Zhaodong Li

Ferroelectric Polarization-Enhanced Photoelectrochemical Water Splitting in TiO2–BaTiO3 Core–Shell Nanowire Photoanodes

The performances of heterojunction-based electronic devices are extremely sensitive to the interfacial electronic band structure. Here we report a largely enhanced performance of photoelectrochemical (PEC) photoanodes by ferroelectric polarization-endowed band engineering on the basis of TiO2/BaTiO3 core/shell nanowires (NWs). Through a one-step hydrothermal process, a uniform, epitaxial, and spontaneously poled barium titanate (BTO) layer was created on single crystalline TiO2 NWs. Compared to pristine TiO2 NWs, the 5 nm BTO-coated TiO2 NWs achieved 67% photocurrent density enhancement. By numerically calculating the potential distribution across the TiO2/BTO/electrolyte heterojunction and systematically investigating the light absorption, charge injection and separation properties of TiO2 and TiO2/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. The ferroelectric polarization could be switched by external electric field poling and yielded PEC performance gain or loss based on the direction of the polarization. This study evidence that the piezotronic effect (ferroelectric or piezoelectric potential-induced band structure engineering) holds great promises in improving the performance of PEC photoelectrodes in addition to chemistry and structure optimization.

Highly Efficient Capillary Photoelectrochemical Water Splitting Using Cellulose Nanofiber‐Templated TiO2 Photoanodes

Light absorption, charge separation, and appropriate interfacial redox reactions are three key aspects that lead to highly effi cient solar energy conversion. [ 5–10 ] Therefore, development of highperformance PEC electrodes has been concentrated largely on engineering the band structure of photoanodes, enlarging semiconductor-electrolyte interfacial area, and enabling rapid charge separation, collection, and transportation. [ 11,12 ] High porosity three dimensional (3D) nanostructures, such as branched nanowire architectures and nanofi ber networks, offer extremely large surface area, excellent charge transport properties, as well as long optical paths for effi cient light absorption. As a result, 3D nanostructures are the current focus of a tremendous surge of interest in PEC photoanode development. [ 10,13 ]

Sequential Infiltration Synthesis of Doped Polymer Films with Tunable Electrical Properties for Efficient Triboelectric Nanogenerator Development

Doping polymer with AlOx via sequential infiltration synthesis enables bulk modification of triboelectric polymers with tunable electric or dielectric properties, which broadens the material selection and achieves a durable performance gain of triboelectric nanogenerators.

Cl-Doped ZnO Nanowires with Metallic Conductivity and Their Application for High-Performance Photoelectrochemical Electrodes

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. ZnO NWs grown by hydrothermal method are pervasively used in optoelectronic, photovoltaic, and piezoelectric energy-harvesting devices. We synthesized in situ Cl-doped ZnO NWs with metallic conductivity that would fit seamlessly with these devices and improve their performance. Possible Cl doping mechanisms were discussed. UV-visible absorption spectroscopy confirmed the visible light transparency of Cl-doped ZnO NWs. Cl-doped ZnO NW/TiO2 core/shell-structured photoelectrochemical (PEC) anode was fabricated to demonstrate the application potential of highly conductive ZnO NWs. Higher photocurrent density and overall PEC efficiency compared with the undoped ZnO NW-based device were achieved. The successful doping and low resistivity of ZnO could unlock the potential of ZnO NWs for applications in low-cost flexible transparent electrodes.

Nanometre-thick single-crystalline nanosheets grown at the water-air interface.

To date, the preparation of free-standing 2D nanomaterials has been largely limited to the exfoliation of van der Waals solids. The lack of a robust mechanism for the bottom-up synthesis of 2D nanomaterials from non-layered materials has become an obstacle to further explore the physical properties and advanced applications of 2D nanomaterials. Here we demonstrate that surfactant monolayers can serve as soft templates guiding the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. One- to 2-nm-thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of micrometres are synthesized at the water–air interface. In this process, the packing density of surfactant monolayers adapts to the sub-phase metal ions and guides the epitaxial growth of nanosheets. It is thus named adaptive ionic layer epitaxy (AILE). The electronic properties of ZnO nanosheets and AILE of other materials are also investigated.

