ni−Co3O4 was significantly larger than that of Co3O4, suggesting higher population of active sites which can be related with the relatively higher surface areas of these materials. However, the enhanced OER activity should not be fully correlated with this factor as the CV curve of Cu−Co3O4 showed nearly identical shape as that of Co3O4, but the former exhibited higher OER activity. The interaction between Co and metal dopants should also be taken into account as the active

remove the template. By considering the results obtained from the electron microscopy studies, we propose that the nanoparticles were first formed on the STL from the thermal decomposition of the metal precursors. Because of the role of the substrate, the particles were well-packed and the “sheetlike” nanostructure was already present at the first stage. Afterward, the tea leaves, which mostly consist of carbon, were combusted at higher temperatures, and thus the nanostructure of the metal oxides was maintained. One key aspect concerning this process is that the decomposition temperature of the metal nitrates has to be higher than the combustion temperature of the tea leaves. Otherwise, the hard template (STL in this case) will vanish prior to the formation of the metal oxides, and this will lead to the formation of larger particles. Therefore, the combustion temperature of the pretreated tea leaves was checked using thermogravimetric analysis. As shown in Figure S6, no clear weight loss was observed at temperatures lower than ∼260 °C. Since the decomposition temperature of metal nitrates was reported to be lower, we could be confident that the formation of interconnected nanoparticles already took place before the removal of the STL template at higher calcination temperatures.29,40 In order to better understand the growth of the cobalt oxide nanoparticles on the STL, the composite obtained after the drying process was also calcined at a relatively lower temperature (225 °C for 1 h under air). This temperature is sufficient for the decomposition of the cobalt precursor while the tea leaves could remain stable according to the TG measurements (Figure S6). As shown in the SEM images (Figure S7), the presence of nanosized cobalt oxide particles on the carbon support can be clearly observed. However, the particle size is much smaller (3−5 nm) compared with the sample calcined at higher temperatures. This can be attributed to the lower calcination temperature and shorter duration. Elemental mapping also suggests the uniform distribution of cobalt atoms in the sample calcined at 225 °C, which confirms that Co3O4 nanoparticles first grow in the structure of the tea leaves. A higher thermal treatment of 550 °C causes the combustion of the leaves, sintering of Co3O4 nanoparticles and formation of a nanosheet structure. After a detailed structural analysis, the catalytic activity of the synthesized materials for electrochemical water oxidation was then evaluated following the benchmark protocol described by Jaramillo’s group.14 The comparison was first made between STL-templated Co3O4 and bulk Co3O4 which was obtained from the direct thermal decomposition of Co(NO3)2·6H2O. As shown in Figure S8, direct calcination of the cobalt precursor E


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ACS Applied Materials & Interfaces property of metal cations can be altered due to the local environment generated by neighboring metal atoms.53 The incorporation of the second metal can also increase the conductivity of catalyst and in turn facilitate the charge transfer.54 The oxygen evolution activity of Co3O4 templated from other two tea species was also tested since they exhibit various BET surface areas (the corresponding values are presented in Figure S5). As plotted in Figure S9, the anodic current prior to the onset (at E < 1.52 V vs RHE) and the current density of OER clearly increase with increasing BET surface area, suggesting higher amount of surface active sites. Higher calcination temperature (750 °C) was also applied to study the effect on the morphology and water oxidation activity of templated Co3O4. As can be seen from the TEM images (Figure S10), calcination at higher temperature results in larger crystallite of Co3O4. This was also confirmed by the narrower reflections from the XRD pattern. In terms of catalysis, the oxygen evolution activity was significantly lower than the counterpart calcined at 550 °C, and this can be attributed to the lower surface area of particles with larger size. Furthermore, the stability of STL-templated Co3O4 was checked by monitoring the applied potential at a constant current density of 10 mA/ cm2. As can be observed from Figure S10, the overpotential was maintained in the range of 0.41−0.42 V during a period of 6 h, and the TEM images show that the morphology of the sample remains unchanged after long-term electrolysis. The electrochemical data presented above suggest that the nanostructured metal oxides prepared from tea templating can be efficient and stable oxygen evolution electrocatalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.T.). ORCID

Harun Tüysüz: 0000-0001-8552-7028 Funding

This work was supported by the MAXNET Energy consortium of Max Planck Society, the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG), and Fonds der Chemischen Industrie (FCI). C. K. Chan acknowledges support from Alexander von Humboldt Foundation Fellowship. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank S. Palm, H. Bongard, and B. Spliethoff for microscopy images and EDX analysis and J. Tseng for Rietveld refinement of XRD patterns.

