Photocatalytic reduction of CO2 over Ag/TiO2 nanocomposites prepared with a simple and rapid silver mirror method

Characterization and Effect of Ag(0) vs. Ag(I) Species and Their Localized Plasmon Resonance on Photochemically Inactive TiO2

Commercial TiO2 (anatase) was successfully modified with Ag nanoparticles at different nominal loadings (1%–4%) using a liquid impregnation method. The prepared materials retained the anatase structure and contained a mixture of Ag0 and AgI species. Samples exhibited extended light absorption to the visible region. The effect of Ag loading on TiO2 is studied for the photocatalytic reduction of CO2 to CH4 in a gas–solid process under high-purity conditions. It is remarkable that the reference TiO2 used in this work is entirely inactive in this reaction, but it allows for studying the effect of Ag on the photocatalytic process in more detail. Only in the case of 2% Ag/TiO2 was the formation of CH4 from CO2 observed. Using different light sources, an influence of the localized surface plasmon resonance (LSPR) effect of Ag is verified. A sample in which all Ag has been reduced to the metallic state was less active than the respective sample containing both Ag0 and Ag+, indicating that a mixed oxidation state is beneficial for photocatalytic performance. These results contribute to a better understanding of the effect of metal modification of TiO2 in photocatalytic CO2 reduction.

The solvent-driven formation of multi-morphological Ag-CeO2 plasmonic photocatalysts with enhanced visible-light photocatalytic reduction of CO2

Ag-CeO2 plasmonic photocatalysts with multiple morphologies were synthesized via a simple solvent-driven method. The phase compositions, morphologies and optical properties of the samples were systematically investigated. A combination of noble metal Ag and semiconductor CeO2 in certain solvents (such as methanol and ethylene glycol) enhanced surface plasmon resonance (SPR), which was attributed to the good dispersion of Ag particles on CeO2 and high Ag0 ratios on the surface. The enhanced SPR effect boosted absorption of incident light and facilitated charge carrier separation and transport efficiency caused by the formation of Schottky barriers, thus promoting VLPCR performance. The optimum ACG sample (ethylene glycol was adopted as the solvent) exhibited the maximum VLPCR activity, achieving a CH4 yield of 100 μmol and a CH3OH yield of 35 μmol per gram of catalyst per hour during 6 h visible-light irradiation.

Plasmon enhanced photocatalytic and antimicrobial activities of Ag-TiO2 nanocomposites under visible light irradiation prepared by DBD cold plasma treatment

Silver nanoparticles (Ag NPs) have been deposited on powder P25 by a novel two-step method involving a precipitation reaction and atmospheric pressure dielectric barrier discharge (DBD) cold plasma treatment without the use of any environmentally and biologically hazardous reducing agents. The silver precursor is formed in the processing of precipitation reaction and then completely reduced to the metallic state by atmospheric pressure DBD cold plasma treatment as proved by X-ray photoelectron spectroscopy, UV-Visible absorption spectra and HRTEM analyses. TEM images indicate that the Ag NPs with average diameter of 3.7 nm were deposited on powder P25 with high dispersion although no reducing agents, stabilizers or surfactants were used. The prepared products show remarkable improvement for methylene blue (MB) photodegradation and effective inhibition of bacterias against Escherichia coli and Staphylococcus aureus.

Nanoplasmonically Engineered Interfaces on Amorphous TiO2 for Highly Efficient Photocatalysis in Hydrogen Evolution

The nanoplasmonic metal-driven photocatalytic activity depends heavily on the spacing between metal nanoparticles (NPs) and semiconductors, and this work shows that ethylene glycol (EG) is an ideal candidate for interface spacer. Controlling the synthetic systems at pH 3, the composite of Ag NPs with EG-stabilized amorphous TiO₂ (Ag/TiO₂-3) was synthesized by the facile light-induced reduction. It is verified that EG spacers can set up suitable geometric arrangement in the composite: the twin hydroxyls act as stabilizers to bind Ag NPs and TiO₂ together and the nonconductive alkyl chains consisting only of two CH₂ are able to separate the two building blocks completely and also provide the shortest channels for an efficient transfer of radiation energies to reach TiO₂. Employed as photocatalysts in hydrogen evolution under visible light, amorphous TiO₂ hardly exhibits the catalytic activity due to high defect density, whereas Ag/TiO₂-3 represents a remarkably high catalytic efficiency. The enhancement mechanism of the reaction rate is proposed by the analysis of the compositional, structural, and optical properties from a series of Ag/TiO₂ composites.

