Amorphous MoSx Coated TiO2 Nanotube Arrays for Enhanced

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C: Energy Conversion and Storage; Energy and Charge Transport x

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Amorphous MoS Coated TiO Nanotube Arrays for Enhanced Electrocatalytic Hydrogen Evolution Reaction Zhongqing Liu, Xiaoming Zhang, Bin Wang, Min Xia, Shiyuan Gao, Xinyu Liu, Ali Zavabeti, Jian Zhen Ou, Kourosh Kalantar-Zadeh, and Yichao Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01678 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Amorphous MoSx Coated TiO2 Nanotube Arrays for Enhanced Electrocatalytic Hydrogen Evolution Reaction Zhongqing Liu1,†,*, Xiaoming Zhang1,†, Bin Wang1, Min Xia2, Shiyuan Gao1, Xinyu Liu3, Ali Zavabeti4, Jian Zhen Ou4, Kourosh Kalantar-zadeh4,5* and Yichao Wang6,*

1

School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, P. R. China

2

School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing100083, P. R. China 3

Qiushi Honors College, Tianjin University, Tianjin 300072, P. R. China

4

School of Engineering, RMIT University, Melbourne Vic 3000, Australia

5

School of Chemical Engineering, University of New South Wales, Sydney, New South Wales

2052, Australia 6

School of Life and Environmental Sciences, Deakin University, Geelong, Victoria 3216,

Australia

†

Z. Liu and X. Zhang contributed equally

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ABSTRACT: Two dimensional (2D) amorphous MoSx (a-MoSx) has been confirmed to be a highly active and economic electrocatalyst for hydrogen evolution reaction (HER). The development of its hybrid cocatalyst is envisioned to bestow more active sites with appropriate crystal engineering and modified electronic properties for enhancing catalytic performance. In this work, a composite cocatalyst comprising of a-MoSx (x = 1.78) and well-ordered anodized TiO2 nanotube arrays (TNAs) is successfully developed through a facile electrodeposition route. The synergistic coupling of the unique vector charge transfer effect of TNAs and proliferation of active sites in a-MoSx derived from space confinement effect and curved interface growth of TNAs lead to a significant enhancement of HER activity, compared to those of other forms of MoS2 based electrodes that have been previously reported. The MoSx/TNAs electrode exhibits the relatively small onset overpotential of 88 mV and presents an overpotential of 157 mV at 10 mA cm-2 HER current density. The composite electrodes also show an excellent stability with no performance degradation after undergoing 1000 times successive linear sweep voltammetry. The deposition of a-MoSx onto the curved sidewall in a confined space of TNAs is demonstrated to be an effective method to induce the growth of amorphous MoSx, leading to an enhanced catalytic activity toward HER.


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1. INTRODUCTION Electrocatalytic water splitting driven by renewable energy sources such as sunlight and wind is believed to be one of the most attractive solutions to global energy and environmental problems.1-3 Currently, the most active catalysts for hydrogen evolution reaction (HER) are noble metals.4-5 However, both high cost and scarcity of these metals have been seriously impeding their large-scale commercial applications. Therefore, developing efficient, durable, and cost-effective HER catalyst to replace precious metals is critical for the future implementation of the hydrogen economy. Recently, remarkable advances have been achieved by utilization of MoS2 as promising nonprecious metal catalysts for HER.6-9 Both experiments and density functional theory calculations demonstrate that catalytic activity of MoS2 primarily originates from the edge sites or defects in the layered structure.10-13 Thus, much effort has been devoted to engineering the fine nanostructure of crystalline MoS2 to maximize the edge exposure.14-18 Nevertheless, even with these meticulous nanostructure engineering efforts, the active edge sites of crystalline MoS2 are hardly abundant enough for catalytic reactions, resulting in a low total electrode catalytic activity. In contrast to the edge sites of crystalline MoS2, amorphous molybdenum sulfide intrinsically possesses HER-active defect sites throughout the entire surface of the catalyst originated from its highly irregular arrangement structure,19-20 thus exhibiting higher catalytic activity toward HER. Furthermore, the facile methods and mild preparation conditions make amorphous MoSx (a-MoSx) highly appealing for HER. However, the poor crystallinity of a-MoSx leads to low electrochemcial stability and slow electron transport in molybdenum sulfide, severely deteriorating the overall catalytic activity toward HER. An effective strategy is to couple a-MoSx with some functional supports so as to promote the electron transport toward the active sites of catalyst.21-22 Titanium dioxide (TiO2) is utilized as one of the most popular molybdenum sulfide supports.23-25 As a popular electrode material, TiO2 has been 3 ACS Paragon Plus Environment


