Light Driven Alternative Plasmonic Catalysis for the Reduction of Heavy Metals in Solution

Visible light driven catalysis has typically been performed with nanomaterials such as gold and silver to take advantage of their high carrier concentration, which produces a Localized Surface Plasmon Response (LSPR) in the interaction with light. Due to their low melting point and expense however, alternatives such as TiN and ZrN are increasingly investigated and gaining interest for the fields of photocatalysis, photochemistry, biophotonics, sensing and waveguiding, among others.<br/>In this work, we present the novel non-thermal plasma synthesis technique to make plasmonic ZrN with particle size of roughly 10 nm.¹ The plasmonic peak is measured in solution to be at around 620 nm in the visible spectrum, making ZrN an attractive and efficient alternative for light driven catalysts. The ZRD and TEM measurements confirm a cubic rock salt structure of ZrN nanoparticles. Catalysis experiments are performed with the goal of reducing heavy metals from water such as platinum and chromium (VI) Chromium (VI) is a known carcinogen², while its reduced species chromium (III) is a nutrient for some plants and is required to metabolize fats and sugars in humans.³ ZrN has shown the ability to reduce chromium (VI) to chromium (III) much faster than TiO2 using only water as an oxidizing agent without requiring the addition of methanol, another known carcinogen, to speed up the reaction time. We find that ZrN is much more efficient in reducing Cr (VI) to Cr (III) compared to the more commonly used semiconductor TiO2, when a broad source light, containing both UV and Visible light is used. For comparison, AU nanoparticles were unable to reduce a measurable amount of chromium (VI) under the same light conditions. Additionally, we also see that ZrN is able to reduce chromium (VI) in the dark by heating the solution in a bath of water while TiO2 and Au were not able to. Other catalysts experiments reducing chromium (VI) were performed in a solution illuminated with the same 500 W HG-XE arc lamp light source (Newport 67005) and a monochromator to spectrally resolve the light to show quantum yield measurements of Cr (VI) reduction using only visible light, convincingly demonstrating that the photocatalysis process is due to the plasmonic response of ZrN. Interestingly, we find that ZrN was able to reduce more Cr (VI) than TiO2 with light at 350 nm as well as in the visible part of the spectrum. Experiments reducing Chloroplatinic acid (H2PtCl6) comparing TiN⁴ and ZrN using the addition of methanol to enhance the reaction were both able to reduce platinum (IV) to platinum, ZrN is much more efficient.<br/>(1) Exarhos, S.; Alvarez-Barragan, A.; Aytan, E.; Balandin, A. A.; Mangolini, L. Plasmonic Core-Shell Zirconium Nitride-Silicon Oxynitride Nanoparticles. <i>ACS Energy Lett.</i> <b>2018</b>, <i>3</i> (10), 2349–2356. https://doi.org/10.1021/acsenergylett.8b01478.<br/>(2) Gibb, H. J.; Lees, P. S. J.; Pinsky, P. F.; Rooney, B. C. Lung Cancer among Workers in Chromium Chemical Production. <i>Am. J. Ind. Med.</i> <b>2000</b>, <i>38</i> (2), 115–126. https://doi.org/10.1002/1097-0274(200008)38:2&lt;115::AID-AJIM1&gt;3.0.CO;2-Y.<br/>(3) Anderson, R. A. Essentiality of Chromium in Humans. <i>Sci. Total Environ.</i> <b>1989</b>, <i>86</i> (1–2), 75–81. https://doi.org/10.1016/0048-9697(89)90196-4.<br/>(4) Barragan, A. A.; Hanukovich, S.; Bozhilov, K.; Yamijala, S. S. R. K. C.; Wong, B. M.; Christopher, P.; Mangolini, L. Photochemistry of Plasmonic Titanium Nitride Nanocrystals. <i>J. Phys. Chem. C</i> <b>2019</b>, <i>123</i> (35), 21796–21804. https://doi.org/10.1021/acs.jpcc.9b06257.