Largely enhanced photocatalytic activity of Au/XS2/Au (X = Re, Mo) antenna-reactor hybrids: charge and energy transfer

An antenna-reactor hybrid coupling plasmonic antenna with catalytic nanoparticles is a new strategy to optimize photocatalytic activity. Herein, we have rationally proposed a Au/XS₂/Au (X = Re, Mo) antenna reactor, which has a large Au core as the antenna and small satellite Au nanoparticles as the reactor separated by an ultrathin two-dimensional transition-metal dichalcogenide XS₂ shell (∼2.6 nm). Due to efficient charge transfer across the XS₂ shell as well as energy transfer via coupling of the Au antenna and Au reactor, the photocatalytic activity has been largely enhanced: Au/ReS₂/Au exhibits a 3.59-fold enhancement, whereas Au/MoS₂/Au exhibits a 2.66-fold enhancement as compared to that of the sum of the three individual components. The different enhancement in the Au/ReS₂/Au and Au/MoS₂/Au antenna-reactor hybrid is related to the competition and cooperation of charge and energy transfer. These results indicate the great potential of the Au/XS₂/Au antenna-reactor hybrid for the development of highly efficient plasmonic photocatalysts.

Ni-Nanocluster Modified Black TiO2 with Dual Active Sites for Selective Photocatalytic CO2 Reduction

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO₂ molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni-nanocluster loaded black TiO₂ (Ni/TiO_2[Vo] ) with built-in dual active sites for selective photocatalytic CO₂ conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO₂ binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO_2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO₂ (P-25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.

Plasmonic surface nanostructuring of Au-dots@SiO2 via laser-irradiation induced dewetting

The in situ observation of Au dot formation and the self-assembly dynamics of Au nanoparticles (NPs) was successfully demonstrated via dewetting of Au thin films on SiO₂ glass substrates under nano-second pulsed laser irradiation using a multi-quantum beam high-voltage electron microscope. Moreover, using electron energy-loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM), the plasmonic properties of the formed Au/SiO₂ nanostructure were analyzed to demonstrate its validity in advanced optical devices. The uniformly distributed Au NPs evolved into a dot alignment through movement and coalescence processes was demonstrated in this in situ observation. We carried out the plasmon-loss images of the plan view and the cross-section of the Au/SiO₂ nanostructures were obtained at the plasmon-loss peak energy for investigate the three-dimensional distribution of surface plasmon. Furthermore, discrete-dipole approximation (DDA) calculations were used to simulate the plasmonic properties, such as the surface plasmon resonance and the surface plasmon field distribution, of isolated single Au/SiO₂ nanostructures. This STEM-EELS-acquired surface plasmon map of the cross-sectional sample is in excellent agreement with the DDA calculations. This results demonstrated the influence of the contact condition between Au NP and SiO₂ glass on the plasmonic properties, and may improve the technology for developing advanced optical devices.

An overview of the reaction conditions for an efficient photoconversion of CO2

Carbon dioxide (CO2) emission is one of the well-known causes of global warming. Photoconversion of CO2 to useful chemical compounds using solar energy is an attractive approach as it reduces the major greenhouse gas and promises a sustainable energy source. This method involves radical-chain reactions that form cation and anion radicals generated as a result of the reaction with photogenerated electrons (e−) and holes (h+) between metal oxide photocatalyst and the reactants. Therefore, the product distribution of a modified photocatalyst even under specific reaction conditions is difficult to predict. The CO2 photocatalytic reduction process is controlled by several conditions such as reactor configuration, photocatalyst type, and nature of the reducing agents. Here, we review the parameters such as temperature, pH, CO2 pressure, type of reductant, role of co-catalysts, dopants, and type of photocatalysts that influence the end products of the photocatalytic reduction of CO2. In this review, the different modifications recommended for the photocatalysts to improve CO2 reduction and receive maximum valuable end product (methane, ethanol, methanol, hydrogen, and carbon monoxide) have been listed. The discussion also includes specific behaviors of photocatalysts which lead to different product distribution. It has been noted that different metal and nonmetal dopants improve the activity of a photocatalyst and influence the end product distribution by altering the active species. Similarly, the key factors, i.e. size, morphology and doping, which have been ruling the photocatalytic activity of CO2 reduction under UV or visible light irradiation have been identified.