A Pedagogical Investigation of Metal Oxide Photocatalysts: From Band Theory to Nanoparticle Synthesis and Structure

by Mitchell Albert-Shapiro

Mentor: Mary Jane Schulz, Chemistry; funding source: Provost’s Office

shapiroalbertmitchellh_39548_2262658_Shapiro-Albert-Summer-Scholars-2020-Poster

My Summer Scholars research topic focused on the materials that my research lab is most interested in: metal oxide photocatalysts, chemical compounds of a metal and oxygen which, when exposed to light, can promote chemical reactions to break down pollutants in water, ultimately providing clean drinking water in places without an electrical grid. While initially I was anticipating to be in the laboratory gathering data on our existing catalyst, the COVID pandemic afforded me an opportunity to delve into both teaching myself the theory of metal-oxide photocatalysts, which include topics that are cursorily addressed in undergraduate chemistry courses, and reading promising publications in the field. What resulted was a series of self-contained powerpoint presentations on the core concepts of metal oxide photocatalysts, aimed at an undergraduate chemistry audience. I set out in my poster to mimic that self-contained style, so that readers could read whichever section(s) interested them, while still addressing a variety of topics. The sections presented include:

• Semiconductors and the PN-Junction – At their core, metal oxide photocatalysts are semiconductors, conducting electricity worse than a conductor, but better than an insulator. This frustratingly vague definition stems from the sheer number of materials which fall under this classification. However, at a molecular level, the basic requirement of semiconductors is a half-filled valence shell; a molecule can have a total of 8 electrons in its outermost shell, so if it has 4 electrons, it can have semiconductor character. This section also explores atomic-level modifications that are made to semiconductor materials to improve their conductivity, as well as what happens when two different semiconductors meet.
• Band Theory and Band Bending – In a semiconductor, each individual atom has a number of allowed energies for its electrons to occupy, called discrete orbitals. In other words, if an electron can only occupy two energies, the electron will never be found between these levels. But when a semiconductor molecule begins to accumulate more and more atoms, these allowed energy levels overlap and blend together into broad bands of allowed states. The difference between the unexcited state, or valence band, and the excited state, or conduction band, is a vital quantity called the band gap – the energy required for an electron to move between bands.
• Quantum Dots and Quantum Confinement – What if the entire semiconductor is only a handful of atoms? Band theory still applies to the semiconductor particle, but there aren’t as many orbitals contributing to the bands, so the bands get thinner, and the band gap gets wider. This allows us to verify the size of our tiny nanoparticle and differentiate it from a bulk material. The decrease in the bands’ widths also squeezes together like charges, the repulsions of which create a higher energy state. This is the quantum confinement phenomenon, and any particle which experiences this squeezing is a quantum dot. The energy of confinement together with the band gap energy complete a picture of the entire electronic environment of the particle.
• Wulff Constructions – The electronic environment is vital to characterizing a nanoparticle, but just as tantamount is the particle’s shape. A particle will have faces of different sizes, the smallest of which will have the highest surface energy. As such, a small face is where chemical reactions are happening. To determine where these small faces might be, and devise the entire shape of the particle, Wulff’s Theorem is applied, which states that the surface energy of a particle is minimized when the distances of each face from the center of the particle are proportional to the surface energy of that face. This allows chemists to create a graph of the shape in two dimensions, which a computer can then extrapolate into three dimensions.
• Nanoparticle Synthesis – With an appreciation of the shape and electronic environment of a semiconductor, it’s time to make a metal-oxide photocatalyst! Here, countless papers reaffirm that there is no right way to make a particle – in fact, in the 20+ papers which contributed to this research, no synthesis was ever repeated exactly. This brief section walks through the very basic classification of the methods used to create a nanoparticle, but there are countless factors to consider in the real world!
• Future Studies – Having able to get back into the laboratory in person, I’ve begun to synthesize nanoparticles and put my knowledge to the test. This section explains the preliminary work I’ve done in both synthesizing and sizing my own metal oxide photocatalyst, and reemphasizes just how valuable these concepts are to a rising materials chemist.