Layer-by-Layer Self-Assembly of CdS Quantum Dots/Graphene Nanosheets Hybrid Films for Photoelectrochemical and Photocatalytic Applications

In recent years, increasing interest has been devoted to synthesizing graphene–semiconductor nanocomposites as efficient photocatalysts for extensive applications. Unfortunately, it is still challenging to make uniform graphene–semiconductor composite films with controllable film thickness and architecture, which are of paramount importance to meet the application requirements. In this work, stable aqueous dispersion of polymer-modified graphene nanosheets (GNs) was prepared via in situ reduction of exfoliated graphite oxide in the presence of cationic poly­(allylamine hydrochloride) (PAH). The resultant water-soluble PAH-modified GNs (GNs-PAH) in conjunction with tailor-made negatively charged CdS quantum dots (QDs) were utilized as nanobuilding blocks for sequential layer-by-layer (LbL) self-assembly of well-defined GNs–CdS QDs hybrid films, in which CdS QDs overspread evenly on the two-dimensional (2D) GNs. It was found that the alternating GNs–CdS QDs multilayered films showed significantly enhanced photoelectrochemical and photocatalytic activities under visible light irradiation as compared to pure CdS QDs and GNs films. The enhancement was attributed to the judicious integration of CdS QDs with GNs in an alternating manner, which maximizes the 2D structural advantage of GNs in GNs–CdS QDs composite films. In addition, photocatalytic and photoelectrochemical mechanisms of the GNs–CdS QDs multilayered films were also discussed. It is anticipated that our work may open new directions for the fabrication of uniform semiconductor/GNs hybrid films for a wide range of applications

Thermodynamically Driven One-Dimensional Evolution of Anatase TiO₂ Nanorods: One-Step Hydrothermal Synthesis for Emerging Intrinsic Superiority of Dimensionality

In photoelectrochemical cells, there exists a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and back-reaction of electrons to recombine with oxidized species on the semiconductor–electrolyte interface, which determines the charge collection efficiency and is strongly influenced by the density and distribution of electronic states in band gap and architectures of the semiconductor electrodes. One-dimensional (1D) anatase TiO₂ nanostructures are promising to improve charge transport in photoelectrochemical devices. However, the conventional preparation of 1D anatase nanostructures usually steps via a titanic acid intermediate (e.g., H₂Ti₃O₇), which unavoidably introduces electronic defects into the host lattice, resulting in undesired shielding of the intrinsic role of dimensionality. Here, we manage to promote the 1D growth of anatase TiO₂ nanostructures by adjusting the growth kinetics, which allows us to grow single-crystalline anatase TiO₂ nanorods through a one-step hydrothermal reaction. The synthesized anatase nanorods possess a lower density of trap states and thus can simultaneously facilitate the diffusion-driven charge transport and suppress the electron recombination. Moreover, the electronically boundary free nanostructures significantly enhance the trap-free charge diffusion coefficient of the anatase nanorods, which enables the emergence of the intrinsic superiority of dimensionality. By virtue of these merits, the anatase nanorods synthesized in this work take obvious advantages over the conventional anatase counterparts in photoelectrochemical systems (e.g., dye-sensitized solar cells) by showing more efficient charge transport and collection and higher energy conversion efficiency

Understanding Chemical Bonding in Alloys and the Representation in Atomistic Simulations

Alloys are widely used in catalysts and structural materials. The nature of chemical bonding and the origin of alloy formation energies, defect energies, and interfacial properties have not been well understood to date but are critical to material performance. In this contribution, we explain the polar nature of chemical bonding and an implementation in classical and reactive atomistic simulations to understand such properties more quantitatively. Electronegativity differences between metal atoms lead to polar bonding, and exothermic alloy formation energies are related to charge transfer between the different elements. These differences can be quantified by atomic charges using pairwise charge increments, determined by matching the computed alloy formation energy to experimentally measured alloy formation energies using pair potentials for the pure metals. The polar character of alloys is comparable to organic molecules and partially ionic minerals, for example, AlNi and AlNi₃ alloys assume significant atomic charges of ±0.40<i>e</i> and +0.60<i>e</i>/–0.20<i>e</i>, respectively. The subsequent analysis of defect sites and defect energies using force-field-based calculations shows excellent agreement with calculations using density functional theory and embedded atom models (EAM). The formation of vacancy and antisite defects is characterized by a redistribution of charge in the first shell of neighbor atoms in the classical models whereby electroneutrality is maintained and charge increments correlate with differences in electronegativity. The proposed atomic charges represent internal dipole and multipole moments, consistent with existing definitions for organic and inorganic compounds and with the extended Born model (Heinz, H.; Suter, U. W. <i>J. Phys. Chem. B</i> <b>2004,</b> <i>108</i> (47), 18341–18352). The method can be applied to any alloy and has a reproducibility of ±10%. In contrast, quantum mechanical charge schemes remain associated with deviations exceeding ±100%. The atomic charges for alloys provide a simple initial measure for the internal electronic structure, surface adsorption of molecules, and reactivity in catalysis and corrosion. The models are compatible with the Interface force field (IFF), CHARMM, AMBER, OPLS-AA, PCFF, CVFF, and GROMOS for reliable atomistic simulations of alloys and their interfaces with minerals and electrolytes from the nanometer scale to the micrometer scale

