Formation of Semimetallic Cobalt Telluride Nanotube Film via Anion

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Formation of semimetallic cobalt telluride nanotube film via anion exchange tellurization strategy in aqueous solution for electrocatalytic applications Supriya Patil, Eun-Kyung Kim, Nabeen K. Shrestha, Jinho Chang, Joong Kee Lee, and Sung-Hwan Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08501 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 3, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Formation of semimetallic cobalt telluride nanotube film via anion exchange tellurization strategy in aqueous solution for electrocatalytic applications

Supriya A. Patil a,‡, Eun-Kyung Kima,‡, Nabeen K. Shresthaa,*, Jinho Changb, Joong Kee Leec,*, Sung-Hwan Hana,*

a

Department of Chemistry, Hanyang University, Seongdong-gu, Haengdang-dong 17, Seoul

133-791, Republic of Korea. b

Sungshin Women's University, School of Biological Science and Chemistry, 55 Dobong-ro

76 ga-gil, Gangbuk-gu, Seoul 142-732, Republic of Korea. c

Energy Storage Research Centre, Korea Institute of Science and Technology,Hwarangno 14-

gil 5, Seongbuk-gu, 136791 Seoul, Republic of Korea.

ABSTRACT Metal telluride nanostructures have demonstrated several potential applications particularly in harvesting and storing green energy. Metal tellurides are synthesized by tellurization process performed basically at high temperature in reducing gas atmosphere, which makes the process expensive and complicated. The development of a facile and economical process for desirable metal telluride nanostructures without complicated manipulation is still a challenge. In an effort to develop an alternative strategy of tellurization, herein we report a thin film formation of self-standing cobalt telluride nanotubes on various conducting and non-conducting substrates using a simple binder-free synthetic strategy based on anion exchange transformation from a thin film of cobalt hydroxycarbonate nanostructures in aqueous solution at room temperature. The nanostructured films before and after ion exchange transformation reaction are characterized using field emission scanning electron microscope, energy dispersive X-ray analyser, X-ray photoelectron spectroscopy, thin film X-ray diffraction technique, high resolution transmission electron microscope, and selected area electron diffraction analysis technique. After the ion exchange transformation of nanostructures, the film shows conversion from insulator to highly electrical conductive semimetallic characteristic. When used as a counter electrode in I3-/I1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

redox electrolyte based dye-sensitized solar cells, the telluride film exhibits an electrocatalytic reduction activity for I3- with a demonstration of solar-light to electrical power conversion efficiency of 8.10%, which is highly competitive to the efficiency of 8.20% exhibited by a benchmarked Pt-film counter electrode. On the other hand, the telluride film electrode also demonstrates electrocatalytic activity for oxygen evolution reaction from oxidation of water.

Keywords: Thin film, cobalt telluride, ion exchange, tellurization, electrocatalysis

1. INTRODUCTION Nanostructured metal chalcogenides have attracted a significant deal of attention due to their promising applications in various fields.1-4 Especially, they have been investigated extensively in sustainable energy conversion and storage devices, owing to their particular and distinctive properties For example, CdTe and Cu(In,-Ga)Se2 nanostructure-based solar cells have demonstrated a comparable quantum efficiencies to the crystalline silicon-based solar cells.5 However, most importantly it should be noted that the cost of the chalcogenidebased solar cells is considerably less as compared to the silicon-based solar cells. This is the key for the extensive study of metal chalcogenides as absorber layer in solar cells and as device architectures.6-12 On the other hand, due to the narrow bandgap for absorption of visible light and a suitable conduction band level, CoTe has also demonstrated a promising visible light driven photocatalyst for reduction of CO2 into methane.13 Similarly, metal chalcogenides have also demonstrated as a promising non-precious metal catalyst for the substitution of noble metal-based electrocatalysts used for the oxygen reduction reaction in fuel cells, hydrogen and oxygen evolution reactions from electrolysis of water and triiodide reduction reaction in dye sensitized solar cells.14-27 As a result of wide applications of metal chalcogenides, the global scientific communities working on various disciplines have been attracted towards the development of a facile synthesis route for various nano-structured chalcogenides, particularly tellurides. Basically, metal tellurides are synthesized using a tellurization process. In a typical tellurization process, metallic tellurium powder is evaporated in reducing gas atmosphere, which then reacts with metal precursors to produce metal tellurides.28 Owing to the complicated and the expensive process, various alternative approaches particularly hydrothermal and solvothermal routes have also been reported for the 2 ACS Paragon Plus Environment

Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synthesis of metal tellurides.27,29-35 However, most of these approaches are also the high temperature and pressure synthesis process. Moreover, majority of these techniques yield the final product as bulk powder form, while thin film form is highly desirable for most of the applications. Considering these drawbacks of the available techniques, there remains a great challenge to develop a facile, efficient and environmentally friendly synthesis route for the production of nanostructured metal chalcogenides particularly in thin film form.

Recently, a simple solution phase based chemical reaction at room temperature leading to anion exchange from metal carbonate and metal oxide solids was demonstrated for the thin film formation of metal sulfides and selenides.9,24 Owing to the simplicity, application of the method to produce other important chalcogenides such as metal tellurides is highly fascinating and significant. However, the similar approach is not applicable for tellurization reaction due to the fact that aqueous solutions of tellurium analogue that are often used as sulfur or selenium containing precursors are either unknown or not readily available. To address the existence of a significant challenge in the design and development of a suitable strategy for synthesis of metal tellurides, the present work is focused on the development of a very facile and environmentally friendly route for synthesis of nanostructured telluride film at room temperature and pressure using less hazardous reagents. Based on the post synthetic chemical transformation of a thin solid film strategy without degrading the film quality, herein we report, for the first time, the fabrication of thin film consisting of self-standing cobalt telluride nanotubes. These nanotubes have been fabricated from aqueous solution of tellurium precursor via the direct anion exchange transformation of 1-D nanostructured cobalt hydroxycarbonate thin films deposited using a simple chemical bath deposition technique. Further, as a concept of application demonstration of the obtained telluride film, we have also investigated the films deposited by the proposed approach for electrocatalytic reduction of triiodide in dye-sensitized solar cells (DSSCs) and for oxygen evolution reaction (OER) from electrooxidation of water.