CONCLUSION It was demonstrated for the first time that by using spent tea leaves as the hard template, Co3O4 and transition metal (Cu, Ni, Fe, Mn) incorporated cobalt oxides could be prepared by a simple, scalable impregnation−calcination procedure. Electron microscopy studies revealed that all products possess a unique nanostructure which was constructed by nanosized crystallites in the size of ∼10 nm. TG and SEM results suggested that the tea leaves first functioned as the hard template for the formation of nanoparticles and then were removed by combustion at higher temperatures. Prepared oxides were then tested for electrochemical water oxidation, and the Cu, Ni, and Fe incorporated cobalt oxides were found to exhibit higher activity than pristine and nontemplated Co3O4. The catalytic data suggest that the tea templating method can be utilized as a facile way to fabricate nanostructured functional catalyst.

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in large scale and from other commercial tea species, thermogravimetric analysis of pretreated tea leaf, SEM image and elemental mapping of intermediate sample, TEM image and oxygen evolution linear scan of Co3O4 obtained from direct thermal decomposition of cobalt nitrate hexahydrate, oxygen evolution linear scan of templated Co3O4 from other two tea species, TEM images, XRD pattern, and oxygen evolution linear scan of templated Co3O4 calcined at 750 °C, controlled-current electrolysis of STL-templated Co3O4 at a current density of 10 mA/cm2 for 6 h, and TEM image after stability test (PDF)

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ABBREVIATIONS STL, spent tea leaves; OER, oxygen evolution reaction; LSV, linear sweep voltammetry; CV, cyclic voltammetry. REFERENCES

(1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Lewis, N. S. Toward Cost-effective Solar Energy Use. Science 2007, 315, 798−801. (3) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (4) O’regan, B.; Grätzel, M. A low-Cost, High-Efficiency Solar Cell based on Dye-sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (5) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (6) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (7) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 45). Prog. Photovoltaics 2015, 23, 1−9. (8) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (9) Gust, D.; Moore, T. A.; Moore, A. L. Solar Fuels via Artificial Photosynthesis. Acc. Chem. Res. 2009, 42, 1890−1898. (10) Deng, X.; Tüysüz, H. Cobalt-Oxide-based Materials as Water Oxidation Catalyst: Recent Progress and Challenges. ACS Catal. 2014, 4, 3701−3714.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12005. Measured, calculated, and difference patterns for STL templated pristine Co3O4, Fe and Mn incorporated Co3O4, calculated lattice parameters and microstrain concentration calculated from Rietveld refinement for STL templated Co3O4, Fe and Mn incorporated Co3O4, elemental analysis of STL-templated mixed oxides by EDX in SEM, XRD patterns of STL-templated Cu− Co3O4 after calcination, N2 sorption isotherms and pore size distribution of STL-templated Co3O4 and mixed oxides, TEM images of STL-templated Co3O4 prepared F


