Yanhao Yu

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.

Evolution of Hollow TiO2 Nanostructures via the Kirkendall Effect Driven by Cation Exchange with Enhanced Photoelectrochemical Performance

Hollow nanostructures are promising building blocks for electrode scaffolds and catalyst carriers in energy-related systems. In this paper, we report a discovery of hollow TiO2 nanostructure evolution in a vapor-solid deposition system. By introducing TiCl4 vapor pulses to ZnO nanowire templates, we obtained TiO2 tubular nanostructures with well-preserved dimensions and morphology. This process involved the cation exchange reaction between TiCl4 vapor and ZnO solid and the diffusion of reactants and products in their vapor or solid phases, which was likely a manifestation of the Kirkendall effect. The characteristic morphologies and the evolution phenomena of the hollow nanostructures from this vapor-solid system were in a good agreement with the Kirkendall effect discovered in solution systems. Complex hollow TiO2 nanostructures were successfully acquired by replicating various ZnO nanomorphologies, suggesting that this unique cation exchange process could also be a versatile tool for nanostructure replication in vapor-solid growth systems. The evolution of TiO2 nanotubes from ZnO NW scaffolds was seamlessly integrated with TiO2 NR branch growth and thus realized a pure TiO2-phased 3D NW architecture. Because of the significantly enlarged surface area and the trace amount of Zn left in the TiO2 crystals, such 3D TiO2 nanoforests demonstrated enhanced photoelectrochemical performance particularly under AM (air mass) 1.5G illumination, offering a new route for hierarchical functional nanomaterial assembly and application.

Development of Lead Iodide Perovskite Solar Cells Using Three-Dimensional Titanium Dioxide Nanowire Architectures

Three-dimensional (3D) nanowire (NW) architectures are considered as superior electrode design for photovoltaic devices compared to NWs or nanoparticle systems in terms of improved large surface area and charge transport properties. In this paper, we report development of lead iodide perovskite solar cells based on a novel 3D TiO2 NW architectures. The 3D TiO2 nanostructure was synthesized via surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique that also implemented the Kirkendall effect for complete ZnO NW template conversion. It was found that the film thickness of 3D TiO2 can significantly influence the photovoltaic performance. Short-circuit current increased with the TiO2 length, while open-circuit voltage and fill factor decreased with the length. The highest power conversion efficiency (PCE) of 9.0% was achieved with ∼ 600 nm long 3D TiO2 NW structures. Compared to other 1D nanostructure arrays (TiO2 nanotubes, TiO2-coated ZnO NWs and ZnO NWs), 3D TiO2 NW architecture was able to achieve larger amounts of perovskite loading, enhanced light harvesting efficiency, and increased electron-transport property. Therefore, its PCE is 1.5, 2.3, and 2.8 times higher than those of TiO2 nanotubes, TiO2-coated ZnO NWs, and ZnO NWs, respectively. The unique morphological advantages, together with the largely suppressed hysteresis effect, make 3D hierarchical TiO2 a promising electrode selection in designing high-performance perovskite solar cells.

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.

Enhanced Photoelectrochemical Performance from Rationally Designed Anatase/Rutile TiO2 Heterostructures

In a photoelectrochemical (PEC) cell for water splitting, the critical issue is charge separation and transport, which is usually completed by designing semiconductor heterojunctions. TiO2 anatase-rutile mixed junctions could largely improve photocatalytic properties, but impairs PEC water splitting performance. We designed and prepared two types of TiO2 heterostructures with the anatase thin film and rutile nanowire phases organized in different sequences. The two types of heterostructures were used as PEC photoanodes for water splitting and demonstrated completely opposite results. Rutile nanowires on anatase film demonstrated enhanced photocurrent density and onset potential, whereas strong negative performance was obtained from anatase film on rutile nanowire structures. The mechanism was investigated by photoresponse, light absorption and reflectance, and electrochemical impedance spectra. This work revealed the significant role of phase sequence in performance gain of anatase-rutile TiO2 heterostructured PEC photoanodes.