2. Experimental Section 2.1 Chemicals All chemicals used in the present work were of reagent grade, and they were used as obtained from the suppliers without further purification. Fluorine doped tin oxide (FTO) glass and plain non-conducting glass substrates used for deposition were first cleaned in detergent water, acetone and isopropanol for 30 min each using an ultrasonic bath. Similarly, Ti-sheets 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(0.3 mm thick obtained from TiTECH, Korea) were cut into desired size (typically 2 cm × 5 cm), and were cleaned by sonicating sequentially for 10 min in acetone, isopropanol and methanol followed by deionized water. These substrates were dried in a stream of argon and stored for further use. Deionized water was used throughout the experiment.

2.2 Synthesis of cobalt hydroxycarbonate film Cobalt hydroxycarbonate film was prepared with slightly modified method reported previously for the preparation of CdCO3 or In(OH)3 film9,33. In a typical synthesis, 20 ml aqueous solutions of 0.15 M cobalt precursor (i.e., Co(NO3)2 or CoCl2 or Co(C5H7O2)2 ) was mixed with an equal volume of 0.4 M aqueous urea in a 50 ml falcon tube. A substrate plate was placed vertically inside the tube with the nonconducting face toward the tube wall followed by sealing the tube and heating the mixture at 90 °C for 6 h. After completion of the reaction, cobalt hydroxycarbonate film formed on the FTO was thoroughly washed with water and dried in a stream of argon.

2.3 Preparation of tellurium precursor solution 1 g of metallic tellurium fine powder was added to 50 ml water with constant stirring and continuous bubbling of argon. After 10 min, 8.5 g NaOH and 12.5 g Rongalite were added, and the stirring was continued for 30 minutes under argon bubbling followed by heating the solution at 100 °C for 45 minutes. After cooling down to room temperature, the solution is kept at dark.

2.4 Synthesis of cobalt telluride thin film The cobalt hydroxycarbonate thin films deposited on FTO, and Ti- substrates were dipped into the freshly prepared telluride precursor solution for various durations to optimize the ion exchange reaction. The optimum time for the ion exchange reaction was determined by investigating the kinetics of the reaction using X-ray diffraction (XRD) technique. The completion of the ion exchange reaction was confirmed by measuring the XRD of the sample ion exchanged at different interval of time until no further change in XRD patterns of the sample was obtained. After completion of the reaction, the film was washed with water and dried in a stream of argon. Following the similar procedure, thin hydroxycarbonate films were also deposited on plain non-conducting glass slides for characterizing crystal structure and conductivity of the film.

4 ACS Paragon Plus Environment

Page 4 of 25

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 Film characterization Hydroxycarbonate and telluride films deposited on the FTO, Ti and glass substrates were employed for various characterizations. The surface topography was examined with a field-emission scanning electron microscopy (FE-SEM, HITACHI S-4800). Crystal structure of the telluride thin film was confirmed using an X-ray diffractometer (Rigaku D/MAX 2600V, Cu kα= 0.15418 nm), high-resolution transmission electron microscopy (HR-TEM, Omega EM) and selected area electron diffraction (SAED) analysis. The chemical composition of the films was examined using an energy dispersive X-ray analyzer (EDX) and X-ray photoelectron spectroscopy (XPS).

In order to evaluate the performance of the nanotubular CoTe films as DSSC counter electrode, DSSC devices were fabricated as described elsewhere.24,36-38 The counter electrode was the nanotubular CoTe film deposited on a FTO substrate, and the solar-to-electric power conversion efficiency of the cells was measured by irradiating the photoanode (0.25 cm2) with 100 mW cm2 white light using a solar simulator (PEC-L01, Peccell). The photocurrent thus produced was measured using a Keithley 2400 source.

For voltammetry, a circular area having 1 cm diameter of the nanotubular CoTe films was exposed to the electrolyte by pressing the nanotubular CoTe film coated substrate against a rubber O-ring with a copper back contact electrode in a three-electrode electrochemical cell using a Pt foil as counter electrode unless stated otherwise, and a Ag/AgCl (3 M KCl) reference electrode. The sweep rate in all voltammetry was 5 mVs-1. Electrochemical impedance spectra (EIS) were recorded at open circuit conditions in the frequency range of 0.01 Hz to 1.5 MHz. All electrochemical measurements were performed using COMPACTSTAT Electrochemical Interface & Impedance Analyser (IVIUM Technologies).

3. RESULTS AND DISCUSSION 3.1 Film formation and morphology Figure 1 shows typical optical images of the cobalt hydroxycarbonate (top images) and cobalt telluride (bottom images) films. The magnified optical views of the hydroxycarbonate and cobalt telluride films are shown in Fig. S1(supporting Information) wherein a uniform and visually crack free deposition can be observed. This suggests the present method as a 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

significant route for synthesis of a uniform and crack free cobalt telluride thin film from the cobalt hydroxycarbonate film via aqueous solution phase ion exchange transformation reaction. The bottom optical images of the telluride films were obtained after reacting the cobalt hydroxycarbonate films (top images) with tellurium precursor solution for 12 h at room temperature. The change in color of the hydroxycarbonate films into dark after the reaction can be clearly noticed, which indicates the existence of chemical transformation reaction. Fig. 2 shows typical FE-SEM images of the as-synthesized hydroxycarbonate films and the same films after chemical transformation reaction. The hydroxycarbonate film formation takes place due to the reaction of Co2+ ions from cobalt precursor and the insitu CO32- and OH- ions released from hydrolysis of urea. The overall net reaction for the formation of cobalt hydroxycarbonate can be shown as below in equation (i). NH2CONH2 + 3H2O + Co2+ → Co(OH)x(CO3)y + 2NH4+

