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Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering Shubhadeep Pal, Mihir Ranjan Sahoo, Vineesh Thazhe Veettil, Kiran Kumar Tadi, Arnab Ghosh, Parlapalli Venkata Satyam, Ravi Kumar Biroju, Pulickel M Ajayan, Saroj Kumar Nayak, and Tharangattu N. Narayanan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00032 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017
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Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering Shubhadeep Pal,1Mihir Sahoo,2 Vineesh T. Veettil,1 Kiran K. Tadi,1Arnab Ghosh,3,4 Satyam Parlappalli,3 Ravi K. Biroju,1 Pulickel M. Ajayan,5 Saroj K. Nayak,2,* and Tharangattu N. Narayanan1,*
1
TIFR-Centre for Interdisciplinary Sciences (TCIS), Tata Institute of Fundamental Research,
Hyderabad - 500075. 2
Indian Institute of Technology (IIT), Bhubaneswar, Odisha - 751013 India.
3
Institute of Physics (IoP), Bhubaneswar, Odisha - 751005, India.
4
Indian Institute of Technology, Kharagpur-721302, India.
5
Department of Materials Science &Nanoengineering, Rice University, Houston, TX - 77005-
1892, USA. (*Corresponding Authors:
[email protected] (
[email protected], TNN),
[email protected] (SKN))
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Demonstrating the coupling of CNTs via Suzuki reaction to make 3D CNTs, and the augmented hydrogen evolution reaction activities of 3D CNTs through band engineering.
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ABSTRACT Controlled assembly of mesoscopic structures can bring interesting phenomena due to their interfaces. Here, carbon nanotubes (CNTs) are cross-coupled via a C-C bonding through Suzuki reaction resulting in to 3-dimensional (3D) CNT sponges, and these 3D CNTs are studied for their efficacy towards electrocatalytic hydrogen evolution reaction (HER) in acidic medium - one of the promising methods for the production of a renewable energy source, hydrogen. Both singleand multiwall- CNTs (SWCNTs and MWCNTs) are studied for the development of 3DSWCNTs and 3DMWCNTs, and these 3D CNTs are found to be HER active with small reaction onset potentials and low charge transfer resistances unlike their uncoupled counter parts. First principle density functional calculations show that combination of electron acceptor and donor bonded to CNT network can provide a unique band structure modulation in the system facilitating the HER reaction. This study can pave the possibilities of band engineering of CNTs via functionalization and cross-coupling reactions.
Keywords: Hydrogen evolution reaction; Carbon nanotubes; Electrocatalysis; Suzuki coupling; Density Functional Theory; Band Engineering.
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1. INTRODUCTION Structural integrity, compactness and favorable mesoporosity of carbon nanotubes (CNTs) are fascinated researchers for their applications in
electronic interconnects and electrochemical
energy storage/conversion devices.[1,2] Attempts towards the formation of covalent junctions among CNTs, to make their 3-dimensional (3D) seamless structures, are intensely pursuing in the last few years, and a few of these attempts were succeeded to bring large scale connectivity among CNTs.
[3]
Elements such as boron, sulfur, phosphorus etc. based atomic welding using
chemical vapor deposition is one such attempts reported by the authors to deliver covalently connected low density CNT sponges.[4] Further, considering multi-walled CNTs (MWCNTs) as analogs of compact organic macro-molecules, the authors were reported the inter-connection of individual MWCNTs via the Suzuki reaction assisted C-C coupling.[5] In the present study, the theoretical possibility for the formation of Suzuki coupled CNTs via a benzene molecule is verified, and the impact of such a coupling in an electrochemical reaction, hydrogen evolution reaction (HER,
) in acidic medium, is tested. The
coupling is found to be changing the density of states (DOS) of individual CNTs in favor of the HER reaction and it also improves the adsorption of the H+ ions, resulting in to an overall improvement in the electron transfer kinetics. Heterogeneous electron transfer (HET) studies on the coupled CNTs are conducted to check the veracity of the theoretical predictions, and the Suzuki coupling is achieved on different CNTs, including two different types of metallic MWCNTs (different dimensions: both longer and shorter and also CNTs obtained from two different vendors) and semiconducting single wall CNTs (SWCNTs, (7,6)). These studies are in tune with the predictions that the coupling can improve the HET kinetics of HER. Further in this study, the roles of residual impurities are taken in to account by successive washing of the samples and further blocking the metallic catalytic sites using potassium thiocyanate (KSCN).[6]
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It has been unequivocally established in the literature that functions of CNTs in catalysis is either as 'true catalysts' or catalyst supports.[7] But, electro-catalytic activities of metal free CNTs are highly debated in the literature.[6,8] Metals are indispensable for the growth of CNTs, and their complete removal from CNTs even after multiple washing processes is questionable.[9] Hence residual impurities impart the catalytic efficiency of most of the CNT based 'metal free' catalysts reported.[10] But, there is a huge demand for the replacement of precious and noble metals based conventional catalysts for the viable use of sustainable futuristic energy systems. Recently, Jiang et al reported that residual metallic impurities become pronounced on CNTs with two-three walls, while the effect of metallic impurities is much lower in SWCNTs/MWCNTs.[6] Further they have proven that the intrinsic activity of CNTs are related to the quantum properties such as charge transfer via tunneling. It is in general agreed that the apparent increase in the electrochemical activities of nitric acid treated SWCNTs is due to the presence of edge like tips produced via the cutting of the CNTs during the acid treatment.[11] But such effects are negligible in MWCNTs and hence metal free MWCNTs will be inactive in electrocatalytic reactions. The role of oxygen containing functional groups in electrocatalysis of CNTs was studied by Gooding et al, and they showed that the oxygenated species (particularly -COOH) in SWCNTs render favorable electrochemical properties.[12] But later it was shown that oxygenated species in graphite and MWCNTs slows down the HET rates, and the enhanced activities reported by others might be caused by the iron based impurities present in the samples.[13] The enhanced activity of oxygenated metallic SWCNTs towards oxygen reduction reaction was reported due to the formation of strained sites formed during the oxidation treatment.[14] Heteroatoms such as boron, nitrogen etc. doped CNTs and other graphitic systems were also reported their enhanced catalytic activities towards various reactions due to the modification of spin and charge density distributions of adjacent carbon atoms.[15]
