Directed Assembly of Cuprous Oxide Nanocatalyst for CO₂ Reduction Coupled to Heterobinuclear ZrOCo^II Light Absorber in Mesoporous Silica

Hierarchical assembly of an oxo-bridged binuclear ZrOCo^II light absorber unit coupled to a cuprous oxide nanocluster catalyst for CO₂ reduction on mesoporous silica support is demonstrated. The proper positioning of the Cu oxide cluster was achieved by photodeposition of a [Cu­(NCCH₃)₄]²⁺precursor by visible light excitation of the ZrOCo charge transfer chromophore, followed by mild calcination at 350 C. Illumination of the Cu_<i>x</i>O_<i>y</i>-ZrOCo unit so formed in the presence of a diethylamine electron donor resulted in the reduction of surface Cu centers to Cu⁰ as demonstrated by the characteristic infrared band of adsorbed ¹³CO probe molecules at 2056 cm^–1. For analogous Cu_<i>x</i>O_<i>y</i>-TiOCo^II units, the oxidation state makeup of the surface Cu centers was dominated by Cu^I, and the Cu⁰, Cu^I, and Cu^II composition was found to depend on the wavelength of MMCT excitation. The observed strong dependence of the CO₂ photoreduction yield on the oxidation state of the surface Cu centers directly proves that CO₂ is reduced on the Cu_<i>x</i>O_<i>y</i> surface, thus establishing that the ZrOCo^II unit functions as light absorber, donating electrons to the Cu_<i>x</i>O_<i>y</i> catalyst on whose surface CO₂ is reduced

Effect of the Agglomerated State on the Photocatalytic Hydrogen Production with in Situ Agglomeration of Colloidal TiO₂ Nanoparticles

The photocatalytic production of H2 in aqueous TiO2 colloid (with methanol as an electron donor) was greatly accelerated by the in situ agglomeration of the colloid although such an agglomeration should reduce the photocatalytic activity in most other cases because of the reduction of the surface area. The in situ agglomeration occurred after an induction period of 3 h and was ascribed to the pH increase which was resulted from the photocatalytic reduction of nitrate (incorporated from the synthetic step of TiO2 sol) to ammonia. The agglomeration occurred at pH close to the isoelectric point of colloidal TiO2 which was 6.9 as measured by the ζ-potential. It is proposed that the charge separation is facilitated by electron hopping from particle to particle when TiO2 nanoparticles are connected with each other within the agglomerates. This behavior was further supported by the photocurrent collection measurement (mediated by the methyl viologen MV2+/MV+ redox couple in the colloidal solution), which also showed a rapid increase in the photocurrent after the agglomeration of TiO2 nanoparticles. When the colloid of TiO2 was initially coagulated at around pH 6, the production of hydrogen increased linearly with time without showing an induction period and the collected photocurrent showed an immediate increase upon irradiation. To understand the role of the agglomerated state, the colloidal TiO2 (well-dispersed) and the suspension of commercial TiO2 (agglomerated) systems were compared and discussed for their photocatalytic behaviors. The present study demonstrates that the degree of agglomeration of TiO2 nanoparticles is a critical parameter in determining the efficiency of the charge separation and the photocatalytic hydrogen production

Spectroelectrochemical Investigation of the Local Alkaline Environment on the Surface-Nanostructured Au for the Conversion of CO₂ to CO

The electrocatalytic conversion of carbon dioxide (CO2) into fuels could potentially achieve a sustainable carbon-based economy. The engineering of nanostructured metal electrodes can enhance their activity and selectivity by controlling their local chemical environment; however, direct observation is challenging. In this study, we investigate the molecular-level reaction mechanism of a nanoporous-structured Au electrode for the conversion of CO2 to carbon monoxide (CO) using surface-enhanced infrared absorption spectroscopy (SEIRAS). We designed a well-structured nanoporous Au layer (with a depth distribution of 56.3 nm) on a Si prism using a high-temperature non-aqueous anodization process and characterized the nanoporous Au electrode using atomic force microscopy (AFM) and X-ray absorption spectroscopy (XAS). The in situ SEIRAS results demonstrated that the nanoporous Au electrode has a dominant active site, promoting the linear CO intermediate and suppressing the bridging CO intermediate when compared with the non-structured Au electrode. We also revealed a high local pH at a reaction potential of −0.9 V and the slow diffusion kinetics of local CO32– at an open-circuit potential. These findings provide deeper insights into the electrochemical kinetics and corresponding mechanisms occurring in the electric double layers and highlight the potential for the design of efficient electrocatalysts for CO2 reduction

