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

Cesium Carbonate Functionalized Graphene Quantum Dots as Stable Electron-Selective Layer for Improvement of Inverted Polymer Solar Cells

Solution processable inverted bulk heterojunction (BHJ) polymer solar cells (PSCs) are promising alternatives to conventional silicon solar cells because of their low cost roll-to-roll production and flexible device applications. In this work, we demonstrated that Cs₂CO₃ functionalized graphene quantum dots (GQDs–Cs₂CO₃) could be used as efficient electron-selective layers in inverted PSCs. Compared with Cs₂CO₃ buffered devices, the GQDs–Cs₂CO₃ buffered devices show 56% improvement in power conversion efficiency, as well as 200% enhancement in stability, due to the better electron-extraction, suppression of leakage current, and inhibition of Cs⁺ ion diffusion at the buffer/polymer interface by GQDs–Cs₂CO₃. This work provides a thermal-annealing-free, solution-processable method for fabricating electron-selective layer in inverted PSCs, which should be beneficial for the future development of high performance all-solution-processed or roll-to-roll processed PSCs

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

High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide

In this work, we present Co₃O₄ quantum dots (QDs) as a highly efficient and stable oxygen evolution reaction (OER) catalyst at neutral pH. The Co₃O₄ QDs with a mean size of 5 nm were synthesized by reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux conditions. The as-synthesized Co₃O₄ QDs show extraordinary water oxidation activity with onset overpotential as low as 398 mV and mass activity as high as 567 A/g (at 1.75 V vs RHE) in a 0.2 M phosphate buffer electrolyte (pH ∼7), which are among the most efficient Earth-abundant OER catalysts at neutral pH reported in the literature, reaching a stable current density of 10 mA/cm² at an overpotential of ∼490 mV with a Tafel slope of 80 mV/decade. Through in-depth investigations by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, the high spin Co²⁺ and Co³⁺ cations on the surface of Co₃O₄ QDs were found to be important to promote the OER kinetics at neutral pH

High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide

In this work, we present Co₃O₄ quantum dots (QDs) as a highly efficient and stable oxygen evolution reaction (OER) catalyst at neutral pH. The Co₃O₄ QDs with a mean size of 5 nm were synthesized by reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux conditions. The as-synthesized Co₃O₄ QDs show extraordinary water oxidation activity with onset overpotential as low as 398 mV and mass activity as high as 567 A/g (at 1.75 V vs RHE) in a 0.2 M phosphate buffer electrolyte (pH ∼7), which are among the most efficient Earth-abundant OER catalysts at neutral pH reported in the literature, reaching a stable current density of 10 mA/cm² at an overpotential of ∼490 mV with a Tafel slope of 80 mV/decade. Through in-depth investigations by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, the high spin Co²⁺ and Co³⁺ cations on the surface of Co₃O₄ QDs were found to be important to promote the OER kinetics at neutral pH

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

Tunneling Interlayer for Efficient Transport of Charges in Metal Oxide Electrodes

Due to the limited electronic conductivity, the application of many metal oxides that may have attractive (photo)-electrochemical properties has been limited. Regarding these issues, incorporating low-dimensional conducting scaffolds into the electrodes or supporting the metal oxides onto the conducting networks are common approaches. However, some key electronic processes like interfacial charge transfer are far from being consciously concerned. Here we use a carbon-TiO₂ contact as a model system to demonstrate the electronic processes occurring at the metal–semiconductor interface. To minimize the energy dissipation for fast transfer of electrons from semiconductor to carbon scaffolds, facilitating electron tunneling while avoiding high energy-consuming thermionic emission is desired, according to our theoretical simulation of the voltammetric behaviors. To validate this, we manage to sandwich ultrathin TiO₂ interlayers with heavy electronic doping between the carbon conductors and dopant-free TiO₂. The radially graded distribution of the electronic doping along the cross-sectional direction of carbon conductor realized by immobilizing the dopant species on the carbon surface can minimize the energy consumption for contacts to both the carbon and the dopant-free TiO₂. Our strategy provides an important requirement for metal oxide electrode design