Photocatalytic Reduction Of Carbon Dioxides By Plasmonic Silver-Based Composite Photocatalyst Under Visible Light Irradiation

Increasing anthropogenic emissions of carbon dioxide into the atmosphere is widely considered as a major cause of global climate change.Reduction of CO2 with H2O to value-added products by using solar energy,which is clean,renewable,and nearly inexhaustible,offers a potential approach for reducing CO2 emissions and for meeting long-term global energy demands.Nonetheless,semiconductor materials that are available have relatively poor photocatalytic efficiency in the visible-light range or have insufficient ability for charge separation.Thus,it is still a challenge to develop new and more efficient visible-light photocatalysts to meet the requirements of future CO2 photoreduction technologies driven by solar energy.Plasmonic photocatalysis has recently come into focus as a very promising technology for high-performance photocatalysis.It affords drastic enhancement of visible-light absorption by localized surface plasmon resonance(LSPR)produced by the collective oscillations of surface electrons of noble metal nanoparticles.LSPR can impart visible-light response to large-band-gap photocatalysts,which is especially important for materials that weakly absorb visible light.Successful applications of plasmon resonance prompted us to prepare new kinds of active and stable visible-light-driven plasmonic photocatalysts for efficient solar-to-chemical energy conversion,namely,AgIO3 or AgX(X=Cl,Br,I)particles with silver nanoparticles dispersed on their surface(abbreviated as Ag/AgIO3 and Ag/AgX,respectively).New types of plasmonic nanocomposite photocatalysts consisting of AgIO3 or AgX(X=Cl,Br,I)with optically active Ag nanoparticles were designed,fabricated,and examined to address the role of plasmon excitations in their performance.Ag supported on AgIO3(Ag/AgIO3),was synthesized through a solid-state ion-exchange reaction between AgNO3 and KIO3 at ambient temperature and pressure,followed by reducing Ag+ions on the surface of AgIO3 particles to AgO by using hydrazine hydrate.Ag supported on AgX(Ag/AgX(X=Cl,Br,I))particles were fabricated with a facile solid-state ion-exchange procedure followed by light-induced reduction of Ag+ions in the surface region of AgX to Ag0 nanoparticles.The resultant catalyst powders were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectra(XPS),ultraviolet-visible diffuse reflection spectra(UV-vis),Brunauer-Emmett-Teller(BET)surface area,and Mott-Schottky techniques to correlate the physicochemical properties of the catalysts with their performance in heterogeneous photo-catalytic reduction of C02.Detailed studies on the photocatalytic reduction behavior of these plasmonic photocatalysts were performed.The particles displayed high activity and stability in the photocatalytic conversion of CO2 with water vapor to CH4 and CO under visible-light irradiation(>400 nm wavelength).Quantum yields(QY),energy returned on energy invested(EROEI),turnover number(TON)and turnover frequency(TOF)of Ag/AgIO3,and QY and EROEI of Ag/AgX were calculated based on the product yields and the active sites of the photocatalysts,and photon quantum number of the incident light.The results indicated that the activity of the as-prepared photocatalysts was much higher than that of N-TiO2.The high activity of photocatalytic reduction of CO2 was attributed to the LSPR exhibited by Ag nanoparticles on the surface of AgIO3 or AgX(X=Cl,Br,I)under visible light irradiation.The mechanism of separation of the photo-generated electrons and holes at the Ag/AgIO3 or Ag/AgX composites was discussed.Semiconductor direct excitation,the direct electron injection from plasmonic Ag nanoparticles to adjacent semiconductors,resonance energy transition,and/or direct photocatalysis by plasmonic Ag nanoparticles may play different roles in different plasmonic photocatalysts,due to the different energy band structure of semiconductors,geometric structures of semiconductors and Ag nanoparticles,etc.In short,due to the LSPR absorption,nanostructured Ag nanoparticles could display distinct absorptions in the visible light region and play a key role in efficient photocatalytic reduction of C02.Our novel photocatalysts could potentially be extended to applications in photo-catalytic reduction of CO2 under sunlight,or removing other types of chemicals in the atmosphere.