Synthesis of Well-Defined, Surfactant-Free Co₃O₄ Nanoparticles:The Impact of Size and Manganese Promotion on Co₃O₄ Reduction and Water Oxidation Activity

Abstract: A surfactant-free synthetic route has been developed to produce size-controlled, cube-like cobalt oxide nanoparticles of three different sizes in high yields. It was found that by using sodium nitrite as salt-mediating agent, near-quantitative yields could be obtained. The size of the nanoparticles could be altered from 11 to 22 nm by changing the cobalt concentration and reaction time. These surfactant-free nanoparticles form ideal substrates for facile deposition of further elements such as manganese. The effect of size of the cobalt oxide nanoparticles and the presence of manganese on the reducibility of cobalt oxide to metallic cobalt was investigated. Similarly, the effect of these parameters was investigated with a visible light promoted water oxidation system with cobalt oxide as catalyst, together with [Ru(bpy) 3] 2+ light harvester dye and an electron acceptor. Graphical Abstract: A novel surfactant-free synthetic route has been developed to produce size-controlled, cube shaped cobalt oxide nanoparticles in high yields. [Figure not available: see fulltext.]. </p

Mechanism of Photocatalytic Reduction of CO2 by Ag3PO4(111)/g-C3N4 Nanocomposite: A First-Principles Study

Density functional theory (DFT) calculations have been performed to investigate the electronic structure and photocatalytic activity of a hybrid Ag3PO4(111)/g-C3N4 structure. Due to Ag(d) and O(p) states forming the upper part of the valence band and C(p), N(p), and Ag(s) the lower part of the conduction band, the band gap of the hybrid material is reduced from 2.75 eV for Ag3PO4(111) and 3.13 eV for monolayer of g-C3N4 to about 2.52 eV, enhancing the photocatalytic activity of the Ag3PO4(111) surface and g-C3N4 sheet in the visible region. We have also investigated possible reaction pathways for photocatalytic CO2 reduction on the Ag3PO4(111)/g-C3N4 nanocomposite to determine the most favored adsorption geometries of reaction intermediates and the related reaction energies. For CO2 reduction, our findings demonstrate that the Ag3PO4(111)/g-C3N4 heterostructure thermodynamically exhibits a higher selectivity toward CH4 production than that of CH3OH. The CO2 reduction process takes place through either HCOOH* or HOCOH* as an intermediate species, where the highest exothermic reaction energy of −2.826 eV belongs to the hydrogenation of t-COOH* to HCOOH* and the lowest reaction energy of −0.182 eV for hydrogenations of CH2O* to CH2OH* and HCO* to c-HCOH*. Our results from charge density difference calculations of the Ag3PO4(111)/Ag/g-C3N4 revealed that the charge transfer between the Ag3PO4(111) slab and g-C3N4 monolayer occurs through mediation of atomic Ag, thus proposing a Z-scheme mechanism. Moreover, a smaller band gap energy of 0.73 eV is calculated for this ternary nanocomposite due to the midgap states of the atomic Ag at the interface. These results provide in depth understanding of the reaction mechanism in the reduction and conversion of CO2 to useful chemicals via an Ag3PO4 and g-C3N4-based nanocomposite photocatalyst under visible light

Implementation of Ag nanoparticle incorporated WO3 thin film photoanode for hydrogen production

WO3 thin film photoanodes containing different concentrations of Ag nanoparticles were synthesized by sol-gel method. Based on UV-visible spectra, presence of a surface plasmon resonance peak at 470 nm of wavelength indicated formation of silver nanoparticles in the WO3 films. According to atomic force microscopy (ATM) analysis, the highest value for surface roughness and the effective surface ratio was observed for the sample containing 2 mol% of Ag. X-ray diffraction (XRD) patterns revealed that WO3 nanocrystalline structure was formed in the monoclinic phase with the average size of about 18.2 nm while Ag nanocrystals were determined in cubic phase. X-ray photoelectron spectroscopy (XPS) showed that Ag exists in a combination of metal/oxide states on the surface. Photoresponse investigation of the synthesized films indicated that the highest photocurrent was obtained for the sample containing 2 mol% Ag with the maximum incident photon to current efficiency (IPCE) of about 20% at 360 nm wavelength. Moreover, measuring the amount of hydrogen produced during water splitting reactions verified that the highest hydrogen production rate (similar to 3 mu mol/h) was obtained for the sample with 2 mol% Ag. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.X111518sciescopu