……………………..(i)

The pink colored insoluble hydroxycarbonate was found to be precipitated on the substrates immersed into the reaction solution, and thus forming hydroxycarbonate film, which is being separated from the aqueous soluble ammonium salt of the reaction shown in equation (i). Interestingly, the SEM images in Fig. 2 show that the morphology of the hydroxycarbonate film is regulated by the type of cobalt precursors used in the reaction (i). For example, nanobeam like structure is observed (Fig. 2a) when chloride precursor was used and nanobelt (Nb) like structure was observed when nitrate (Fig. 2b) and actylacetone (Fig. 2c) precursors were used. In addition, size of these nanostructural building blocks of the hydroxycarbonate film is different in each case. The top optical images of the hydroxycarbonate films synthesized from different cobalt precursors have displayed different color from light to dark pinkish and yellowish (Fig. 1 and Fig. S1). The diffuse reflectance measurement of these hydroxycarbonate films shown in Fig. S2 demonstrates that all the three hydroxycarbonate films synthesized using three different cobalt precursors have similar characteristic reflation spectra. However, it is important to note that the spectra have exhibited different degree of reflections. This suggests that the different color observed for the above mentioned hydroxycarbonate films could be due to different degree of reflection of light from the film surface, which might have caused by the different surface roughness due to different shaped nanostructural building blocks present in the film. The above observed on the dictation of the cobalt precursors over the morphology of the hydroxycarbonate can be explained on the basis of Pearson’s acid base principle, which states that hard acids react preferentially with 6 ACS Paragon Plus Environment

Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hard bases, and soft acids react preferentially with soft bases.34 Based on this classification of metal ions and ligands (counter ions), the affinity of counter ions to cobalt ions is on the following order: Cl- > NO3- > C5H7O2-. That means the chloride ions bind cobalt ions strongly than the nitrate and actylacetone ions, and thereby it controls the local concentration of the cobalt ions available for the reaction (i). Consequently, the controlled release of OH- and CO32- ions by the slow hydrolysis of urea and the reaction with different cobalt precursors might have kinetically controlled the morphology of the cobalt hydroxycarbonate film. Generally, complexing agents and/or surfactants are used to control the kinetics of a reaction in order to achieve the controlled shaped nanomaterials. However, the present strategy can be more advanced in a sense that this approach avoids the contamination from the external shape controlling reagents, which could otherwise have adverse influences on the functional property of the final targeted nanostructures.

When the hydroxycarbonate film was immersed into the tellurium precursor solution, the film turned gradually into black as shown in Fig. 1. The observed change in color of the hydroxycarbonate films into dark is plausibly due to the transformation of hydroxycarbonate into telluride, which can be represented by the reaction shown below in equation (ii). Co(OH)x(CO3)y + Te2- → CoTe + xH2O + yCO2

...............................(ii)

The surface topography of the films after the above anion exchange transformation reaction is shown in the bottom row of Fig. 2. It is very interesting to note that regardless of the morphology of the hydroxycarbonate film, the final shape of the telluride obtained from the ion exchange reaction is more or less similar. That is - all the telluride films possess the hallow tube like nanostructure with rough surface. In addition, a clear hollow tubular nanostructure can be seen at the broken part of the structure in inset image of Fig. 2d and in Fig. S3b. It should be also noted that the dimension of the telluride nanotubes and their arrangement in the film are directly related to that of the corresponding hydroxycarbonate nanostructures. In other words, after the ion exchange transformation reaction, the thicker and longer hydroxycarbonate nanostructures have transferred into the thicker and longer telluride nanotubes, while the thinner and shorter hydroxycarbonate nanostructures have transferred into the thinner and shorter telluride nanotubes. Meanwhile, the films have also maintained the more or less similar arrangement of the nanostructural building blocks in the films. For simplicity, further detail characterizations of the films were performed taking the film synthesized using CoCl2 precursor as an example. One of the technologically significance of 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the proposed method to synthesize nanostructured telluride film is that irrespective of the electrical conductivity and surface properties of the substrates, the cobalt telluride nanotubular film can be deposited on any materials such as metals, glasses, plastics, papers or textiles, etc. The only limitation is that these substrates should not react and should resist up to 90 °C (i.e. the condition used for the deposition of hydroxycarbonate film) in the reaction solution. To illustrate the ability of the proposed method to deposit nanotubular telluride film on various substrates, apart from conductive FTO coated glass substrate (Fig. 2) and Ti-foil (Fig. S4), non-conductive cellulose filter paper and polypropylene papers were also used as substrates (Fig. S4), which may find potential applications on flexible electronic and opto-electronic devices.

The anion exchange transformation kinetics for the formation of nanotubular telluride structure was studied qualitatively by recording XRD patterns of the hydroxycarbonate film after various interval of reaction in tellurium precursor solution as presented in Fig. 3a. As evident in the XRD patterns, a clear and gradual change with the disappearance of some hydroxycarbonate peaks and appearance of new peaks in XRD patterns of the ion exchanged hydroxycarbonate film at different interval of reaction time can be observed. The transformation reaction at the early stage is drastically faster (see the XRD patterns of the sample obtained at 0 min and 10 min of the ion exchange reaction) as tellurium ions get large number of available reaction sites on the surface of the hydroxycarbonate structures. Once the surface of the hydroxycarbonate nanostructures is completely covered by the telluride skin, further ion exchange reaction takes place only by the diffusion of the tellurium ions inside the telluride skinned hydroxycarbonate structures. Consequently, the kinetic of the ion exchange reaction is gradually decelerated, which can also be realized from the XRD patterns of the ion exchanged samples shown in Fig. 3a. The XRD patterns of the hydroxycarbonate film (0 minsample) in Fig. 3b show the formation of orthorhombic cobalt basic carbonate phase before the ion exchange reaction. The molecular composition of this carbonate phase is reported as Co(CO3)0.35Cl0.20(OH)1.10 (JCPDS No.: 38-0547). Further, the XRD patterns reveal that after 12 h of ion exchange reaction, no further development/disappearance of XRD peaks takes place (Fig. 3a). This indicates the completeness of the ion exchange reaction in 12 h when approximately 14 µm thick hydroxycarbonate films (Fig. 4) are immersed in the tellurium ion precursor solution. The XRD patterns of the ion exchanged film for 12 h (Fig. 3b) confirm the conversion of the hydroxycarbonate film into hexagonal CoTe (JCPDS 34-0420). Fig. 4 shows the cross-sectional SEM views of the cobalt hydroxycarbonate film and CoTe film 8 ACS Paragon Plus Environment