Stable Quantum Dot Photoelectrolysis Cell for Unassisted Visible Light Solar Water Splitting

Sunlight is an ideal source of energy, and converting sunlight into chemical fuels, mimicking what nature does, has attracted significant attention in the past decade. In terms of solar energy conversion into chemical fuels, solar water splitting for hydrogen production is one of the most attractive renewable energy technologies, and this achievement would satisfy our increasing demand for carbon-neutral sustainable energy. Here, we report corrosion-resistant, nanocomposite photoelectrodes for spontaneous overall solar water splitting, consisting of a CdS quantum dot (QD) modified TiO₂ photoanode and a CdSe QD modified NiO photocathode, where cadmium chalcogenide QDs are protected by a ZnS passivation layer and gas evolution cocatalysts. The optimized device exhibited a maximum efficiency of 0.17%, comparable to that of natural photosynthesis with excellent photostability under visible light illumination. Our device shows spontaneous overall water splitting in a nonsacrificial environment under visible light illumination (λ > 400 nm) through mimicking nature’s “Z-scheme” process. The results here also provide a conceptual layout to improve the efficiency of solar-to-fuel conversion, which is solely based on facile, scalable solution-phase techniques

Identification of Surface Reactivity Descriptor for Transition Metal Oxides in Oxygen Evolution Reaction

A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), the surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performance still remains a great challenge. Here we show the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (M_CUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of M_CUS, and thus the increase in M_CUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. The electronic origin of the surface reactivity can be well explained by a new model proposed in this work, wherein the energy of the highest-occupied d-states relative to the Fermi level determines the intermediates’ bonding strength by affecting the filling of the antibonding states. Our model for the first time well describes the reactivity trends among TMOs, and would initiate viable design principles for, but not limited to, OER catalysts

In Situ Spectroscopic Identification of μ‑OO Bridging on Spinel Co₃O₄ Water Oxidation Electrocatalyst

The formation of μ-OO peroxide (Co–OO–Co) moieties on spinel Co₃O₄ electrocatalyst prior to the rise of the electrochemical oxygen evolution reaction (OER) current was identified by in situ spectroscopic methods. Through a combination of independent in situ X-ray absorption, grazing-angle X-ray diffraction, and Raman analysis, we observed a clear coincidence between the formation of μ-OO peroxide moieties and the rise of the anodic peak during OER. This finding implies that a chemical reaction step could be generally ignored before the onset of OER current. More importantly, the tetrahedral Co²⁺ ions in the spinel Co₃O₄ could be the vital species to initiate the formation of the μ-OO peroxide moieties

Sulfur-Mediated Self-Templating Synthesis of Tapered C‑PAN/g‑C₃N₄ Composite Nanotubes toward Efficient Photocatalytic H₂ Evolution

Hollow one-dimensional (1-D) nanostructures have drawn great attention in heterogeneous photocatalysis. Herein, we report that tapered polyacrylonitrile-derived carbon (C-PAN)/g-C₃N₄ composite nanotubes can be synthesized through a facile sulfur-mediated self-templating method via thermal condensation of polyacrylonitrile (PAN), melamine, and sulfur. The hollow tapered C-PAN/g-C₃N₄ composite nanotubes exhibit superior photocatalytic H₂ evolution performance under visible light irradiation. The 5 wt % C-PAN/g-C₃N₄ composite nanotubes show a 16.7 times higher photocatalytic H₂ evolution rate than that of pure g-C₃N₄, which is even 4.7 times higher than that of a 5 wt % C-PAN/g-C₃N₄ nanosheet composite obtained without sulfur. The hollow nanotubular composite structure provides g-C₃N₄ with higher specific surface area, enhanced light absorption, and better charge carrier separation and transfer, which synergistically contribute to the superior photocatalytic activity. Our work provides a new strategy to develop carbon-based architected photocatalysts