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Here a new insight into the manipulation of electrocatalytic activities of CNTs is brought by the covalent C-C coupling modification through Suzuki reaction. Electrophilic (halogenation) and nucleophilic (boric acid functionalization) 'metal free' CNTs (metals were removed by successive washing in nitric acid at elevated temperatures (60 oC, and the residual metals (Fe, Co and Ni) were quantified in parts per million (ppm) level) were developed via chemical functionalization and palladium assisted cross-coupling reaction is employed to make interconnected CNTs. The chemical treatment resulted in to a stable suspension of coupled-CNTs without their aggregation through extended
stacking, another challenge in the practical
applications of CNTs - the lack of surfactants free inks.[16] The HER catalytic activities of CNTs on each stage of functionalization is also tested. Though the effect of various functional groups in CNTs towards catalysis is reported previously,[14] covalently linked CNTs are not studied for their catalytic efficacies. Further, a generalized picture towards the modification of catalytic activities of CNTs by coupling, irrespective of their dimensional variations and electronic properties, is depicted here. The theoretical study presented here also shows the possibilities of enhancing the catalytic activities of CNTs through functionalization via band engineering, but without adversely affecting the electronic properties of CNTs, another bottleneck in the development of CNT based devices.[17]
2. EXPERIMENTAL SECTION 2.1 CNT Purification and Suzuki Coupling. As purchased CNTs (100 mg) are refluxed in a round bottom flask containing nitric acid, HNO3 (5 mg/ml) at a temperature ~120 oC for purification. Two types of MWCNTs are used for the present study. The MWCNTs obtained from CheapTubes.com having 10-30 µm length and 20-30 nm diameter, and those obtained from Sigma Aldrich having 5-7 µm length and 110-170 nm diameter are used. SWCNTs are obtained from Sigma Aldrich with diameter 0.7-1.1 nm and chirality (7,6). SWCNT (7,6) (Sigma Aldrich 704121) was synthesized using Co-Mo catalyst ACS Paragon Plus Environment 6
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containing 90 weight% carbon). All the CNTs are treated with nitric acid (HNO3) to remove the residual impurities within the tubes (detailed procedure is given in the experimental section), and this will eventually functionalize the walls with -COOH and OH groups. Further functionalization steps and Suzuki coupling procedure are as follows:[5] The functionalized tubes are dispersed and stirred in 50 ml thionylchoride (SOCl) in 2 ml of 1,2dichlorobenzene (C6H4Cl2) solvent at 80 oC for one day. This will leads to the halogenation of CNTs (chlorination). A part of the chlorinated CNTs are further treated with benzene 1,4diboronic acid (C6H4[B(OH)2]2) via Suzuki reaction. In this reaction, 40 mg chlorinated tubes is mixed in 30 mL toluene (C7H8) containing 160 mg cesium carbonate (Cs2CO3) and 20 mg of C6H4[B(OH)2] under inert (N2) atmosphere. After 1 hour of mixing, 30 mg of tetrakis (triphenylphosphine) palladium (0) (Pd[(C6H5)3P]4) is added to this mixture. This mixture is kept under stirring at temperature ~ 100 oC for 5 days. A spongy like CNT residue is obtained after this reaction protocol (a photograph of the residue is shown in supporting information figure S1) and this is similar to that the authors reported previously for MWCNTs sponges (here called 3D SWCNT/ 3D MWCNT)[5]. The 3D material is washed with methanol (CH3OH) and type I DI water for several times via vacuum filtration. This will help to remove the unreacted chlorinated tubes through water washing. Then the 3D CNTs are thoroughly sonicated in dimethyl formamide (DMF, C3H7NO) and isopropyl alcohol (IPA, C3H7OH) mixture (1:1) to remove the residual organic components. The 3D CNTs are dried and treated with water, and then dispersed in water/IPA mixture (3:1) for further analysis. 2.2. Structural Characterization. CNTs are characterized using JEOL JEM 2010 field emission gun transmission electron microscope (TEM) with an accelerating voltage of 200 kV. Residual metal levels in the CNTs are analyzed by ICPE- 9820 instrument from Shimadzu company. FTIR is measured in transmittance
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mode on a Bruker (model: Alpha) spectrometer. Renishaw inVia Raman spectrometer with 532 nm and 633 nm laser is used at 50X for 10 s.
2.3 Electrochemical Studies All the electrochemical studies are carried out at room temperature using a Biologic potentiostat model SP-300. Electrochemical experiments are conducted in three electrode systems using Ag/AgCl (saturated KCl) as reference electrode and platinum (Pt) as counter electrode (graphite is also used as counter electrode to check the possibilities of Pt effects via the Pt leaching out during the study while using Pt counter electrode, and but similar results were obtained. In order to prove this, all the electrochemical impedance spectroscopy (EIS) studies are shown using graphite counter electrode). The working electrode is a modified 3 mm glassy carbon electrode (GCE) and the electrolyte used is 0.5 M H2SO4. The working electrode is polished thoroughly in alumina polishing pad with aluminum nanoparticle (