Page 8 of 25

Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


obtained from the cobalt hydroxycarbonate film by anion exchange transformation reaction for 12 h. In both cases, self-standing 1-D nanostructures can be observed. However, as discussed above in Fig. 2 and Fig. S3, the hydroxycarbonate film is consisted of nanobeams, whereas the anion exchanged film is consisted of hallow nanotubular structures. The formation of hallow structure after ion exchange reaction can be ascribed to Kirkendall effect.24 The nanostructured films were further investigated using TEM and SAED. In line with SEM images in Fig. 2 and Fig. S3, the formation of hallow tubular nanostructure of the cobalt telluride after anion exchange reaction is also confirmed by TEM image shown in Fig. 5a. The SAED pattern of this nanotube is shown in inset image, which reveals its crystalline structure, and it is in line with the XRD patterns shown in Fig. 3b. In addition, HR-TEM in Fig. 5b also shows the crystalline nanotubes with lattice space of 2.85 Å, which corresponds to the lattice distance of (101) crystal plan in CoTe. This further confirms the formation of CoTe from the cobalt hydroxycarbonate by the proposed anion exchange transformation strategy. Although, in general only high temperature reaction favors the crystalline product, it is very interesting to note that even after the ion exchange transformation reaction at room temperature, the as-obtained film is still crystalline. The formation of CoTe was also confirmed by determination of chemical compositional of the tellurides film deposited on a plain glass substrate using XPS and EDX analysis. (Fig. S5a) shows Co 2p core level XPS spectra of the cobalt hydroxycarbonate film, and CoTe film obtained after anion exchange reaction for 12 h. The Co 2p1/2 and Co 2p3/2 peaks located at the binding energies of 797.22 eV and 781.21 eV, respectively reveal the Co2+ oxidation state13 in both films before after the anion exchange reaction. Further, the same divalent oxidation state of cation before and after the ion exchange reaction of the film suggests that the anion after the anion exchange reaction should also be divalent in nature. This implies that the tellurium in the anion exchanged film should be in divalent oxidation state. To confirm the oxidation state of tellurium of the anion exchanged film, Te 3d core level XPS spectrum was investigated. Fig. S5b shows that Te 3d3/2 and Te 3d5/2 peaks are located at the binding energies of 583.43 eV and 572.98 eV, respectively. This result confirms the Te2- oxidation sate of tellurium in the anion exchanged film.13 In addition, the EDX elemental analysis (Fig. S6) demonstrates that the anion exchanged film consists of cobalt and tellurium in approximately equal atomic ratio. Based on the results of XPS and EDX, formation of CoTe film after anion exchange reaction of the cobalt hydroxycarbonate film has been confirmed.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.2 Electrochemical characterization and applications of nanotubular CoTe film The above results demonstrate that crystalline CoTe nanotubes can be grown in the form of a uniform film directly on a conducting and non-conducting surface using the proposed anion exchange transformation strategy. Such CoTe films on a conducting substrate could be used as electrode materials for electrochemical devices. As a concept of application demonstration, CoTe film obtained from the anion exchange of cobalt hydroxycarbonate film synthesized by using cobalt chloride precursor has been illustrated here as a promising electrocatalyst in synthesis of green energy- for example in photovoltaics and water splitting device. For application in such devices, the film should have appreciable electrical conductivity. Hence, it is worthwhile to measure the electrical conductivity of the CoTe film. Fig. 6a shows the conductivity of the cobalt hydroxycarboate and CoTe films measured between two points in terms of current-voltage curve. Obviously, in contrast to the hydroxycarbonate film, the CoTe film exhibits its highly conductive semimetallic characteristic, which suggests its potentiality as an electrode material. Recently, some metal chalcogenides such as CoS, MoS2, NiS, WS2, CoSe, NiSe2, FeS2 and CoTe have been investigated as electrocatalysts for reduction of triiodide, and they have demonstrated a very promising for replacing the expensive Pt-counter electrode in I3-/I- redox shuttle mediated DSSCs.19-26,33,39 The electroctalytic reduction of triiodide by the Pt and CoTe film electrodes is investigated using cyclic voltammetry of a dummy cell consisting of two identical electrodes with the configuration of electrode/I3-/I- redox electrolyte/electrode. The cyclic voltammograms in Fig. 6b show a pair of reversible redox peaks correspondig to I3- + 3e- ↔ 3I- redox reaction. The redox peak positions at the Pt and the CoTe film electrodes are closer suggesting that thermodynamically they have competitive electrocatalytic activity for the above redox reaction. In addition, the similar peak current densities exhibited by the cyclic voltammograms of the Pt- and CoTe- film electrodes reveal that kinetically the above redox reaction is also highly competitive at the both electrodes. Further, the slopes of Tafel polarization curves (Fig. 6c) exhibited by the both film electrodes are closer. This suggests that the degree of their electrocatalytic activity for triiodide reduction is more or less the same. A real practical application of the CoTe film as a counter electrode in a DSSC is demonstrated by the current-voltage (I-V) curves in Fig. 6d. The detail photovoltaic profiles obtained from the I-V curves are tabulated in Table 1. As expected from the above electrochemical characterization of the CoTe-film and Pt-film electrodes, the DSSCs based on these Pt and CoTe films as counter electrodes have demonstrated the photovoltaic performance very close to each other. The Pt-film counter electrode based cell exhibited the 10 ACS Paragon Plus Environment

Page 10 of 25

Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solar to electric power conversion efficiency of 8.20% with a short circuit density (Jsc) of 20.31 mA cm-2, and an open-circuit potential (Voc) of 0.63 V. Similarly, the CoTe-film counter electrode based cell exhibited the power conversion efficiency of 8.10% with Jsc of 21.40 mA cm-2, and Voc of 0.62 V. The above photovoltaic performance suggests that the nanotubular CoTe film architectured by the proposed anion exchange transformation approach can be a potential candidate for substituting the expensive Pt-counter electrode.