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widely used in many electrochemical applications, due to its low cost, non-toxicity, environmental friendly nature as well as its excellent chemical stability.26-28 Among TiO2 support materials with various morphologies,29-32 TiO2 in nanotube arrays form (TNAs) possesses a large surface area and distinctive 3D well-ordered nanotube structure, which are favourable for the loading of catalyst materials and contributing to a fast electron transfer from the electrode to the active sites of catalysts owing to a vector charge transfer effect.33 The curved growth interface and confined space of tube structured materials such as TNAs are likely to further facilitate the formation of amorphous MoSx (a-MoSx) with more active catalytic sites on the tubes, which can lead to enhanced electrocatalytic performance.4,

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In the hybrid

materials, the slow interface kinetics can be effectively compensated by a large number of active sites resulting from abundant defects and/or vacancies with high density on the catalyst surface.34 Besides, the well–ordered and vertically aligned nanotubes might be more favorable for mass transport and charge transfer than disordered support materials, so efficient charge transfer, constant reactant supply and product removal can be ensured. In this study, we demonstrate a strategy to anchor a-MoSx onto TNAs via a facile electrodeposition method. The microstructure, chemical composition and electrochemical performance of the resultant MoSx/TNAs electrodes are comprehensively investigated and results are compared to related investigations. The MoSx/TNAs present an excellent electrocatalytic activity toward HER with outstanding stability, thanks to the large number of active sites of MoSx with high sulphur vacancies and synergistic coupling between a-MoSx and TNAs.

2. EXPERIMENTAL SECTION


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2.1. Synthesis of TNAs and MoSx/TNAs composites. The anatase TiO2 nanotube arrays (TNAs) with a vertical orientation used here were prepared using an electrochemical anodization technique followed by a heat treatment, which is similar to that in our previous work.33, 35-36 In specific, a Ti foil (0.5 mm thick, Ti 99.8%) served as the working electrode and a graphite foil as the counter electrode. Electrochemical anodization was carried out at a constant voltage of 60 V for 4 hours at a room temperature. After ultrasonic washing in 10% H2O2 for 10 minutes, calcination was conducted at 500 ℃ for 2 hours with a ramp-up and ramp-

down of 10 ℃ min–1 each in air to transform TNAs from the amorphous state into the anatase

phase. Synthesis of MoSx/TNAs composites were taking advantage of electrochemical deposition of MoSx onto the TiO2 nanotubes, in which MoSx were selectively deposited on electrically accessible sites of the Ti–based TNAs electrode. Therefore, the electronic charge transfer from TNAs to the deposited MoSx can be guaranteed.37 During the electrodeposition of MoSx, a mixture of 1 mM Na2MoO4 and 6 mM Na2S was used as the electrolyte. 5% (v/v) of HCl was added in a dropwise manner to the above mixture solution to adjust the pH of solution to ~8.0. In the pH adjustment, electrolyte solution experienced color changes from transparent to light red and ultimately wine red. The chemical reaction in this process is described using following equation:38 Na2 MoO4 + 8HCl + 4Na2 S → Na2 MoS4 + 8NaCl + 4H2 O

(1)

Formation of MoSx/TNAs via electrochemical deposition was performed using a standard three–electrode setup in the electrolyte containing Na2MoS4. The TNAs was working as the working electrode. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum sheet was the counter electrode. In the deposition, constant voltage (–1.2 V vs SCE) was applied to the system and the duration of the deposition was 900 s. This time and applied voltage were chosen based on optimal experimental measurements, as shown in Fig. S1, Supporting Information. Selection of electrodeposition time is demonstrated in Supporting 5 ACS Paragon Plus Environment


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Information Note S1. Electrodeposition of MoSx (x = 2-a, 0

Amorphous MoSx Coated TiO2 Nanotube Arrays for Enhanced

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