Superior Electron Transport and Photocatalytic Abilities of Metal-Nanoparticle-Loaded TiO₂ Superstructures

Metal–semiconductor nanocomposites have been widely employed for designing efficient optoelectronic devices and catalysts. The performance of such nanocomposites is significantly influenced by both the method of preparation and the electronic and morphological structures of metals and semiconductors. Here, we have synthesized novel nanocomposites containing plate-like anatase TiO2 mesocrystal superstructures and noble metal (Au, Pt) nanoparticles. These metal nanoparticles were preferentially photodeposited on the edge of TiO2 mesocrystals. The electron transport and photocatalytic properties of the novel nanocomposites were subsequently studied. Single-molecule fluorescence spectroscopy measurements on a single particle directly revealed that most of the photogenerated electrons could migrate from the dominant surface to the edge of the TiO2 mesocrystal with the reduction reactions mainly occurring at its lateral surfaces containing {101} facets. The loading of metal nanoparticles on the superstructure of TiO2 was found to greatly improve the photogenerated charge separation efficiency allowing significant (more than 1 order of magnitude) enhancement of the photocatalytic reaction rate in organic degradation reactions. These outstanding features allowed significantly reduced consumption (ca. 10% of that of typical TiO2 nanocrystal samples) of the Au or Pt loading on the TiO2 mesocrystal while maintaining the same photocatalytic activity

Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCo^II Unit Coupled to Ir Oxide Nanocluster Catalyst

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo^II group coupled to an iridium oxide nanocluster (IrO_<i>x</i>) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO_<i>x</i> water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo^II metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir­(acac)₃] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo^II–IrO_<i>x</i> units in the SBA-15 pores loaded with a mixture of ¹³CO₂ and H₂O vapor resulted in the formation of ¹³CO and O₂ monitored by FT-IR and mass spectroscopy, respectively. Use of ¹⁸O labeled water resulted in the formation of ¹⁸O₂ product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO_<i>x</i> catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O₂ product from the silica pores

Superior Electron Transport and Photocatalytic Abilities of Metal-Nanoparticle-Loaded TiO₂ Superstructures

Metal–semiconductor nanocomposites have been widely employed for designing efficient optoelectronic devices and catalysts. The performance of such nanocomposites is significantly influenced by both the method of preparation and the electronic and morphological structures of metals and semiconductors. Here, we have synthesized novel nanocomposites containing plate-like anatase TiO₂ mesocrystal superstructures and noble metal (Au, Pt) nanoparticles. These metal nanoparticles were preferentially photodeposited on the edge of TiO₂ mesocrystals. The electron transport and photocatalytic properties of the novel nanocomposites were subsequently studied. Single-molecule fluorescence spectroscopy measurements on a single particle directly revealed that most of the photogenerated electrons could migrate from the dominant surface to the edge of the TiO₂ mesocrystal with the reduction reactions mainly occurring at its lateral surfaces containing {101} facets. The loading of metal nanoparticles on the superstructure of TiO₂ was found to greatly improve the photogenerated charge separation efficiency allowing significant (more than 1 order of magnitude) enhancement of the photocatalytic reaction rate in organic degradation reactions. These outstanding features allowed significantly reduced consumption (ca. 10% of that of typical TiO₂ nanocrystal samples) of the Au or Pt loading on the TiO₂ mesocrystal while maintaining the same photocatalytic activity

Investigation of the AgCl Formation Mechanism on the Ag Wire Surface for the Fabrication of a Marine Low-Frequency-Electric-Field-Detection Ag/AgCl Sensor Electrode

One of the most widely used electric field sensors for low-frequency electric field detection (LFEFD) in seawater uses the Ag/AgCl electrode. The surface structure of the electrode including AgCl layers plays a critical role in the electrode’s electrochemical performance required for the sensor. In this study, the sequential AgCl formation process under the constant current was examined on the Ag wire in an electrode size for actual applications, and an optimal electrode surface structure was suggested for the LFEFD Ag/AgCl sensor. Upon mild anodization (0.2 mA/cm2) in 3.3 M KCl solution that permits us to follow the AgCl formation process manageably, Ag dissolution from the wire surface begins leaving cavities on the surface, with the accompanied growth of initial Ag grains. During this period, AgCl deposits in sizes of about several micrometers to 10 μm with crystal planes also form primarily along scratch lines on the wire surface, but in a partial scale. Then, with further anodization, the assumed thin AgCl deposits start to form, covering a large portion of the wire surface. They grow to become deposits in sizes of about several micrometers to 10 μm with no clear facet planes next to one another and are connected to form the network structure, representing the main developing mode of the AgCl deposits. While they cover all the surface, AgCl deposits also form on the surface of the already formed ones, making multiple AgCl layers. All these deposits develop through the nucleation process with a relatively high surface energy barrier, and their formation rate is solely controlled by the release rate of Ag+ from the wire, thus by the applied current magnitude. The Ag/AgCl electrode with a thick AgCl layer and many holes in the AgCl surface structure like microchannels is considered to work effectively for the LFEFD sensor in terms of both detection sensitivity and service lifetime

