Comparison of Photocatalytic Activities of TiN and Zrn Nanoparticles Incorporated into TiO2matrix Under Visible Excitation
Authors: Baturina Olga A , Boltersdorf Jonathan , Epshteyn Albert , Purdy Andrew , Simpkins Blake , Leff Asher C , Forcherio Gregory T. , Santiago Eva , Govorov Alexander O. 
Published: December 2020

TiN and ZrN belong to a new class of refractory transition metal nitride plasmonic materials that exhibit a localized surface plasmon resonance in visible and near-infrared spectral regions [1, 2]. TiN and ZrN exhibit an optical response analogous to that of the extensively studied plasmonic Au nanostructures thereby offering inexpensive, chemically and mechanically robust material alternative to Au for plasmon-mediated photocatalytic reactions. When plasmonic nanoparticle (NP) is integrated with a semiconducting TiO2 support, properties of the interface become important. Prior research highlighted two significant differences between the TiN/TiO2 and Au/TiO2 interfaces [3]. First, TiN does not form a Schottky barrier with TiO2, leading to poor separation of photogenerated carriers in the absence of an external bias. However, under positive potential bias, higher efficiencies of hot carrier collection are observed for TiN/TiO2 system as compared to Au/TiO2 [4]. Second, photocarriers generated via interband transitions in TiN may play a major role in photoelectrochemical reactions at the TiN/TiO2 interface, in contrast to the intraband transitions in Au NPs at the Au/TiO2 interface [4, 5]. Herein, we extend our scope to the ZrN/TiO2 interface that is predicted to benefit from higher rates of hot carrier generation in ZrN (vs TiN) [6].

The plasmonically-sensitized TiN/TiO2 and ZrN/TiO2 photocatalysts were investigated for methanol (CH3OH) photoelectrochemical oxidation under visible excitation. The effect of plasmonic NPs loading, applied potential, pH and CH3OH concentration were examined. Near-and far-field electrodynamic simulations and quantum calculations were performed to facilitate interpretation of photoelectrochemical experiments. Using COMSOL software, we computed both free-standing TiN and ZrN NPs, as well as embedded into a TiO2 matrix. The rates of resonant optical generation of over-barrier hot electrons were calculated using the quantum formalism.

ZrN powder prepared in lab and commercial TiN NPs (PlasmaChem) were dispersed into a P25 TiO2 matrix by ultrasonically agitating metal nitride NPs and TiO2 powders in H2O overnight. ZrN NPs were synthesized by ammonolysis of Zr(NMe2)4 and subsequent annealing at 950 °C under NH3 flow; clean Zr(NMe2)4 was prepared following a literature protocol [7]. Electrochemical experiments were conducted in a three-electrode photoelectrochemical cell. Thin films of TiN/TiO2 and ZrN/TiO2 deposited onto fluorine-tin-oxide (FTO)-coated glass served as working electrodes. Platinum foil and Ag/AgCl in 3 M NaCl (BioLogic, Inc) were used as a counter and reference electrodes, respectively. Nine single-color LED lights were employed for illumination. The intensities varied between 10-100 mW/cm2, and 300-480 mW/cm2 in the wavelength range of 490-670 and 730-960 nm, respectively.

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