(Invited) Comparison of Photocatalytic Activities of TiN and Zrn Nanoparticles Incorporated into TiO2matrix Under Visible Excitation

Abstract

Refractory transition metal nitrides such as TiN and ZrN belong to a new class of plasmonic materials that exhibit a plasmon resonance in visible and near-infrared spectral regions [1, 2]. The similarity of the optical appearance between that of Au and TiN and ZrN makes one wonder whether these inexpensive, chemically and mechanically robust materials can be used in place of Au for plasmon-mediated photocatalytic reactions. When integrated with a semiconducting TiO2 support, there are two important differences between the TiN/TiO2 and Au/TiO2 interfaces. First, TiN does not form a Schottky barrier with TiO2, leading to poor separation of photogenerated carriers in absence of an external bias. Under positive potential bias however, higher efficiencies of hot electron collection by TiO2 are observed for TiN/TiO2 systems as compared to Au/TiO2 systems [3]. Second, photocarriers generated across 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 nanoparticles (NPs) [4]. Here, 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) [5].We investigate TiN/TiO2 and ZrN/TiO2 photocatalysts for methanol (CH3OH) photoelectrochemical oxidation under visible excitation. The effect of variables such as plasmonic catalyst loading, applied potential, pH and CH3OH concentration are 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 ones embedded into a TiO2 matrix. In the following step, we calculated the rates of resonant optical generation of over-barrier hot electrons 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; clean Zr(NMe2)4 was prepared following a literature protocol. Electrochemical experiments were conducted in a three-electrode photoelectrochemical cell. Thin films of TiN/TiO2 and ZrN/TiO2 were deposited onto fluorine-tin-oxide (FTO)-coated glass as a working electrode. 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.