Engineers at Lehigh University are the first to utilize a single enzyme biomineralization process to create a quantum confined CdS/reduced graphene oxide (CdS/rGO) catalyst that uses the energy of captured sunlight to split water molecules to produce hydrogen.
The synthesis process—catalyzed by the single enzyme cystathionine γ-lyase (CSE)—is performed at pH 9 in a buffered aqueous solution, under ambient conditions, and utilizes the low-cost precursors Cd acetate, L-cysteine, graphene oxide, and a poly-L-lysine linker molecule. This method overcomes the sustainability and scalability challenges of previously reported methods. A paper on their work is published in the RSC journal Green Chemistry.
Solar-driven water splitting is a promising route towards a renewable energy-based economy. The generated hydrogen could serve as both a transportation fuel and a critical chemical feedstock for fertilizer and chemical production. Both of these sectors currently contribute a large fraction of total greenhouse gas emissions.
One of the challenges to realizing the promise of solar-driven energy production is that, while the required water is an abundant resource, previously-explored methods utilize complex routes that require environmentally-damaging solvents and massive amounts of energy to produce at large scale. The expense and harm to the environment have made these methods unworkable as a long-term solution.
The Lehigh University team harnessed a biomineralization approach to synthesizing both quantum confined nanoparticle metal sulfide particles and the supporting reduced graphene oxide material to create a photocatalyst that splits water to form hydrogen.
Our water-based process represents a scalable green route for the production of this promising photocatalyst technology.—Steven McIntosh, Professor in Lehigh’s Department of Chemical and Biomolecular Engineering
Over the past several years, McIntosh’s group has developed a single enzyme approach for biomineralization—the process by which living organisms produce minerals of size-controlled, quantum confined metal sulfide nanocrystals.
In a previous collaboration with Christopher J. Kiely, Harold B. Chambers Senior Professor in Lehigh’s Department of Materials Science and Engineering, the lab successfully demonstrated the first precisely controlled, biological way to manufacture quantum dots.
Their one-step method began with engineered bacterial cells in a simple, aqueous solution and ended with functional semiconducting nanoparticles, all without resorting to high temperatures and toxic chemicals.
Other groups have experimented with biomineralization for chemical synthesis of nanomaterials. The challenge has been achieving control over the properties of the materials such as particle size and crystallinity so that the resulting material can be used in energy applications.—Leah C. Spangler, lead author and currently a Postdoctoral Research Fellow at Princeton University
Spangler was able to tune the group’s established biomineralization process to not only synthesize the cadmium sulfide nanoparticles but also to reduce graphene oxide to the more conductive reduced graphene oxide form.
Spangler then bound the two components together to create a more efficient photocatalyst consisting of the nanoparticles supported on the reduced graphene oxide.
CSE actively decomposes L-cysteine to generate reactive HS− in aqueous solution at pH 9. Careful selection and control of the synthesis conditions enable both reduction of graphene oxide to rGO, and control over the mean CdS nanocrystal size. The CdS is conjugated to the rGO via a poly-L-lysine crosslinker molecule introduced during rGO formation. The completed CdS/rGO photocatalyst is capable of producing H2, without the aid of a noble metal co-catalyst, at a rate of 550 μmol h−1 g−1 for an optimized CdS/rGO ratio. This rate is double that measured for unsupported CdS and is comparable to CdS/rGO photocatalysts produced using more typical chemical synthesis routes.—Spangler et al.
This material is based on work supported by the National Science Foundation (NSF).
Leah C. Spangler, Joseph P. Cline, John D. Sakizadeh, Christopher J. Kiely and Steven McIntosh (2019) “Enzymatic synthesis of supported CdS quantum dot/reduced graphene oxide photocatalysts” Green Chemistry doi: 10.1039/C9GC00097F