Skip to main content

Synthesis of Hybrid Metal-Semiconductor Tetrapod Photocatalysts for Improved Water Splitting

Periodic Reporting for period 1 - HMST-PC (Synthesis of Hybrid Metal-Semiconductor Tetrapod Photocatalysts for Improved Water Splitting)

Reporting period: 2019-01-01 to 2020-12-31

The direct harvesting of sunlight to generate fuels or electricity represents a highly attractive means to produce clean energy, with the input being a free and unlimited natural resource. While modern solar cells are capable of directly generating electricity from sunlight, low efficiencies and challenges storing the power for later use points to a need for new technologies. To this end, “solar-to-fuel” generating systems are designed to store energy from sunlight in the form of chemical bonds which can be later broken with mild external stimulus to provide energy on-demand. Of these, the most studied system is the photoinduced solar water splitting reaction, wherein liquid H2O is broken down into hydrogen gas (H2) and oxygen gas (O2) using semiconductor photocatalyst. In recent years, semiconductor nanoparticles (SC-NPs) have been extensively studied as photosensitizers in solar energy-to-fuel conversion systems, but no system exists that can efficiently achieve both H2 and O2 evolution from H2O. The overall objective of this proposal as written was to explore enabling opportunities towards and develop, a family of hybrid metal-semiconductor nanocrystals that would serve as stand-alone visible-light photocatalysis for the efficient generation of fuels from sunlight. Building upon a rich body of literature related to nanoscale photocatalysis and energy storage, the proposed work was designed to make significant contributions to the action field by addressing key challenges hindering the efficient use of nanoscale photocatalysts to date, and integrating these advances into a proof-of-concept energy conversion system.
At the beginning of the project, and throughout, semiconductor tetrapod materials were a primary research focus, due to their exemplary light absorption properties and a possibility for enhanced charge separation that would lead to more efficient catalysis. As a result, protocols to prepare CdSe-seeded CdS-shell tetrapods with high activity toward photocatalytic hydrogen evolution were initially developed based on earlier literature reports, with a focus on fine-tuning internal energetics and tetrapod arm length to lead to optimal catalysis. These tetrapods were then modified with hydrogen evolution catalysts, namely, gold nanoparticles. For gold nanoparticles, we found an over 10x increase in activity towards hydrogen evolution when gold was selectively deposited at the tips of the tetrapod arms compared to nanorod control samples. Furthermore, new methods for the modification of CdSe-seeded CdS-tetrapods with Pt-nanoparticles were developed, enabling selective platinum nanoparticle deposition onto the tips of the CdS arms. While not pursued at this stage, an effort is currently ongoing based on this work, towards the goal of comparing hydrogen evolution activity between tetrapod samples with differing internal energetics and selective Pt-nanoparticle tips.

While tetrapods remained a key focus throughout the effort, leading to the abovementioned dissemination and research progress as a result of the first half of the project, we found there was difficulty in preparing these materials with appropriate energetics at the quantity/quality level required for rapid screening of conditions required to move forward with the action. Since multiple work efforts of the project would rely on having large batches of these primary materials for synthetic modifications and subsequent characterizations as photocatalysts, the decision was made to also begin to focus on systems that are easier to prepare that could be used to address the same questions/goals as the proposal targeted with the tetrapod materials. As a result, we transitioned to also incorporate the use of nanorod architectures, as these are well established and easily accessible through known protocols. This transition to model systems that were easier to prepare and analyze proved vital as we moved forward with the project.

Indeed, during the project, we found that it was necessary to break down the water-splitting reaction into its constituent components, focusing in one branch on solving key bottlenecks in the production of efficient hydrogen evolution catalysts that have efficient charge separation and also hole extraction at neutral pH, and in the other branch focusing on the coupling of photoactive nanoparticles to known oxygen evolution catalysts (noble metal-based, and biological) to address the challenge of coupling oxygen evolution to existing solar-to-hydrogen generating systems. It was envisioned that solving these challenges separately with well-defined systems would afford the greatest opportunity for achieving the overall goals of the project. At the conclusion of the project, we have tackled the issue of hole extraction in semiconductor photocatalysts by developing the first protocol for control of CdSe-seeded CdS-shell nanorod widths, demonstrating a previously unreported and unexpected "sweet spot" in nanorod shell thickness that enabled up to 100% photon-to-hydrogen conversion at neutral pH. This effort is currently being prepared for submission to ACS Energy Letters and has identified an ideal material for coupling to existing oxygen evolution systems that rely on efficient hole extraction. In addition, we have one manuscript currently submitted and in review, with recommendations to accept upon revisions, to the Journal of Materials Chemistry A on the coupling of metallic nanoparticles to spinach-derived photosystem II, achieving simultaneous oxygen evolution under irradiation and the highest photocurrent production reported for photosystem II-based bioconjugates. In addition, we began efforts to couple these two projects together towards the goal of producing an all-in-one water-splitting catalyst, and these efforts are continuing in the Amirav lab at the Technion Institute as a result of the work enabled by this project.
Numerous advances beyond the state of the art were made in the project. These are listed below:
1) CdSe-seeded CdS tetrapods with 2.3 nm seeds were prepared, with selective Pt-tips, and their catalytic activity tested
2) Protocols were developed for facile tuning of CdSe-seeded CdS nanorod width
3) Nanorods with 100% photon-to-hydrogen conversion at neutral pH were prepared
4) A biohybrid photocatalytic oxygen evolution and photocurrent production system was invented utilizing novel chemistry to link photoactive nanoparticles to photosystem II

Additionally, it is expected that an all-in-one water-splitting photocatalyst will be prepared by combining the systems developed in this project in the near future.

Potential impacts for the project relate to the vital need for alternative energy sources that limit our need for environmentally harmful fossil fuel utilization.
This work highlights the promise of coupling robust chemical methods, precision nanochemistry, and abundant biological materials to generate fuels from water, which will have broad-reaching societal impacts as we work towards carbon neutral economies and green energy systems.
Summary image