As a counter half reaction in production of hydrogen- a green fuel from water, studies on oxygen evolution reaction (OER) from oxidation of water, and the catalysts involved in OER have scientific as well as industrial importance. Although ruthenium40 and iridium41 based materials have demonstrated as the best electrocatalyst for OER, recently various low cost materials such as NiS39, N2 doped reduced graphene oxide-Co3O4 composite42, Ni(OH)2 hallow spheres43, Fe and Ni-Fe based oxide films44-46 have also demonstrated the competitive electrocatalytic performance for OER. Therefore, in another experiment, the feasibility of CoTe nanotubular film on a Ti-substrate was investigated briefly for its electrocatalytic performance on water oxidation. For comparison, similar 1-D nanostructured films of cobalt hydroxycarbonate before anion exchange reaction, and Co3O4 obtained by thermal oxidation of the hydroxycarbonate film were also investigated. Fig. 7a shows the linear sweep voltamograms exhibited by the cobalt hydroxycarbonate, CoTe, and Co3O4 film electrodes in 1 M KOH (pH 14) aueous electrolyte. Here, the experimentally applied potential (EAg/AgCl) is converted into the poteantial in reversible hydrogen electrode (RHE) scale using the equation: E(RHE) = EAg/AgCl + 0.059 pH + E°Ag/AgCl, where E°Ag/AgCl = 0.21 V at 25°C. The onset potential, which is the bias potential for initiation of OER is determined by the intersection of the vertical branch and the horizontal branch of the voltammograms at zero current density. Thus, the onset potential for OER at the hydroxycarbonate film electrode is found to be at 1.78 V (vs RHE). Based on +1.23 V as the standard potential for the initiation of OER at 25 °C and pH 0 ([H+] = 1.0 M), an extra electrochemical potential (i.e., overpotential) of 0.55 V is needed to initiate the OER at the hydroxycarbonate film electrode, which represents a kind of bigger loss and nonideality in the electrochemical process. However, after thermal oxidation of the hydroxycarbonate film into Co3O4, the over potential to initiate the OER is reduced to 0.37 V. This value is smaller than the over potential reported for the initiation of OER from a ultrathin porous nanoplate structured Co3O4 electrode.47 Most importantly, it should be noted that after anion exchange transformation of cobalt hydroxycarbonate nanorods into CoTe nanotubular film, the over potential for initiation of the OER is further reduced to 0.33 V, and 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the electrode has achieved 10 mA.cm-2 of current density at the moderate over potential of 0.37 V for OER (shown by dotted line in Fig. 7a). In contrast, the hydroxycarbonate and Co3O4 nanorod film electrodes achieved 10 mA.cm-2 of OER current density at the relatively larger over potential of 0.67 V and 0.50 V, respectively. It should be noted that the BET surface area (Fig. S7) of Co3O4 (13.61 m2g-1) and CoTe (14.33 m2g-1) are closer, which suggests that the higher OER current density exhibited by the nanotubular CoTe film in Fig. 7a should be attributed to the material property as well as the hollow tubular structures of the CoTe. Such hallow structures plays a key role in penetration of electrolyte. Meanwhile, the semimetallic character of the CoTe tubular structures establish a facile electron path between the electrolyte and electrode. This accelerates the charge transfer reaction across the electrode/electrolyte interface, and thereby this leads to the enhanced electrocatalytic activity. This argument is in good aggrement with the experimentally measured charge transfer resitance shown by the EIS in Fig. 7b. The EIS demonstartes a considerbly samller charge transfer reasistance between the CoTe film electrode and electrolyte as exhibited by the remarkably small semicircle exhibited by the CoTe film electrode as compared to the Co3O4 film and cobalt hydroxycarbonate film electrodes. Based on the above OER performance (Fig. 7a), the electrocatalytic activity of the nanotubular CoTe film electrode towards OER is closer to the NiS1.03 film electrode reported recently39, which has demonstrated OER performance competitive to some of the best OER catalysts such as N2 doped reduced graphene oxideCo3O4 composite, RuO2 and Ni(OH)2 hallow spheres.40,42,43 Fig. S8 shows the electrochemical stability of the CoTe nanotubular film after long run time of electrolysis for OER, which reveals a slow degradation of the CoTe film with the electrolysis time. Further optimization of the nanotubular CoTe film electrode towards OER is underway.

4. CONCLUSIONS The current work demonstrates a very facile and low temperature chemical transformation strategy route to fabricate a binder-free thin film electrode consisting of selfstanding CoTe nanotubes. This binder-free thin film electrode demonstrated the competitive electrocatalytic performance for reduction of triiodide in a dye-sensitized-solar cell, and is considered as a promising non-precious metal catalyst for substituting Pt-counter electrode in dye-sensitized-solar cells. In addition, the nanotubular CoTe film electrode also demonstrated the eletrocatalytic activity towards OER, which is competitive to some of the best OER catalysts reported earlier. It is believed that the ability of the proposed aqueous solution phase anion exchanged transformation strategy to generate 1-D nano-scaled structures of cobalt 12 ACS Paragon Plus Environment

Page 12 of 25

Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


telluride offers new opportunities to synthesize various metal telluride nanostructures, and thus opens new prospects to extend their potential applications in various fields. ■ASSOCIATED CONTENT Supporting Information Additional optical & SEM images, diffuse reflectance spectra, XPS spectra, EDS spectra & EDS profile, and N2 adsorption/desorption isotherms. ■ AUTHOR INFORMATION Author Contributions ‡ These authors contributed equally to this work.