Simultaneous Enhancement of Charge Separation and Hole Transportation in a TiO2–SrTiO3 Core–Shell Nanowire Photoelectrochemical System

Efficient charge separation and transportation are key factors that determine the photoelectrochemical (PEC) water-splitting efficiency. Here, a simultaneous enhancement of charge separation and hole transportation on the basis of ferroelectric polarization in TiO2 -SrTiO3 core-shell nanowires (NWs) is reported. The SrTiO3 shell with controllable thicknesses generates a considerable spontaneous polarization, which effectively tunes the electrical band bending of TiO2 . Combined with its intrinsically high charge mobility, the ferroelectric SrTiO3 thin shell significantly improves the charge-separation efficiency (ηseparation ) with minimized influence on the hole-migration property of TiO2 photoelectrodes, leading to a drastically increased photocurrent density ( Jph ). Specifically, the 10 nm-thick SrTiO3 shell yields the highest Jph and ηseparation of 1.43 mA cm-2 and 87.7% at 1.23 V versus reversible hydrogen electrode, respectively, corresponding to 83% and 79% improvements compared with those of pristine TiO2 NWs. The PEC performance can be further manipulated by thermal treatment, and the control of SrTiO3 film thicknesses and electric poling directions. This work suggests a material with combined ferroelectric and semiconducting features could be a promising solution for advancing PEC systems by concurrently promoting the charge-separation and hole-transportation properties.

Biocompatibility and in vivo operation of implantable mesoporous PVDF-based nanogenerators

The rapid developments of implantable biomedical electronics give rise to the motivation of exploring efficient and durable self-powered charging system. In this paper, we report a mesoporous polyvinylidene fluoride (PVDF)-based implantable piezoelectric nanogenerator (NG) for in vivo biomechanical energy harvesting. The NG was built with a sponge-like mesoporous PVDF film and encapsulated by polydimethylsiloxane (PDMS). After embedding this NG into rodents, a Voc of ~200 mV was produced from the gentle movement of rodent muscle. Meanwhile, no toxicity or incompatibility sign was found in the host after carrying the packaged NG for 6 weeks. Moreover, the electric output of this NG was extremely stable and exhibited no deterioration after 5 days of in vivo operation or 1.512 × 108 times mechanical deformation. This NG device could practically output a constant voltage of 52 mV via a 1 µF capacitor under living circumstance. The outstanding efficiency, magnificent durability and exceptional biocompatibility promise this mesoporous PVDF-based NG in accomplishing self-powered bioelectronics with potentially lifespan operation period.

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.

Air-Stable Porous Fe2N Encapsulated in Carbon Microboxes with High Volumetric Lithium Storage Capacity and a Long Cycle Life

The development of inexpensive electrode materials with a high volumetric capacity and long cycle-life is a central issue for large-scale lithium-ion batteries. Here, we report a nanostructured porous Fe2N anode fully encapsulated in carbon microboxes (Fe2N@C) prepared through a facile confined anion conversion from polymer coated Fe2O3 microcubes. The resulting carbon microboxes could not only protect the air-sensitive Fe2N from oxidation but also retain thin and stable SEI layer. The appropriate internal voids in the Fe2N cubes help to release the volume expansion during lithiation/delithiation processes, and Fe2N is kept inside the carbon microboxes without breaking the shell, resulting in a very low electrode volume expansion (the electrode thickness variation upon lithiation is ∼9%). Therefore, the Fe2N@C electrodes maintain high volumetric capacity (1030 mA h cm-3 based on the lithiation-state electrode volume) comparable to silicon anodes, stable cycling performance (a capacity retention of over 91% for 2500 cycles), and excellent rate performance. Kinetic analysis reveals that the Fe2N@C shows an enhanced contribution of capacitive charge mechanism and displays typical pseudocapacitive behavior. This work provides a new direction on designing and constructing nanostructured electrodes and protective layer for air unstable conversion materials for potential applications as a lithium-ion battery/capacitor electrode.