(Invited) Photoelectrochemical Methanol Oxidation Under Visible and UV Excitation of TiO2-Supported TiN and Zrn Plasmonic Nanoparticles

Abstract

TiN and ZrN nanoparticles (NPs) are an emerging class of plasmonic materials that can be used for harvesting energy of visible light in order to drive chemical reactions on the photocatalyst surface [1-3]. These refractory NPs effectively absorb light over l = 450-1200 nm, covering most of the solar spectrum. Both TiN and ZrN have the optical appearence of gold, but with the advantages of thermal stability, corrosion resistance and low cost. Previous theoretical and experimental research have shown that TiN does not exhibit as strong of a plasmonic resonance as gold (broader response, sample variability due to defect-dependent metallicity) [2, 4]. ZrN is expected to have a sharper blue-shifted local surface plasmon resonance (LSPR) response in comparison to TiN [4, 5]. Indeed, this theoretical prediction was verified by observation of a sharp plasmonic maximum for ZrN NPs decorated by a thin layer of SiN [4]. However, ZrN NPs appear to be less chemically inert owing to zirconium’s strong affinity for oxygen. A layer of dielectric ZrO2 forms on the surface of ZrN NPs even in the presence of trace oxygen. The oxide layer has a detrimental effect on the LSPR, leading to its broadening and red-shifting [5].Herein both experimental and computational approaches are used in order to optimize the performance of TiO2-supported TiN and ZrN NPs towards photoelectrochemical methanol (CH3OH) oxidation under visible excitation. UV excitation is utilized in order to provide complementary information on the interaction between photogenerated carriers at the plasmonic NP/semiconductor interface.The effects of plasmonic catalyst loading and applied potential were examined. Reaction products were determined by gas chromatography-mass spectrometry analysis of ZrN/TiO2 and TiN/TiO2 aqueous suspensions. Optical spectra for both TiN and ZrN NPs were computed using COMSOL including calculations of these NPs in solution and embedded in a TiO2 matrix. To examine the effect of oxide layer, the NPs were decorated with the corresponding oxide layers.ZrN NPs were synthesized by ammonolysis of Zr(NMe2)4. Clean Zr(NMe2)4 was prepared following a protocol from the literature [6]. In-house made ZrN powder and commercial TiN NPs (PlasmaChem) were dispersed into a P25 TiO2 matrix via the sonochemically mediated mixing of the transition metal nitride and TiO2 NPs in 50:50 (v/v) H2O:ethanol mixture overnight. The resulting photocatalysts contained 0.5 - 5 wt. % loadings of TiN and ZrN on TiO2. The ZrN/TiO2, TiN/TiO2 and bare TiO2 films for photoelectrochemical experiments were prepared by drop-casting 60 mL of ink comprising 20 mg of the photocatalyst, 30 ml of 5 wt% Nafion solution (Ion Power), 2.96 ml H2O and 0.74 ml iso-propanol, on FTO slide (coated surface area of 0.385 cm2). Experiments were conducted in a three-electrode photoelectrochemical cell. The photocatalyst films deposited on FTO-coated glass substrates served as working electrodes, while Pt foil and Ag/AgCl in 3 M NaCl (BioLogic, Inc) were used as 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.Our results indicate that optical properties and photocatalytic activity of ZrN/TiO2 are strongly affected by ZrN surface oxidation and agglomeration. We found that under visible illumination, both in-house synthesized 17 nm ZrN and commercial 30 nm TiN NPs promote TiO2 activity for CH3OH oxidation, while under visible + UV excitation, an inhibition effect is observed. The differences between the TiN/TiO2 and ZrN/TiO2 interfaces are discussed and the mechanisms of promotion/inhibition of TiO2 photocatalytic activity by ZrN and TiN NPs are proposed.