Synthesis of Mesoporous Cobalt Boride by Strictly

Chemical Science View Article Online EDGE ARTICLE View Journal A mesoporous non-precious metal boride system: synthesis of mesoporous cobalt boride by strictly Cite this: DOI: 10.1039/c9sc04498a † All publication charges for this article controlled chemical reduction have been paid for by the Royal Society ab a ac a a a of Chemistry Bo Jiang,‡ Hui Song,‡ Yunqing Kang, Shengyao Wang, Qi Wang, Xin Zhou, Kenya Kani,d Yanna Guo,a Jinhua Ye, a Hexing Li, c Yoshio Sakka,b Joel Henzie *a and Yamauchi Yusuke *de Generating high surface area mesoporous transition metal boride is interesting because the incorporation of boron atoms generates lattice distortions that lead to the formation of amorphous metal boride with unique properties in catalysis. Here we report the first synthesis of mesoporous cobalt boron amorphous alloy colloidal particles using a soft template-directed assembly approach. Dual reducing agents are used Received 6th September 2019 to precisely control the chemical reduction process of mesoporous cobalt boron nanospheres. The Accepted 14th November 2019 Earth-abundance of cobalt boride combined with the high surface area and mesoporous DOI: 10.1039/c9sc04498a Creative Commons Attribution 3.0 Unported Licence. nanoarchitecture enables solar-energy efficient photothermal conversion of CO2 into CO compared to rsc.li/chemical-science non-porous cobalt boron alloys and commercial cobalt catalysts. Introduction conditions and strong reducing agents.17–19 The combination of these factors makes the creation of high surface area meso- Colloidal metal nanoparticles have interesting electronic prop- porous transition metals via so-templated chemical synthesis erties that enable a wide range of applications in heterogeneous very challenging despite all their potential advantages in terms catalysis, energy storage and conversion, fuel cells and chemical of economic cost. 1–5 This article is licensed under a sensing. Generating even higher surface area metallic archi- Alloying precious metals with transition metals is a common tectures increases the material utilization efficiency of the metal strategy to stabilize transition metals against oxidation and still and exposes additional active sites for higher efficiency catal- access the unique electronic properties of the metal for catalysis.6–10 But most reports on mesoporous metal colloids utilize ysis. Recently, transition metal catalysts that incorporate boron Open Access Article. Published on 15 November 2019. Downloaded 1/15/2020 12:06:34 PM. precious metals such as Pt, Pd, and Ir and their alloys.11–16 There (B) have gained a lot of attention because: (i) B can act as have been few examples of mesoporous architectures composed a donor or acceptor depending on metal concentration and help 20,21 of nonprecious transition metals, despite their abundance in partially stabilize the transition metal against oxidation and the Earth's crust. This is due in part to the sensitivity of tran- (ii) B can generate lattice strain which affects the electronic sition metals to oxidation in air or solvents and their relatively structure of the metal and helps form lower-coordination 22–24 low redox potential, which requires more strenuous reaction unsaturated sites. Transition metal boride alloys and boron-doped metals exhibit enhanced activity/selectivity in versatile catalytic reactions such as the hydrogenation of cin- 23 24 aWorld Premier International (WPI) Research Center for Materials Nanoarchitectonics namaldehyde, electrochemical water splitting, electro- 25 26 (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki catalytic CO2 reduction, the oxygen reduction reaction, and 305-0044, Japan. E-mail: [email protected] the electro-oxidation of formic acid.27 For example, boron- bResearch Center for Functional Materials, National Institute for Materials Science doped Cu catalysts exhibit high selectivity for electrocatalytic (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan reduction of CO into C products (Faraday efficiency 79%) c 2 2 The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, P. because B modi es the local electronic structure of copper and 22 R. China creates positive valence sites. And amorphous cobalt boride dSchool of Chemical Engineering and Australian Institute for Bioengineering and (Co–B) has been reported as an exceptionally efficient catalyst Nanotechnology (AIBN), The University of Queensland, Brisbane, Queensland 4072, for hydrogenation of cinnamaldehyde, due to the lattice strain Australia. E-mail: [email protected] that B generates in the crystal structure of the Co metal.23 These e Department of Plant and Environmental New Resources, Kyung Hee University, 1732 