Preparation And Photocatalytic Hydrogen Evolution Of Noble-Metal Modified Carbon Nitride

The conversion and storage of solar energy to hydrogen energy utilizing suitable semiconductor photocatalysts holds significant potential for transforming the energy structure and achieving carbon neutrality.Among various semiconductor photocatalysts,the carbon nitride（PCN）has attracted tremendous attention owing to its facile synthesis,suitable band structure,excellent physicochemical stability and abundant precursors.However,the pure PCN,prepared by thermal polymerization of precursor,always suffers from slow reaction kinetics of photocatalytic H₂ evolution,mainly because of the rapid recombination of photogenerated charge carriers,poor electrical conductivity and lack of active sites.Therefore,to improve the separation and migration of photogenerated charge carriers as well as accelerate H₂ evolution reaction,in this dissertation,a series of high efficiency noble-metal cocatalysts were constructed on the PCN surface by wet-chemical impregnation and high-temperature reduction methods.Besides,the interface effect between Pt-based noble-metal and PCN on the migration behavior of photogenerated charges and H₂ evolution process was analyzed in depth,which provides a theoretical basis for constructing high-performance PCN-based photocatalysts.To achieve broad-spectrum photocatalytic H₂ evolution,a hybrid co-catalysts system comprising Au nanoparticles（Au NPs）and Pt single atoms（Pt SAs）were constructed on the PCN surface(Pt SAs-Au_2.5/PCN)using a two-step loading strategy.That part of Pt SAs were distributed near the Au NPs without alloying was verified by the corresponding characterizations.As confirmed by photo-electrochemical measurements,when exposed to short-wavelength light,the PCN is photoexcited to generate charge carriers,while the Au NPs and Pt SAs not only promote the separation and migration of charge carriers,but also act as active sites to accelerate H₂ evolution,leading to enhanced H₂ evolution activity for Pt SAs-Au_2.5/PCN with the H₂ evolution rate of 8.8 mmol g^-1 h^-1 at 420 nm;Under long-wavelength light,plasmonic hot-electrons were generated on the surface of Au NPs due to its localized surface plasmon resonance,and the Pt SAs trap the plasmonic hot electrons that jump over the Schottky barrier to participate in H₂ evolution,leading to the H₂ evolution rate of Pt SAs-Au_2.5/PCN reaches 264μmol g^-1 h^-1 at 550 nm.Therefore,benefitting from the synergistic effect between Au NPs and Pt SAs,Pt SAs-Au_2.5/PCN exhibits good photocatalytic H₂ evolution activity under whole visible spectral region with the H₂ evolution rate of 13.7 mmol g^-1 h^-1.The Pt co-catalysts were prepared on carbon nitride nanosheets（PCNS）surface by wet-chemical impregnation and high-temperature reduction methods,and the effects of reduction temperature on the morphology of Pt cocatalyst were systematically investigated.At low temperature,Pt atoms bond with N atoms to form Pt SAs cocatalysts,while at high temperature,partial Pt SAs agglomerate into Pt nanoparticles（Pt NPs）as the result of Pt-N bond cleavage.Meanwhile,the Pt NPs and the nearby residual Pt SAs constitute the analogous core-shell Pt NPs-Pt SAs cocatalysts.Compared with isolated Pt SAs,the synergistic electron effects exist in adjacent Pt NPs/Pt SAs sites:with the PCNS as medium,the electronic interaction between Pt NPs and Pt SAs tailors the d-band electron of Pt NPs-Pt SAs sites with downward shift of d-band center,resulting in fast H₂evolution process.Besides,the strong metal-support interaction between Pt NPs/Pt SAs and PCNS accelerates photoexcited electron transfer from PCNS to Pt NPs/Pt SAs sites to participate in H₂ evolution reaction.Consequently,benefitting from the synergistic electron effects in adjacent Pt NPs/Pt SAs sites,the Pt NPs-Pt SAs/PCNS exhibits excellent photocatalytic H₂ evolution activity with the H₂ evolution rate of 15.75 mmol g^-1 h^-1 under visible light and 37.8 mmol g^-1 h^-1 under full spectrum.To enhance the utilization of charge carriers in the inner of PCN,the PCN-C-Pd heterojunctions and interlayer Pd SAs were constructed on the surface and interior space of PCN,respectively,via wet-chemical impregnation and high-temperature reduction methods.The in-situ carbonization process of local PCN catalyzed by Pd NPs was systematically investigated,resulting in the formation of nitrogen-doped carbon at Pd NPs-PCN interface.The influence mechanism of PCN-C-Pd Schottky junctions and interlayer Pd SAs on the separation/transfer behaviors of photogenerated charge carriers was deeply revealed:the PCN-C-Pd NPs Schottky junctions on PCN surface compel electrons flow from the PCN to Pd NPs,while the interlayer Pd SAs act as vertical charge channels greatly elevates the interlayer charge mobility,leading to unimpeded photoelectron transport from interior and surface of PCN to Pd NPs surface for H₂evolution reaction.Besides,for the PCN-C-Pd heterojunction,the introduction of nitrogen-doped carbon also regulates the electronic structure of Pd NPs with downward shift of d-band center,resulting in fast H₂ evolution process.Consequently,the synergistic effect between PCN-C-Pd Schottky junctions and interlayer Pd SAs on the charge transfer behaviors greatly elevates the photocatalytic H₂ evolution activity of PCN with the H₂evolution rate of 83.34 mmol g^-1 h^-1 under simulated sunlight.