Role of Interparticle Charge Transfers in Agglomerated Photocatalyst Nanoparticles: Demonstration in Aqueous Suspension of Dye-Sensitized TiO₂

The interparticle charge transfer within the agglomerates of TiO₂ nanoparticles in slurries markedly enhanced the dye-sensitized production of H₂ under visible light. By purposely decoupling the light absorbing part of Dye/TiO₂ from the active catalytic center of Pt/TiO₂, the role of bare TiO₂ nanoparticles working as a mediator that connects the above two parts in the agglomerates was investigated systematically. The presence of mediator in the agglomerate facilitated the charge separation and the electron transfer from Dye/TiO₂ to Pt/TiO₂ through multiple grain boundaries and subsequently produced more hydrogen. The dye-sensitized reduction of Cr­(VI) to Cr­(III) was also enhanced when Dye/TiO₂ nanoparticles were agglomerated with bare TiO₂ nanoparticles. The charge recombination between the oxidized dye and the injected electron was retarded in the presence of bare TiO₂ nanoparticles, and this retarded recombination on Dye/TiO₂ was confirmed by using transient laser spectroscopy. This phenomenon can be rationalized in terms of an interparticle Fermi level gradient within the agglomerates, which drives the charge separation

Gas Diffusion through Nanoporous Channels of Graphene Oxide and Reduced Graphene Oxide Membranes

Recently, graphene oxide (GO) has been investigated as a class of molecular filters for selective gas and ion transport. However, detailed transport mechanisms have been poorly understood thus far. Here, we report the gas transport behavior of noninterlocked GO and reduced GO (rGO) membranes, which contain nanoporous gas diffusion channels generated by the adjacent edges of GO and rGO sheets. Both membranes exhibited Knudsen gas diffusion behavior; however, the separation factors of these membranes exceeded the theoretical Knudsen separation factors for gas/CO2 selectivities of various gas mixtures owing to extremely low CO2 permeance. The unique transport features of the low CO2 permeance were explained by the blocking effect of CO2 adsorbed in the nanoporous diffusion channels because of the high CO2 affinity of the edges of GO and rGO sheets. Furthermore, the rGO lamellar structure generally shows impermeable interlayer spacing, indicating that the only gas diffusion channel is the nanopores created by neighboring the edges of the rGO sheets. Notably, both membranes maintained a higher H2/CO2 separation factor than the theoretical Knudsen selectivity, including the measurements of mixed-gas permeation experiments. This study provides insight that further GO modification may improve the gas separation performance suitable for specific separation processes

Visible-Light Activation of a Dissolved Organic Matter–TiO₂ Complex Mediated <i>via</i> Ligand-to-Metal Charge Transfer

Given the widespread use of TiO2, its release into aquatic systems and complexation with dissolved organic matter (DOM) are highly possible, making it important to understand how such interactions affect photocatalytic activity under visible light. Here, we show that humic acid/TiO2 complexes (HA/TiO2) exhibit photoactivity (without significant electron–hole activation) under visible light through ligand-to-metal charge transfer (LMCT). The observed visible-light activities for pollutant removal and bacterial inactivation are primarily linked to the generation of H2O2 via the conduction band. By systematically considering molecular-scale interactions between TiO2 and organic functional groups in HA, we find a key role of phenolic groups in visible-light absorption and H2O2 photogeneration. The photochemical formation of H2O2 in river waters spiked with TiO2 is notably elevated above naturally occurring H2O2 generated from background organic constituents due to LMCT contribution. Our findings suggest that H2O2 generation by HA/TiO2 is related to the quantity and functional group chemistry of DOM, which provides chemical insights into photocatalytic activity and potential ecotoxicity of TiO2 in environmental and engineered systems