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Electrocatalytic Activities for Water Oxidation. Chem. Mater. 2013, 25, 4926−4935. (31) Xiao, C.; Lu, X.; Zhao, C. Unusual Synergistic Effects upon Incorporation of Fe and/or Ni into Mesoporous Co3O4 for Enhanced Oxygen Evolution. Chem. Commun. 2014, 50, 10122−10125. (32) Dong, Y.; He, K.; Yin, L.; Zhang, A. A Facile Route to Controlled Synthesis of Co3O4 Nanoparticles and Their Environmental Catalytic Properties. Nanotechnology 2007, 18, 435602. (33) Zhang, Y.; Rosen, J.; Hutchings, G. S.; Jiao, F. Enhancing Photocatalytic Oxygen Evolution Activity of Cobalt-Based Spinel Nanoparticles. Catal. Today 2014, 225, 171−176. (34) Manova, E.; Paneva, D.; Kunev, B.; Estournès, C.; Rivière, E.; Tenchev, K.; Léaustic, A.; Mitov, I. Mechanochemical Synthesis and Characterization of Nanodimensional Iron-Cobalt Spinel Oxides. J. Alloys Compd. 2009, 485, 356−361. (35) Manova, E.; Tsoncheva, T.; Paneva, D.; Mitov, I.; Tenchev, K.; Petrov, L. Mechanochemically Synthesized Nano-Dimensional Iron− Cobalt Spinel Oxides as Catalysts for Methanol Decomposition. Appl. Catal., A 2004, 277, 119−127. (36) Blakemore, J. D.; Gray, H. B.; Winkler, J. R.; Müller, A. M. Co3O4 Nanoparticle Water-Oxidation Catalysts Made by Pulsed-Laser Ablation in Liquids. ACS Catal. 2013, 3, 2497−2500. (37) Schüth, F. Endo- and Exotemplating to Create High-SurfaceArea Inorganic Materials. Angew. Chem., Int. Ed. 2003, 42, 3604−3622. (38) Lu, A. H.; Schüth, F. Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials. Adv. Mater. 2006, 18, 1793−1805. (39) Wan, Y.; Zhao, D. On the Controllable Soft-Templating Approach to Mesoporous Silicates. Chem. Rev. 2007, 107, 2821−2860. (40) Rumplecker, A.; Kleitz, F.; Salabas, E.-L.; Schüth, F. Hard Templating Pathways for the Synthesis of Nanostructured Porous Co3O4. Chem. Mater. 2007, 19, 485−496. (41) Hameed, B. Spent Tea Leaves: A New Non-Conventional and Low-Cost Adsorbent for Removal of Basic Dye from Aqueous Solutions. J. Hazard. Mater. 2009, 161, 753−759. (42) Lavecchia, R.; Pugliese, A.; Zuorro, A. Removal of Lead from Aqueous Solutions by Spent Tea Leaves. Chem. Eng. Trans. 2010, 19, 73−78. (43) World Tea Production and Trade: Current and Future Development; Food and Agriculture Organization of the United Nations, Rome, 2015. (44) Wei, C.; He, W.; Zhang, X.; Liu, S.; Jin, C.; Liu, S.; Huang, Z. Synthesis of Biocarbon Coated Li3V2(PO4)3/C Cathode Material for Lithium Ion Batteries using Recycled Tea. RSC Adv. 2015, 5, 28662− 28669. (45) Bajpai, S. K.; Jain, A. Removal of Copper (II) from Aqueous Solution using Spent Tea Leaves (STL) as a Potential Sorbent. Water SA 2010, 36, 221−228. (46) Amarasinghe, B.; Williams, R. Tea Waste as a Low Cost Adsorbent for the Removal of Cu and Pb from Wastewater. Chem. Eng. J. 2007, 132, 299−309. (47) Tüysüz, H.; Liu, Y.; Weidenthaler, C.; Schüth, F. Pseudomorphic Transformation of Highly Ordered Mesoporous Co3O4 to CoO via Reduction with Glycerol. J. Am. Chem. Soc. 2008, 130, 14108− 4110. (48) Gu, D.; Jia, C.-J.; Weidenthaler, C.; Bongard, H.-J.; Spliethoff, B.; Schmidt, W.; Schüth, F. Highly Ordered Mesoporous CobaltContaining Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide. J. Am. Chem. Soc. 2015, 137, 11407− 11418. (49) Wang, H. Y.; Hsu, Y. Y.; Chen, R.; Chan, T. S.; Chen, H. M.; Liu, B. Oxygen Evolution Reaction: Ni3+-Induced Formation of Active NiOOH on the Spinel Ni−Co Oxide Surface for Efficient Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5, 1500091. (50) Karak, T.; Bhagat, R. M. Trace Elements in Tea Leaves, Made Tea and Tea Infusion: A review. Food Res. Int. 2010, 43, 2234−2252. (51) Xia, T.; Shi, S.; Wan, X. Impact of Ultrasonic-Assisted Extraction on the Chemical and Sensory Quality of Tea Infusion. J. Food Eng. 2006, 74, 557−560.