Optimal Ag Concentration for H2 Productionvia Ag:TiO2 Nanocomposite Thin Film Photoanode

TiO2 thin films containing different concentrations of Ag nanoparticles have been synthesized by sol-gel method. According to UV-visible spectra, presence of an intense surface plasmon resonance peak at 490 nm of wavelength indicated formation of silver nanoparticles in the TiO2 films. Based on atomic force microscopy (AFM) analysis, the surface roughness and the effective surface ratio increased by increasing the Ag mol%. Moreover, scanning electron microscopy (SEM) images showed formation of Ag nanoparticles on the surface for the samples containing high Ag concentration. X-ray diffraction (XRD) patterns revealed that the size of Ag nanocrystals increased by increasing the Ag content in the films while the nanocrystalline size of TiO2 reduced in the presence of silver nanoparticles. Based on x-ray photoelectron spectroscopy (XPS) data, a stoichiometric chemical composition was detected for TiO2 while, Ag presented in a combination a metal/oxide states on the surface. Studying photoresponse of the samples showed that the highest photocurrent was obtained for the sample containing 1 mol% Ag. By measuring the photovoltage versus time, it was found that addition of silver nanoparticles to the TiO2 layer resulted in reduction of the transient time of the photogenerated carriers in the samples. Impedance spectroscopy determined a slight decrease in charge transfer resistance by addition of Ag to the films. Moreover, measuring the amount of hydrogen produced during water splitting reactions verified that the highest quantum yield of 9.6% was obtained for the sample with 1 mol% Ag. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.X112827sciescopu

To What Extent Surface Morphology Can Influence the Photoelectrochemical Performance of Au:WO3 Electrodes?

Considering hydrogen as a future fuel, development of clean energy sources based on solar power is the main human challenge in recent years. Here, for the first time, Au:WO3 photoanodes are synthesized with different Au concentrations and then applied for photoelectrochemical (PEC) water splitting. A comprehensive statistical study on the prepared photoanode surface is conducted to understand the correlation between surface morphology and PEC activity, using atomic force microscopy (AFM). The results clearly justified the maximum surface area observed for the film containing 1 mol % Au. Additionally, X-ray diffraction (XRD) analysis determined that Au nanocrystals have been formed in cubic structure with the size of 2952 nm. X-ray photoelectron spectroscopy (XPS) revealed that the presence of Au in a combined metal/oxide state strongly affects on the Au:WO3 photoanode performance. Photoresponse investigation of the synthesized films showed that the highest photocurrent was obtained for the sample containing 1 mol% gold with the maximum incident photon to current efficiency (IPCE) of about 20% at 360 nm wavelength. In addition, measuring the amount of hydrogen produced in the water splitting reaction supports the result that the sample containing 1 mol% Au exhibits the highest hydrogen production rate (similar to 3 mu mol/h) as compared to other samples.X111010sciescopu

Persistent Quantum Coherence and Strong Coupling Enable Fast Electron Transfer across the CdS/TiO2 Interface: A Time-Domain ab Initio Simulation

Fast transfer of photoinduced electrons and subsequent slow electron–hole recombination in semiconductor heterostructures give rise to long-lived charge separation which is highly desirable for photocatalysis and photovoltaic applications. As a type II heterostructure, CdS/TiO2 nanocomposites extend the absorption edge of the light spectrum to the visible range and demonstrate effective charge separation, resulting in more efficient conversion of solar energy to chemical energy. This improvement in performance is partly explained by the fact that CdS/TiO2 is a type II semiconductor heterostructure and CdS has a smaller energy band gap than UV-active TiO2. Ultrafast transient absorption measurements have revealed that electrons generated in CdS by visible light can quickly transfer into TiO2 before recombination takes place within CdS. Here, using time-domain density functional theory and nonadiabatic molecular dynamics simulations, we show how electronic subsystems of the CdS and TiO2 semiconductors are coupled to their lattice vibrations and coherently evolve, enabling effective transfer of photoinduced electrons from CdS into TiO2. This very fast electron transfer, and subsequent slow recombination of the transferred electrons with the holes left in CdS, is verified experimentally through the proven efficient performance of CdS/TiO2 heterostructures in photocatalysis and photovoltaic applications