© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION Wafer Scale Phase-Engineered 1T- and 2H-MoSe 2 /Mo Core–Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydrogen Evolution Reaction Yindong Qu, Henry Medina, Sheng-Wen Wang, Yi-Chung Wang, Chia-Wei Chen, Teng-Yu Su, Arumugam Manikandan, Kuangye Wang, Yu-Chuan Shih, Je-Wei Chang, Hao-Chung Kuo, Chi-Yung Lee, Shih-Yuan Lu, Guozhen Shen, Zhiming M. Wang,* and Yu-Lun Chueh* Y. D. Qu, Prof. Z. M. Wang Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu 610054, P. R. China E-mail: zhmwang@uestc.edu.cn Y. D. Qu, Prof. Z. M. Wang School of Microelectronics and Solid-State Electronics University of Electronic Science and Technology of China Chengdu 610054, P. R. China Dr. H. Medina, Y.-C. Wang, C.-W. Chen, T.-Y. Su, A. Manikandan, K. Wang, Y.-C. Shih, Prof. C.-Y. Lee, Prof. Y.-L. Chueh Department of Materials Science and Engineering National Tsing Hua University Hsinchu 30013, Taiwan E-mail: ylchueh@mx.nthu.edu.tw S.-W. Wang, Prof. H.-C. Kuo Department of Photonics and Institute of Electro-Optical Engineering National Chiao Tung University Hsinchu 30010, Taiwan J.-W. Chang, Prof. S.-Y. Lu Department of Chemical Engineering National Tsing-Hua University Hsinchu 30013, Taiwan Prof. G. Shen State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Science Beijing 100083, China DOI: 10.1002/adma.201602697 efficient H 2 generation. [2,3] Platinum (Pt)-based electrodes, as the ideal electrodes for the HER process, can provide a Tafel slope of ≈30 mV dec -1 , which is close to the theoretical limit of 29 mV dec -1 because of a hydrogen absorption Gibbs free energy (ΔG H* ) of zero. [2,4] The high cost and steady increase in the global demand for Pt raises concerns for its use in energy production. [5] Therefore, the search for new materials as the ideal electrode for the HER process is imperative. As an alter- native, transition metal dichalcogenides (TMDs) have recently received great attention as potential catalysts for HER. [6–10] Computational studies have identified that TMDs have as an excellent electrocatalytic activity at edge sites with the ΔG H* located at +0.08 eV for the HER process. [11] However, most of the experimental results from different TMDs acting as a catalyst exhibit Tafel slopes larger than 38 mV dev -1 , implying that the HER process is controlled by the Volmer–Heyrovsky mechanism. To improve the HER performance, many efforts have been dedicated to expose more edge sites by engi- neering various morphologies of TMDs, such as core–shell MoO 3 -MoS 2 nanowires, [6] double-gyroid MoS 2 , [7] and vertically aligned MoSe 2 . [9] However, these may lead to inefficient elec- tron transfer because of the semiconducting property of the 2H (trigonal prismatic) phase. To solve this problem, the TMDs are often coupled with a conductive substrate, such as carbon nanotube, [12] gra- phene, [13–16] or 3D metals, [17,18] which not only separate the TMDs to expose more active sites but also accelerate the elec- tron transfer between catalysts and electrode. It has been demonstrated that MoS 3 /Vulcan C composites have reached a Tafel slope of 36 mV dec -1 because of an increase in electron transfer. [19] In a different approach, MoS 2 /CoSe 2 structures were used as a catalyst by taking advantage of the synergistic effects between MoS 2 and CoSe 2 with increased catalytic sites and reduced free energy so that the Tafel slope of 36 mV dec -1 can also be achieved. [20] Both cases indicate that a Tafel slope < 38 mV dec -1 can indicate the potential application of TMDs in efficient HER, although it is not yet close to the theoretical limit, indicating that the HER process is still controlled by elec- trochemical desorption (Volmer–Heyrovsky mechanism) rather than Volmer–Tafel recombination. In this regard, we demon- strate the MoSe 2 /Mo core–shell 3D-hierarchical nanostructures by a low-temperature plasma-assisted selenization process on the Mo 3D-hierarchical nanostructures with controlled shapes Electrochemical water splitting is an ideal eco-friendly way to produce clean energy though hydrogen (H 2 ) production. [1] As part of the process of hydrogen generation known as hydrogen evolution reaction (HER), an electrode acting as a catalyst is needed for efficiently converting a pair of protons and elec- trons into H 2 . In general, the Tafel slope is used to evaluate the efficiency of the HER process. The HER process is slow because hydrogen absorption on the active sites (Volmer reac- tion) will normally result in a Tafel slope > 116 mV dec -1 , although it can be enhanced with a Tafel slope < 38 mV dec -1 by ion exchange via electrochemical desorption (Heyrovsky reaction). The most efficient and fastest HER process should be determined by hydrogen recombination (Tafel reaction) with a Tafel slope of ≈29 mV dec -1 , which is desirable for more Adv. Mater. 2016, DOI: 10.1002/adma.201602697 www.advmat.de www.MaterialsViews.com