Corresponding Author [email protected] ; [email protected] ; [email protected]

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This research was supported by the KIST Institutional Program (2E23964) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013009768).

■ REFERENCES (1)

Yuan, M.; Mitzi, D. B. Solvent Properties of Hydrazine in the Preparation of Metal

Chalcogenide Bulk Materials and Films. Dalton Trans. 2009, 31, 6065–6236. (2)

Antunez, P. D.; Buckley, J. J.; Brutchey, R. L. Tin and Germanium Monochalcogenide

IV-VI Semiconductor Nanocrystals for Use in Solar Cells. Nanoscale 2011, 3, 2399–2411. (3)

Gao, M. R.; Jiang, J.; Yu ,S. H. Solution-Based Synthesis and Design of Late

Transition Metal Chalcogenide Materials for Oxygen Reduction Reaction (ORR). Small 2012, 8, 13–27. (4)

Lai, C. H.; Lu, M. Y.; Chen, L. J. Metal Sulfide Nanostructures: Synthesis, Properties

and Applications in Energy Conversion and Storage. J. Mater. Chem. 2012, 22, 19–30. (5)

Hillhouse, H. W.; Beard, M. C. Solar Cells from Colloidal Nanocrystals: Fundamental,

Materials, Devices, and Economics. Curr. Opin. Colloid Interface Sci. 2009, 14, 245-259.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(6)

Zhang, G. Q.; Finefrock, S.; Liang, D. X.; Yadav, G. G.; Yang, H. R.; Fang, H. Y.;

Wu, Y. Semiconductor Nanostructure-Based Photovoltaic Solar Cells. Nanoscale 2011, 3, 2430–2443. (7)

Kamat, P. V.; Tvrdy, K.; Baker, D. R.;

Radich, J. G. Beyond Photovoltaics:

Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells. Chem. Soc. Rev. 2010, 110, 6664–6688. (8)

Lim, I.; Shinde, D. V.; Patil, S. A.; Ahn , D. Y.; Lee, W.; Shrestha, N. K.; Lee, J.

K.; Han, S. -H. Interfacial Engineering of CdO–CdSe 3D Microarchitectures with in situ Photopolymerized PEDOT for an Enhanced Photovoltaic Performance. Photochem. . Photobiol. 2015, 91, 780–785. (9)

Shinde, D. V.; Lim, I.; Lee, J. -K.; Sung, M. Mo.; Mane, R. S.; Han, S. -H.

Photoelectrochemical Cells by Design: 3D Nanoporous CdO–CdSe Architectures on ITO. J. Mater. Chem. A, 2013, 1, 10436–10441. (10)

Lee, W.; Lee, J.; Yi, W.; Han, S. -H. Electric-Field Enhancement of Photovoltaic

Devices: A Third Reason for the Increase in the Efficiency of Photovoltaic Devices by Carbon Nanotubes. Adv. Mater. 2010, 22, 2264–2267. (11)

Guo, C. X.; Guai, G. H.; Li, C. M. Graphene Based Materials: Enhancing Solar

Energy Harvesting. Adv. Energy Mater. 2011, 1, 448–452. (12)

Qurashi, A. Metal Chalcogenide Nanostructures for Renewable Energy Applications.

Wiley & Sons , Inc. Publications: New Jersey, 2014. (13)

Khan, M. S.; Ashiq, M. N.; Ehsan, M. F.; He, T.; Ijaz, S. Controlled Synthesis of

Cobalt Telluride Superstructures for The Visiblelight Photo-Conversion of Carbon Dioxide into Methane. Appl. Catal. A-Gen. 2014, 487, 202–209. (14)

Feng, Y. J.; Alonso-Vante, N. Nonprecious Metal Catalysts for the Molecular Oxygen-

Reduction Reaction. Phys. Status Solidi B 2008, 245, 1792–1806. (15)

Gewirth, A.; Thorum, M. S. Electroreduction of Dioxygen for Fuel-Cell Applications:

Materials and Challenges. Inorg. Chem. 2010, 49, 3557–3566. (16)

Morozan,

A.

B.;

Palacin,

S.

Low-Platinum and Platinum-Free Catalysts for

the Oxygen Reduction Reaction at Fuel Cell Cathodes. Energy Environ. Sci. 2011, 4, 1238– 1254. (17)

Chen, Z. W.; Higgins, D.; Yu, A. P.; Zhang, L.; Zhang J. J. A Review on Non-

Precious Metal Electrocatalysts for PEM Fuel Cells. Energy Environ. Sci. 2011, 4, 3167– 3192.


Page 14 of 25

Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18)

Zhao, W; Zhang C.; Geng. F.; Zhuo, S.; Zhang, B. Nanoporous Hallow Transition

Metal Chalcogenide Nanosheets Synthesized via the Anion-exchange Reaction of Metal Hydroxides with Chalcogenide Ions. ACS Nano 2014, 8,10909-10919. (19)

Wu, M.; Wang, Y.; Lin, X.; Yu, N.; Wang, L.; Wang, H.; Hagfeldt, A.; Ma, T.