reports indicate that incorporation of boron into metals stabi- Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea lizes a lot of the favorable electronic properties of transition † Electronic supplementary information (ESI) available: Details of experimental procedures and additional experimental data. See DOI: 10.1039/c9sc04498a metal catalysts in nanomaterial systems. Therefore, adapting ‡ These authors contributed equally to this work. these methods to generate mesoporous transition metal borides This journal is © The Royal Society of Chemistry 2019 Chem. Sci. View Article Online Chemical Science Edge Article is a promising approach to access these kinds of catalytic solution, which drives the micellization of PS-b-PEO due to the properties in an ultra-high surface area system. low solubility of the PS segments in water. The optical signature Here we describe a simple synthesis method to generate of this phenomenon could be observed using the Tyndall effect † mesoporous amorphous cobalt boron (a-CoBx) alloy colloidal (Fig. S1 ). The resulting micelles were composed of PS cores particles using block copolymer micelle templates as pore- surrounded by PEO shells with an average diameter of 12.7 nm, directing agents. This method largely relies on the use of as shown in the TEM images using the phosphotungstic (PW) dual-reducing agents (i.e., dimethylamine borane and sodium acid negative staining technique (Fig. S2†). This mixture in the borohydride) to precisely control the reduction process and ask was then degassed and purged under vacuum using facilitate the incorporation of B into the metal. Sodium boro- a Schlenk line to remove dissolved oxygen from the solution and hydride plays a key role in triggering the initially rapid nucle- backlled with the inert gas Ar to help protect the obtained ation of Co metal clusters/nanocrystals, while dimethylamine samples from oxidization. The reaction took place immediately borane assists in the formation of the mesoporous structure as when a small amount of NaBH4 was added into the above the Co metal is deposited around the micelle templates. Both solution. This solution was further incubated at room temper- sodium borohydride and dimethylamine borane serve as the B- ature for 1 h. Finally, the product was isolated by several source and form an amorphous cobalt boron alloy. The ob- consecutive washing/centrifugation cycles with acetone. Omit- tained mesoporous a-CoBx alloys were then examined as cata- ting the PS-b-PEO from the synthesis generated non-porous lysts for the photo-thermal conversion of CO2 with H2 to cobalt boron, indicating the essential role of the polymer as investigate the structure- and composition-dependent catalytic a pore-directing agent in the reaction (Fig. S3†). activity of the mesoporous network. The results show that Generally, the formation of mesoporous metals is accom- mesoporous a-CoBx nanoparticles have a higher performance plished through the cooperative assembly of reduced nano- toward photothermal assisted reduction of CO2 to CO versus crystals and micelles; learning how to chemically control the non-porous a-CoBx and commercial Co catalysts. deposition process is a key point in the synthesis of mesoporous Creative Commons Attribution 3.0 Unported Licence. metals. NaBH4 is a commonly used reducing agent in the synthesis of all kinds of metal nanocrystals in aqueous systems. Results and discussion However, in most cases it is simply too strong a reducing agent Fig. 1a illustrates the formation of mesoporous a-CoB nano- to generate mesoporous structures and typically can only x † particles using so sacricial pore-directing agents. In a typical generate non-porous metals (Fig. S4a ). One can use weaker synthesis, 10 mg of the block polymer PS-b-PEO was completely reducing agents like dimethylamine borane (DMAB), but they dissolved in an aprotic polar solvent N,N-dimethylformamide are not strong enough to reduce the Co metal precursor † (DMF) (1.5 mL) as unimers because of the high solubility of (Fig. S4b ). In this study, we examined how to combine NaBH4 both hydrophilic PEO and hydrophobic PS segments in DMF. and DMAB as dual-reducing agents to drive the reduction of Co This article is licensed under a † Next an aqueous solution of 2.0 mL cobalt acetate (0.06 M) and while still generating the mesoporous structure (Fig. S5 ). 3 mL dimethylamine borane (0.5 M) was added into this Open Access Article. Published on 15 November 2019. Downloaded 1/15/2020 12:06:34 PM. Fig. 1 Illustration and microscopic characterization of mesoporous a-CoBx alloy nanospheres. (a) An illustration describing the formation process. (b) SEM, (c) TEM, (d) HAADF-TEM, (e and f) elemental

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