Date of Award

Fall 2018

Project Type

Thesis

Program or Major

Chemistry

Degree Name

Master of Science

First Advisor

Christine A Caputo

Second Advisor

Roy Planalp

Third Advisor

John Tsavalas

Abstract

Black phosphorus (BP) is a semiconducting allotrope of phosphorus that forms buckled 2D layers. Originally discovered by Percy William Bridgman in 1914, the research community mostly ignored this material until 2014 when unexplored properties and applications became more apparent. This resurgence in interest in this material is being dubbed the “renaissance of black phosphorus”.

Black phosphorus find itself alongside the likes of graphene in an interesting category of materials known as layered semiconductors. These 2D materials are characterized by strong intralayer covalent bonds and weak interlayer Van der Waals forces. Since these interlayer forces are so weak, single or few layers of these materials can be isolated in nanoflake or nanoparticle form (sometimes referred to as a quantum dot) and express different properties than their bulk counterparts. For example, black phosphorus has a tunable band-gap, dependent on its layer count, that ranges from 0.3-2.0 eV. This bandgap sits right between the experimentally observed bandgaps of graphene and TMDs. These types of properties are of interest for the photocatalysis community.

Despite recent advances in generating and utilizing BP materials for photocatalytic systems there are still a few successes utilizing black phosphorus quantum dots (BPQDs) and a co-catalyst. Quantum dots offer electronic and morphology differences compared to their nanoflake counterparts. BPQDs have been shown to have stronger absorption of solar light and larger surface areas which lead to more efficient photocatalysis. Modified BP systems using co-catalysts and surface functionalization for the photogeneration of hydrogen gas have been explored in systems using BP nanoflakes but, a modified BPQD system has yet to be reported.

Platinum is a commonly employed and well-studied catalyst for the reduction of protons to hydrogen gas since it’s conduction band is lower than most photosensitizers while being high enough to still efficiently drive the reaction.8 The reduction of platinum onto other surfaces or to form nanoparticles can be easily achieved. For example, either H2PtCl6 or K2PtCl6 can be chemically reduced by NaBH4 in the presence of a capping agent like citric acid to produce platinum nanoparticles that will not aggregate for months.9 This reduction has also been carried out using photo-activated semi-conducting material as the reducing agent resulting in the platinum precursor to form nanoparticles on the surface of the semi-conductor.

Herein, a system utilizing BPQDs and platinum nanoparticles for the photo-production of hydrogen gas will be presented. BPQDs were synthesized from bulk BP crystals via ultrasonication liquid-exfoliation in NMP. BPQD particle size and morphology was explored through SEM and AFM imaging techniques. Photo-physical properties of BPQDs was observed through UV-vis spectroscopy, fluorescence spectroscopy, and methyl viologen based “electron transfer probe” experiments. Colloidal platinum nanoparticles were synthesized through the chemical reduction of H2PtCl6 via NaBH4 in the presence of citric acid. Platinum was also photochemically reduced by BPQDs to produce a composite BP-Pt material. The photocatalytic capabilities of BPQDs in the presence of colloidal platinum nanoparticles and surface bound platinum were compared. Alternative strategies to achieving hydrogen fuel generation using BPQDs is discussed.

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