Economical

and Effective Sulfide Catalysts for Dye-Sensitized Solar Cells as Counter

Electrodes. Phys. Chem. Chem. Phys. 2011, 13, 19298-19301. (19) Sun, H.; Qin, D.; Huang, S.; Guo, X.; Li, D.; Luo, Y.; Meng, Q. Dye-Sensitized Solar Cells with NiS Counter Electrodes Electrodeposited by a Potential Reversal Technique. Energy Environ. Sci. 2011, 4, 2630-2637. (20)

Kung, C. -W.; Chen, H. -W.; Lin, C. -Y.; Huang, K. -C.; Vittal, R.; Ho, K. -C. CoS

Acicular Nanorod Arrays for the Counter Electrode of an Efficient Dye-Sensitized Solar Cell. ACS Nano, 2012, 6, 7016-7025. (21) Gong, F.; Wang, H.; Xu, X.; Zhou, G.; Wang, Z. In Situ Growth of Co0.85Se and Ni0.85Se on Conductive Substrates as High-Performance Counter Electrodes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 10953-10958. (22)

Guo, J.; Shi, Y.; Chu, Y.; Ma, T. Highly Efficient Telluride Electrocatalysts for Use

as Pt-Free Counter Electrodes in Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 10157-10159. (23)

Gong, F.;

Xu, X.; Li, Z. Q.;

Zhou, G.;

Wang, Z. S. NiSe2 as an Efficient

Electrocatalyst for a Pt-Free Counter Electrode of Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 1437-1439. (24) Patil, S. A.; Shinde, D. V.; Lim, I.; Cho, K.; Bhande, S. S.; Mane, R. S; Shrestha, N. K.; Lee, J. K.; Yoon, T. H.; Han, S. -H. An Ion Exchange Mediated Shape-Preserving Strategy for Constructing 1-D Arrays of Porous CoS1.0365 Nanorods for Electrocatalytic Reduction of Triiodide. J. Mater. Chem. A 2015, 3, 7900–7909. (25) Wang, Y. -C.; Wang, D. -Y.; Jiang, Y. -T.; Chen, H. -A.; Chen, C. -C.; Ho, K. -C.; Chou, H. -L.; Chen, C.- W. FeS2 Nanocrystal Ink as a Catalytic Electrode for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2013, 52, 6694-6698. (26) Gao, M. -R.; Xu, Y. -F.; Jiang, J.; Yu, S. -H. Nanostructured Metal Chalcogenides: Synthesis Modification and Applications in Energy Conversion and Storage Devices. Chem. Soc. Rev. 2013, 42, 2986-3017. (27) Muhler, M.; Bensch, W.; Schur, M. Preparation, Crystal Structures, Experimental and Theoretical Electronic Band Structures of Cobalt Tellurides in the Composition Range 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CoTe1.3–CoTe2. J. Phys. Condens.Matter. 1998, 10, 2947–2962. (28) Sun, Z.; Liufu S.; Chen, X.; Chen L. Tellurization: An Alternative Strategy to Construct Thermoelectric Bi2Te3 Films. J. Phys. Chem. C 2011, 115, 16167–16171. (29) Xie, Y.; Li, B.; Su, H.; Liu, X.; Qian, Y. Solvothermal Route to CoTe2 nanorods. Nanostruct.Mater. 1999, 11, 539–544. (30) Jiang, L.; Zhu, Y. -J. A General Solvothermal Route to the Synthesis of CoTe, Ag2Te/Ag, and CdTe Nanostructures with Varied Morphologies. Eur. J. Inorg. Chem. 2010, 8, 1238–1243. (31) Jiang, L.; Zhu Y.-J.; Cui, J. -B. Nanostructures of Metal Tellurides (PbTe, CdTe, CoTe2, Bi2Te3, and Cu7Te4) with Various Morphologies: A General Solvothermal Synthesis and Optical Properties. Eur. J. Inorg. Chem. 2010, 19, 3005–3011. (32) Wu, G.; Cui, G.; Li, D.; Shen, P. -K.; Li, N. Carbon-supported Co1.67Te2 Nanoparticles as Electrocatalysts for Oxygen Reduction Reaction in Alkaline Electrolyte. J. Mater. Chem. 2009, 19, 6581–6589. (33) Min, Y.; Moon, G. D.; Kim, C. -E.; Lee, J. -H.; Yang, H.; Soon, A.; Jeong, U. SolutionBased Synthesis of Anisotropic Metal Chalcogenide Nanocrystals and Their Applications. J. Mater. Chem. C, 2014, 2, 6222–6248. (34) Shinde, D. V.; Ahn, D. Y.; Jadhav, V. V.; Lee, D. Y.; Shrestha, N. K.; Lee, J. K.; Lee, H. Y.; Mane, R. S.; Han, S.-H. A Coordination Chemistry Approach for Shape Controlled Synthesis of Indium Oxide Nanostructures and Their Photoelectrochemical Properties. J. Mater. Chem. A 2014, 2, 5490–5498. (35) Ji, X.; Tritt,

T. M.; Zhaob, X.; Kolisc, J. W. Solution Chemical Synthesis of

Nanostructured Thermoelectric Materials. J. S. C. Acad. Sci. 2008, 6, 1- 9. (36) Lee, D. Y.; Lim, I.; Shin, C. Y.; Patil, S. A.; Lee, W.; Shrestha, N. K.; Lee, J. K.; Han S.H. Facile Interfacial Charge Transfer across Hole Doped Cobalt-based MOFs/TiO2 NanoHybrids Making MOFs Light Harvesting Active Layers in Solar Cells. 2015, : DOI: 10.1039/c5ta07180a.