(11) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (12) McCrory, C. C.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347−4357. (13) Jung, S.; McCrory, C. C.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation reaction. J. Mater. Chem. A 2016, 4, 3068− 3076. (14) Nakamura, R.; Frei, H. Visible Light-Driven Water Oxidation by Ir Oxide Clusters Coupled to Single Cr Centers in Mesoporous Silica. J. Am. Chem. Soc. 2006, 128, 10668−10669. (15) Nakagawa, T.; Beasley, C. A.; Murray, R. W. Efficient ElectroOxidation of Water near Its Reversible Potential by a Mesoporous IrOx Nanoparticle Film. J. Phys. Chem. C 2009, 113, 12958−12961. (16) Maeda, K.; Domen, K. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. C 2007, 111, 7851−7861. (17) Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612−13614. (18) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy) Hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549−7558. (19) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured α-NickelHydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077− 7084. (20) Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B. In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36−39. (21) Chen, R.; Wang, H. Y.; Miao, J.; Yang, H.; Liu, B. A Flexible High-Performance Oxygen Evolution Electrode with Three-Dimensional NiCo2O4 Core-Shell Nanowires. Nano Energy 2015, 11, 333− 340. (22) Hutchings, G. S.; Zhang, Y.; Li, J.; Yonemoto, B. T.; Zhou, X.; Zhu, K.; Jiao, F. In Situ Formation of Cobalt Oxide Nanocubanes as Efficient Oxygen Evolution Catalysts. J. Am. Chem. Soc. 2015, 137, 4223−4229. (23) Menezes, P. W.; Indra, A.; Sahraie, N. R.; Bergmann, A.; Strasser, P.; Driess, M. Cobalt−Manganese-Based Spinels as Multifunctional Materials that Unify Catalytic Water Oxidation and Oxygen Reduction Reactions. ChemSusChem 2015, 8, 164−171. (24) Song, F.; Hu, X. Ultrathin Cobalt−Manganese Layered Double Hydroxide is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136, 16481−16484. (25) Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das, C.; Tallarida, M.; Schmeißer, D.; Strasser, P.; Driess, M. Unification of Catalytic Water Oxidation and Oxygen Reduction Reactions: Amorphous Beat Crystalline Cobalt Iron Oxides. J. Am. Chem. Soc. 2014, 136, 17530−17536. (26) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780−786. (27) Kanan, M. W.; Yano, J.; Surendranath, Y.; Dinca, M.; Yachandra, V. K.; Nocera, D. G. Structure and Valency of a Cobalt− Phosphate Water Oxidation Catalyst Determined by In Situ X-Ray Spectroscopy. J. Am. Chem. Soc. 2010, 132, 13692−13701. (28) Kanan, M. W.; Nocera, D. G. In situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (29) Grewe, T.; Deng, X.; Tüysüz, H. Influence of Fe Doping on Structure and Water Oxidation Activity of Nanocast Co3O4. Chem. Mater. 2014, 26, 3162−3168. (30) Grewe, T.; Deng, X.; Weidenthaler, C.; Schüth, F.; Tüysüz, H. Design of Ordered Mesoporous Composite Materials and Their G


Research Article

ACS Applied Materials & Interfaces (52) Wang, X.; Yan, C.; Sumboja, A.; Lee, P. S. High Performance Porous Nickel Cobalt Oxide Nanowires for Asymmetric Supercapacitor. Nano Energy 2014, 3, 119−126. (53) Smith, R. D.; Prévot, M. S.; Fagan, R. D.; Trudel, S.; Berlinguette, C. P. Water Oxidation Catalysis: Electrocatalytic Response to Metal Stoichiometry in Amorphous Metal Oxide Films Containing Iron, Cobalt, and Nickel. J. Am. Chem. Soc. 2013, 135, 11580−11586. (54) Li, Y.; Hasin, P.; Wu, Y. NixCo3−xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution. Adv. Mater. 2010, 22, 1926−1929.

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