Electron Microscopy Observation of TiO2 Nanocrystal Evolution in High-Temperature Atomic Layer Deposition

Understanding the evolution of amorphous and crystalline phases during atomic layer deposition (ALD) is essential for creating high quality dielectrics, multifunctional films/coatings, and predictable surface functionalization. Through comprehensive atomistic electron microscopy study of ALD TiO2 nanostructures at designed growth cycles, we revealed the transformation process and sequence of atom arrangement during TiO2 ALD growth. Evolution of TiO2 nanostructures in ALD was found following a path from amorphous layers to amorphous particles to metastable crystallites and ultimately to stable crystalline forms. Such a phase evolution is a manifestation of the Ostwald-Lussac Law, which governs the advent sequence and amount ratio of different phases in high-temperature TiO2 ALD nanostructures. The amorphous-crystalline mixture also enables a unique anisotropic crystal growth behavior at high temperature forming TiO2 nanorods via the principle of vapor-phase oriented attachment.

Piezotronic-Enhanced Photoelectrochemical Reactions in Ni(OH)2-Decorated ZnO Photoanodes

Controlling the interface electronic band structure in heterostructures is essential for developing highly efficient photoelectrochemical (PEC) photoanodes. Here, we presented an enhanced oxygen evolution reaction (OER) by introducing the piezotronics concept, i.e., piezoelectric polarization (Ppz)-induced band engineering. In a Ni(OH)2-decorated ZnO photoanode system, appreciably improved photocurrent density of sulfite (SO3(2-)) and hydroxyl (OH(-)) oxidation reactions were obtained by physically deflecting the photoanode. Both theoretical and experimental results suggested that the performance enhancement was a result of the piezoelectric Ppz-endowed enlargement of the built-in electric field at the ZnO/Ni(OH)2 interface, which could drive an additional amount of photoexcited charges from ZnO toward the interface for OER. This strategy demonstrates a new route for improving the performance of inexpensive catalysts-based solar-to-fuel production.

Cellulose nanofiber-templated three-dimension TiO2 hierarchical nanowire network for photoelectrochemical photoanode

Three dimensional (3D) nanostructures with extremely large porosity possess a great promise for the development of high-performance energy harvesting and storage devices. In this paper, we developed a high-density 3D TiO2 fiber-nanorod (NR) heterostructure for efficient photoelectrochemical (PEC) water splitting. The hierarchical structure was synthesized on a ZnO-coated cellulose nanofiber (CNF) template using atomic layer deposition (ALD)-based thin film and NR growth procedures. The tubular structure evolution was in good agreement with the recently discovered vapor-phase Kirkendall effect in high-temperature ALD processes. The NR morphology was formed via the surface-reaction-limited pulsed chemical vapor deposition (SPCVD) mechanism. Under Xenon lamp illumination without and with an AM 1.5 G filter or a UV cut off filter, the PEC efficiencies of a 3D TiO2 fiber-NR heterostructure were found to be 22-249% higher than those of the TiO2-ZnO bilayer tubular nanofibers and TiO2 nanotube networks that were synthesized as reference samples. Such a 3D TiO2 fiber-NR heterostructure offers a new route for a cellulose-based nanomanufacturing technique, which can be used for large-area, low-cost, and green fabrication of nanomaterials as well as their utilizations for efficient solar energy harvesting and conversion.

Cellulose‐Based Nanomaterials for Energy Applications

Cellulose is the most abundant natural polymer on earth, providing a sustainable green resource that is renewable, degradable, biocompatible, and cost effective. Recently, nanocellulose-based mesoporous structures, flexible thin films, fibers, and networks are increasingly developed and used in photovoltaic devices, energy storage systems, mechanical energy harvesters, and catalysts components, showing tremendous materials science value and application potential in many energy-related fields. In this Review, the most recent advancements of processing, integration, and application of cellulose nanomaterials in the areas of solar energy harvesting, energy storage, and mechanical energy harvesting are reviewed. For solar energy harvesting, promising applications of cellulose-based nanostructures for both solar cells and photoelectrochemical electrodes development are reviewed, and their morphology-related merits are discussed. For energy storage, the discussion is primarily focused on the applications of cellulose-based nanomaterials in lithium-ion batteries, including electrodes (e.g., active materials, binders, and structural support), electrolytes, and separators. Applications of cellulose nanomaterials in supercapacitors are also reviewed briefly. For mechanical energy harvesting, the most recent technology evolution in cellulose-based triboelectric nanogenerators is reviewed, from fundamental property tuning to practical implementations. At last, the future research potential and opportunities of cellulose nanomaterials as a new energy material are discussed.

Hierarchical Branched Vanadium Oxide Nanorod@Si Nanowire Architecture for High Performance Supercapacitors

Vanadium oxide (VOx ) nanorods are uniformly synthesized on dense Si nanowire arrays. This 3D hierarchical nanoarchitecture offers a novel high-performance supercapacitor electrode design with significantly improved specific capacitance and high-rate capability.