Amorphous copper oxide (Cu(II)) nanoclusters function as efficient electrocatalysts for the reduction of carbon dioxide (CO2) to carbon monoxide (CO). In addition to promoting electrocatalytic activity, Cu(II) nanoclusters act as efficient cocatalyts for CO2 photoreduction when grafted onto the surface of a semiconductor (light harvester), such as niobate (Nb3O8(-)) nanosheets. Here, the photocatalytic activity and reaction pathway of Cu(II)-grafted Nb3O8(-) nanosheets was investigated using electron spin resonance (ESR) analysis and isotope-labeled molecules (H2(18)O and (13)CO2). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen ((18)O2) and then reduce CO2 to produce (13)CO. ESR analysis confirmed that excited holes in the valence band of Nb3O8(-) nanosheets react with water, and that excited electrons in the conduction band of Nb3O8(-) nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO2 into CO. The Cu(II) nanocluster-grafted Nb3O8(-) nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO2 photoreduction systems.
Titanium dioxide (TiO2), as an excellent photocatalyst, has been intensively investigated and widely used in environmental purification. However, the wide band gap of TiO2 and rapid recombination of photogenerated charge carriers significantly limit its overall photocatalytic efficiency. Here, efficient visible-light-active photocatalysts were developed on the basis of TiO2 modified with two ubiquitous nanoclusters. In this photocatalytic system, amorphous Ti(IV) oxide nanoclusters were demonstrated to act as hole-trapping centers on the surface of TiO2 to efficiently oxidize organic contaminants, while amorphous Fe(III) or Cu(II) oxide nanoclusters mediate the reduction of oxygen molecules. Ti(IV) and Fe(III) nanoclusters-modified TiO2 exhibited the highest quantum efficiency (QE = 92.2%) and reaction rate (0.69 μmol/h) for 2-propanol decomposition among previously reported photocatalysts, even under visible-light irradiation (420-530 nm). The desirable properties of efficient photocatalytic performance with high stability under visible light with safe and ubiquitous elements composition enable these catalysts feasible for large-scale practical applications.
We successfully clarified the mechanisms of visible-light-driven photocatalytic reactions of Fe(III)-grafted TiO 2 (Fe/TiO 2 ) and Fe(III)-grafted Ru-doped TiO 2 (Fe/ Ru:TiO 2 ). ESR spectroscopy revealed that the visible-light response of the Fe/TiO 2 photocatalyst resulted in the direct charge transfer from the valence band of TiO 2 to the grafted Fe ions. For the Fe/Ru:TiO 2 photocatalyst, acceptor levels were formed by doping Ru ions in the lattice of TiO 2 , and the electrons at the acceptor levels excited on visible-light irradiation readily transfer to Fe ions. Since a longer wavelength light generated the conduction band electrons, we also proposed a two-step electron excitation from valence band to the conduction band through defect levels such as oxygen vacancy. As a result, a part of photogenerated electrons in the conduction band transfer to the grafted Fe ions. Therefore, the Fe/Ru:TiO 2 photocatalyst showed a higher activity because such two kinds of indirect charge transfer to the grafted Fe ions occurred in addition to the direct interfacial charge transfer observed for Fe/TiO 2 . Moreover, chemiluminescence photometry confirmed that the grafted Fe ions function as a promoter to reduce O 2 into H 2 O 2 via two-electron reduction. Therefore, the acceleration in the reduction of O 2 with doping Ru and grafting Fe ions allows a larger number of holes to oxidize organic compounds, resulting in the higher photocatalytic activity.
The mechanochemical surface functionalization of iron oxides with disordered lattices on bare iron (Fe) particles was investigated using simple milling processes to clarify the formation mechanism of the oxide layer and investigate the near-surface models with different states. The homogeneous α-Fe particles at the milling equilibrium were first prepared under an argon atmosphere. After the subsequent milling reaction of the particles with oxygen molecules, the surface analyses by X-ray diffraction and Raman and X-ray photoelectron spectroscopies revealed that the near-surface layers consisted of two iron oxide phases (α-Fe 2 O 3 and Fe 3 O 4 ) through oxygen atom diffusion, and the α-Fe 2 O 3 was dominantly grown on the near surface. During the initial reaction, the signals from an electron spin resonance suggested the dangling bond formation on α-Fe 2 O 3 . The oxygen atoms effectively induce disordered lattices in the local area to form oxidized Fe 3+ clusters, and the geometric distortion formed the dangling bonds, which were theoretically supported by a molecular orbital calculation to elucidate the increase in the unpaired electron sites on the α-Fe 2 O 3 . Therefore, the defective Fe 3+ ions induced by the lattice mismatching between the clusters and bare α-Fe are found to form the disordered lattice that contains the oxygen atoms with unpaired electrons, which are successfully induced by the near-surface strain based on the simple mechanochemical reactions. The patterns of surface activation of the Fe particle surfaces by oxidization will be capable of novel chemical reactions by selective oxygen insertion as well as deep oxidation.
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