Page 16 of 25

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Lim, I.; Yoon, S. J.; Lee, W.; Nah, Y.-C.; Shrestha, N. K.; Ahn H.; S.-H.; Han. Interfacially Treated Dye-Sensitized Solar Cell with in Situ Photopolymerized Iodine Doped Polythiophene. ACS Appl. Mater. Interfaces, 2012, 4, 838–841. (38) Lim, I.; Kim; E.-K.; S. A.; Patil, Ahn, D. Y.; W.; Lee, Shrestha, N. K.; Lee, J. K.; Seok, W. K.; Cho, C.-G.; Han, S.-H. Indolocarbazole based Small Molecules: An Efficient Hole Transporting Material for Perovskite Solar Cells. RSC Adv., 2015, 5, 55321-55327. (39) Shinde, D. V.; Patil, S. A.; Cho, K.; Ahn, D.Y.; Shrestha, N. K.; Lee, J. K.; Mane, R. S.; Han, S. -H. Revisiting Metal Sulfide Semiconductors: A Solution-Based General Protocol for Thin Film Formation, Hall Effect Measurement, and Application Prospects Adv. Funct. Mater. 2015, 25, 5739–5747. (40) Gao, M. -R.; Cao, X.; Gao, Q.; Xu, Y. -F.; Zheng, Y. -R.; Jiang, J.; Yu S. -H. NitrogenDoped Graphene Supported CoSe2 Nanobelt Composite Catalyst for Efficient Water Oxidation. ACS Nano 2014, 8, 3970-3978. (41) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (42) 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. (43) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured α-Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084. (44) Chen, M.; Wu, Y.; Han, Y.; Lin, X.; Sun, J.; Zhang, W.; Cao R. An Iron-based Film for Highly Efficient Electrocatalytic Oxygen Evolution from Neutral Aqueous Solution. ACS Appl. Mater. Interfaces 2015, 7, 21852−21859. (45) Wu, Y.; Chen, M.; Han, Y.; Luo, H.; Su, X.; Zhang, M.-T.; Lin, X.; Sun, J.; Wang, L.; Deng, L.; Zhang, W.; Cao, R. Fast and Simple Preparation of Iron-Based Thin Films as Highly Efficient Water-Oxidation Catalysts in Neutral Aqueous Solution. Angew. Chem. Int. Ed. 2015, 54, 4870 –4875. (46) Qi, J.; Zhang, W.; Xiang, R.; Liu, K.; Wang, H.-Y.; Chen M.; Han, Y.; Cao R. Porous Nickel–Iron Oxide as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction. Adv. Sci. 2015, 2, 1500199- 15001207.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(47) Zhou, X.; Xia, Z.; Tian, Z.; Ma,Y.; Qu, Y. Ultrathin Porous Co3O4 Nanoplates as Highly Efficient Oxygen Evolution Catalysts. J. Mater. Chem. A, 2015, 3, 8107–8114.


Page 18 of 25

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Photovoltaic profiles obtained from I-V curves for a Pt and CoTe film as counter electrode in a DSSC.

Counter electrode

Voc

Jsc

FF

Eff

Pt

0.63

20.31

0.63

8.20

Pt-dark

0.13

0.07

0.32

0.00

CoTe

0.62

21.40

0.60

8.10

CoTe-dark

0.16

0.07

0.33

0.00

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 1. Optical images of cobalt hydroxycarbonate film on FTO glass substrate using (a) CoNO3, (b) CoCl2, and (c) Co(C5H7O2)2 precursors. (d), (e), and (f) are the optical images of the same corresponding films after conversion into CoTe by anion exchange transformation reaction for 12 h at room temperature.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2. FE-SEM images of cobalt hydroxycarbonate film on a FTO glass substrate using (a) CoCl2, (b) CoNO3, and (c) Co(C5H7O2)2 precursors. (d), (e), and (f) are the FE-SEM images of the same corresponding films after conversion into CoTe nanotubes by anion exchange transformation reaction for 12 h at room temperature. Inset images show the magnified view of the respective films.


Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(a)

(iv)

(iii)

(ii)

(i)

Fig. 3. (a) XRD patterns of the cobalt hydroxycarbonate film (obatained using CoCl2 precursor) before and after anion exchange transformation reaction at different intervals. (b) (i) JCPDS (38-0547) lines of cobalt hydroxycarbonate, (ii) XRD patterns of cobalt hydroxycarbonate film, (iii) JCPDS (34-0420) lines of CoTe, and (iv) XRD patterns of nanotubular CoTe film obtained by anion exchanged transformation reaction of the same cobalt hydroxycarbonate for 12 h at room temperature.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

Fig. 4. Cross-sectional SEM images of (a) cobalt hydroxycarbonate film obatained using CoCl2 precursor, and (b) nanotubular CoTe film obtained after anion exchange transformation reaction of the same cobalt hydroxycarbonate film for 12 h at room temperature.

(a)

(b)

Fig. 5. (a) TEM image, and (b) HR-TEM of CoTe nanotubular film obtained by anion exchange transformation reaction of the cobalt hydroxycarbonate nanorod film (obatained using CoCl2 precursor) for 12 h at room temperature. Inset in “a” shows SAED patterns of the CoTe nanotube.

Fig. 5. (a) TEM image, and (b) HR-TEM of nanotubular CoTe film obtained by anion exchange transformation reaction of the cobalt hydroxycarbonate nanorod film (obatained using CoCl2 precursor) for 12 h at room temperature. Inset in “a” shows SAED patterns of the CoTe nanotube.


Page 22 of 25

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c)

(d)

Fig. 6. (a) I-V curve of (i) cobalt hydroxycarbonate obatained using CoCl2 precursor, and (ii) nanotubular CoTe film on a nonconducting glass substrate. (b) Cyclic voltammograms, and (c) Tafel polarization curves exhibited by Pt and nanotubular CoTe films on FTO substrate in I3-/I- redox electrolyte. (d) Photovoltaic I-V curves of DSSCs with Pt and nanotubular CoTe films on FTO substrate as counter electrodes in dark and under 1 sun (100 mW cm-2) illumination conditions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 24 of 25

(b) )

Fig. 7. (a) Linear sweep voltammograms, and (b) electrochemical impedance spectra exhibited by cobalt hydroxycarbonate nanobeam film obatained using CoCl2 precursor, Co3O4 nanorod film obtained by thermal oxidation of the same cobalt hydroxycarbonate film, and nanotubular CoTe films obtained by anion exchange transformation reaction of the same cobalt hydroxycarbonate nanorod film on Ti-substrate in 1 M KOH aqueous solution. Inset shows the enlarged impedance spectrum exhibited by nanotubular CoTe film. Sweep rate: 5 mVs-1, counter electrode: Pt.


Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC 202x78mm (96 x 96 DPI)

ACS Paragon Plus Environment

Formation of Semimetallic Cobalt Telluride Nanotube Film via Anion

Recommend Documents