Research Article www.acsami.org
Toward Inexpensive Photocatalytic Hydrogen Evolution: A Nickel Sulfide Catalyst Supported on a High-Stability Metal−Organic Framework Aaron W. Peters,† Zhanyong Li,† Omar K. Farha,*,†,‡ and Joseph T. Hupp*,† †
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
‡
S Supporting Information *
ABSTRACT: Few-atom clusters composed of nickel and sulfur have been successfully installed into the Zr(IV)-based metal−organic framework (MOF) NU-1000 via ALD-like chemistry (ALD = atomic layer deposition). X-ray photoelectron spectroscopy and Raman spectroscopy are used to determine that primarily Ni2+ and S2− sites are deposited within the MOF. In a pH 7 buffered aqueous solution, the porous catalyst is able to produce H2 gas at a rate of 3.1 mmol g−1 h−1 upon UV irradiation, whereas no H2 is generated by irradiating bare NU-1000. Upon visible light irradiation, little H2 generation was observed; however, with the addition of an organic dye, rose bengal, NiS-AIM can catalyze the production of H2 at an enhanced rate of 4.8 mmol g−1 h−1. These results indicate that ALD in MOFs (AIM) can engender reactivity within high surface area supports for applications in the solar fuels field. KEYWORDS: metal−organic framework, nickel sulfide, hydrogen evolution, atomic layer deposition, photocatalysis
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INTRODUCTION
dramatically increase the activity of transition metal sulfides in the water reduction reaction.8 Metal−organic frameworks (MOFs), a class of materials composed of metal ions/clusters and organic linkers,9−12 offer an exceptional opportunity as potential scaffolds for catalytic materials. Thanks in part to their high surface area13 and chemical stability,14 MOFs have been used as supports to deposit highly dispersed and well-isolated catalysts.15−17 Researchers have consequently tried to take advantage of these porous materials for hydrogen evolution studies.18−20 Most commonly, catalytic metal nanoparticles have been deposited on, or included within, porous frameworks.21−25 Alternatively, known molecular catalysts have been functionalized and then used as MOF linkers.26,27 Despite widespread use of metal chalcogenides in energy-related applications,28,29 metal sulfide-functionalized MOF materials have thus far been understudied. The few published instances have mainly focused on mimicking [FeFe]-hydrogenase30 by incorporating a molecular iron carbonyl catalyst in the MOF linker. For example, the Ott31 and Feng32 groups have tethered a [FeFe]hydrogenase mimic to the linker of Zr-based MOFs through bridging sulfur bonds. However, by binding the catalyst through this manner, the sulfides are unfortunately precluded from participating in the catalysis.18 Recently, we have reported atomic layer deposition (ALD) in MOFs (AIM)33−36 as an
Because of increasing global energy demand as well as unprecedented concentrations of CO2 in the atmosphere resulting from the burning of fossil fuels, extensive research has been conducted to develop renewable energy technologies, of which solar energy plays a prominent role.1 Numerous avenues for harvesting sunlight have been proposed such as solar-to-electricity2 and direct solar-to-fuel3 conversion. Despite great progress made in these research areas, these technologies (solar fuels in particular) still fall short of widespread economic feasibility.4 Needed to decrease the cost of solar fuel generation (e.g., water-splitting reaction) are efficient and inexpensive catalysts for water oxidation and reduction half-reactions.5 Metal sulfides are an intriguing class of non-precious metal catalysts for the hydrogen evolution reaction owing to their “just right” binding energy of protons to undercoordinated active sites.6 In fact, some metal sulfides (e.g., MoxSy, NixSy, and CoxSy) exhibit similar turnover frequencies on their most catalytically active sites as expensive precious metals (e.g., platinum) for H2 production.7 An ongoing challenge, however, is to preferentially present high-activity sites in ways that retain catalyst stability and reactant accessibility. A common method to increase the activity of a heterogeneous catalyst is to decrease the size of the catalyst, thereby increasing the number of exposed active surface sites and/or enhancing the intrinsic activity due to quantum size effects. In this vein, nanostructuring of transition metal sulfides by surface site engineering or templated growth has been shown to © XXXX American Chemical Society
Received: April 20, 2016 Accepted: July 19, 2016
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DOI: 10.1021/acsami.6b04729 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Idealized representation of NU-1000 after imparting NiSx functionality via AIM. In the photocatalytic reaction, the pyrene-based linker acts as a UV sensitizer, which then transfers an electron into the nickel sulfide-functionalized node (proposed structure shown in dotted circle) and subsequently reduces protons/water to hydrogen gas. 3 × 40 mL of acetone over the course of 24 h to ensure complete exchange of the DMF solvent with acetone. This is to facilitate the removal of residual solvent molecules as any residue DMF molecules can cause pore collapse during the thermal activation process. The powder was then thermally activated under vacuum at 120 °C for 18 h or until the outgas rate was
