Plasmon Induced Photocatalysis Based On Transition Metal Nitrides

Surface plasmons are a collective electronic oscillation generated by the coupling of free electrons and electromagnetic fields in metal nanostructures.When it comes to a two-dimensional interface,a propagating surface plasmon is formed,whereas,for metallic nanostructures with dimensions much smaller than the incident wavelength,a"non-propagating"localized surface plasmon is formed.Surface plasmons confine light fields and energy to subwavelength scales,forming a huge electromagnetic field enhancement in local space,which can significantly promote the interaction of light and matter and have important research value in many disciplines.Especially for localized surface plasmon resonance,the applications are more extensive because it can be directly excited by light and the resonant frequency can be tuned by the size,geometric configuration,aggregation mode,and medium environment of the metal nanostructures.In metallic nanostructures,the non-radiative relaxation process of plasmon can generate high-energy"hot electrons",which convert light energy into energy carried by hot electrons that can be used to drive or facilitate a variety of chemical reactions related to energy conversion processes.Based on this,the concept of plasmonic photocatalysis came into being and became a frontier research field of the intersection of physics and chemistry.The noble metals gold（Au）and silver（Ag）have excellent plasmonic properties and their resonance frequencies correspond to the wavelength range of sunlight,therefore they have been the most commonly used materials for surface plasmon studies.Transition metal nitrides of the IVB,VB,and VIB groups have similar excellent plasmonic properties as the noble metals Au and Ag in the ultraviolet-visible-near infrared region and are chemically stable,work-function tunable,and resource-rich,making them a new class of plasmonic materials of great research value.Our work focuses on the design,synthesis,and photocatalytic properties of novel plasmonic materials of non-noble metal niobium nitride（Nb N）,zirconium nitride（Zr N）,and titanium nitride（Ti N）.The main contents are as follows:（1）The Nb N nanoflower with broadband absorption properties was controllably prepared.By loading nickel（Ni）nanoparticles as active sites,an Nb N-Ni"antenna-reactor"plasmonic photocatalytic system was constructed,and its photocatalytic performance for enhancing ammonia borane hydrolysis was investigated.The Nb N nanoflowers were synthesized by controlled nitridation of niobium oxide（Nb₂O₅）nanoflowers,and the ammoniation process resulted in Nb N exhibiting a rich pore structure.The strong broadband optical absorption of Nb N nanoflowers is a consequence of the substantial plasmon coupling of discrete surface plasmon resonance modes and,more importantly,the multiple reflections and absorptions of incident photons within the hierarchical nanoflower structures.Under the visible light（λ>420nm）and near-infrared light（λ>780 nm）irradiation,the catalytic performance of ammonia borane hydrolysis was significantly improved.The catalytic rate of the Ni/Nb N catalyst with an optimum Ni loading（2 wt%）was increased by a factor of 4.6compared to the dark condition under visible light irradiation and by a factor of 2.7under near-infrared light irradiation.Furthermore,the reaction kinetics of ammonia borane hydrolysis were studied in detail.Firstly,the potential barrier required for the reaction was lowered by 0.09 e V under the visible light illumination.Secondly,the catalytic rate and light intensities display a transition from a linear to a super-linear relationship.Further,it exhibits a larger kinetic isotope effect under visible light irradiation.These evidences suggest that the primary reaction mechanism for the catalytic enhancement is due to plasmon-induced hot carriers rather than the photothermal effect.（2）By designing and constructing the ohmic metal-semiconductor heterojunction of Nb N/Nb₂O₅,the hydrogen（H₂）evolution performance in photocatalytic water splitting was studied.After high-temperature calcination,the Nb₂O₅ precursor exhibited mesogenic properties.By controlling the ammoniation process,the Nb₂O₅ surface is partially ammonized,resulting in an in situ ohmic contact interface between Nb N and Nb₂O₅.Both the metal and the semiconductor are exposed through mechanical grinding.In the half-cut metal-semiconductor heterostructure,Nb N enhances the light absorption of the sample and generates hot electrons transfer into the conduction band of Nb₂O₅upon excitation by light,participating in the half-reaction of water splitting.Due to the ohmic contact between Nb N and Nb₂O₅,the photogenerated carriers are effectively separated,thereby facilitating the photocatalytic reaction.Compared with pure Nb N,the H₂ evolution rate of Nb N/Nb₂O₅ heterostructure is increased by a factor of 4.2.Furthermore,the hot carrier transfer process and mechanism at the interface are analyzed by photoelectric tests.（3）By optimizing the ammoniation conditions,the high-quality crystallized Zr N nanoparticles were obtained.After decorating with platinum（Pt）nanoparticles,an"antenna-reactor"structure of Zr N-Pt was constructed.Combined with theoretical calculations,the photocatalytic nitrogen fixation performance of the Zr N-Pt composite was investigated.In the photocatalytic nitrogen fixation reaction,the pure Zr N surface facilitates the absorption of N₂,while the Pt site is favorable for the adsorption of H.Therefore,the synergistic effect of the dual sites formed by Zr N nanoparticles and Pt enables the Zr N-Pt"antenna-reactor"structure to achieve photocatalytic nitrogen fixation performance under visible light irradiation at room temperature and pressure.Furthermore,the photoelectric test results show that the Zr N-Pt composite structure has a lower impedance and a higher photocurrent response than Zr N,indicating that the"antenna-reactor"structure of Zr N-Pt can efficiently facilitate the transfer of hot electrons.（4）With the synthesis of monodisperse titanium dioxide（TiO₂）nanocrystals,the core-shell structure was constructed in two ways to effectively alleviate the high temperature sintering problem of transition metal nitrides.Firstly,the monodisperse TiO₂ nanocrystals were uniformly coated with silicon dioxide（SiO₂）by the inverse microemulsion method,and the core-shell structure of Ti N@SiO₂ was constructed by high-temperature ammoniation.The doping of molybdenum（Mo）changes the electronic structure of Ti N,resulting in a blue shift in the absorption spectrum.Furthermore,the core-shell structure of Ti N@C was constructed by high-temperature carbonization and ammonification utilizing the polymer chains uniformly covered on the surface during the synthesis of TiO₂ nanocrystals.On this basis,an"antenna reactor"structure was constructed with Ti N@C coupling light energy and Pt,rhodium（Rh）,and ruthenium（Ru）as reaction centers,and the photocatalytic nitrogen fixation performance was realized under visible light irradiation.