Secondary Branching and Nitrogen Doping of ZnO Nanotetrapods: Building a Highly Active Network for Photoelectrochemical Water Splitting

A photoanode based on ZnO nanotetrapods, which feature good vectorial electron transport and network forming ability, has been developed for efficient photoelectrochemical water splitting. Two strategies have been validated in significantly enhancing light harvesting. The first was demonstrated through a newly developed branch-growth method to achieve secondary and even higher generation branching of the nanotetrapods. Nitrogen-doping represents the second strategy. The pristine ZnO nanotetrapod anode yielded a photocurrent density higher than those of the corresponding nanowire devices reported so far. This photocurrent density was significantly increased for the new photoanode architecture based on the secondary branched ZnO nanotetrapods. After N-doping, the photocurrent density enjoyed an even more dramatic enhancement to 0.99 mA/cm² at +0.31 V vs Ag/AgCl. The photocurrent enhancement is attributed to the greatly increased roughness factor for boosting light harvesting associated with the ZnO nanotetrapod branching, and the increased visible light absorption due to the N-doping induced band gap narrowing of ZnO

Mesoporous TiO₂ Single Crystals: Facile Shape‑, Size‑, and Phase-Controlled Growth and Efficient Photocatalytic Performance

In this work, we have succeeded in preparing rutile and anatase TiO₂ mesoporous single crystals with diverse morphologies in a controllable fashion by a simple silica-templated hydrothermal method. A simple in-template crystal growth process was put forward, which involved heterogeneous crystal nucleation and oriented growth within the template, a sheer spectator, and an excluded volume, i.e., crystal growth by faithful negative replication of the silica template. A series of mesoporous single-crystal structures, including rutile mesoporous TiO₂ nanorods with tunable sizes and anatase mesoporous TiO₂ nanosheets with dominant {001} facets, have been synthesized to demonstrate the versatility of the strategy. The morphology, size, and phase of the TiO₂ mesoporous single crystals can be tuned easily by varying the external conditions such as the hydrohalic acid condition, seed density, and temperature rather than by the silica template, which merely serves for faithful negative replication but without interfering in the crystallization process. To demonstrate the application value of such TiO₂ mesoporous single crystals, photocatalytic activity was tested. The resultant TiO₂ mesoporous single crystals exhibited remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single-crystal nature, and the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. It was found that {110} facets of rutile mesoporous single crystals can be considered essentially as reductive sites with a key role in the photoreduction, while {001} facets of anatase mesoporous single crystals provided oxidation sites in the oxidative process. Such shape- and size-controlled rutile and anatase mesoporous TiO₂ single crystals hold great promise for building energy conversion devices, and the simple solution-based hydrothermal method is extendable to the synthesis of other mesoporous single crystals beyond TiO₂

A Quasi-Quantum Well Sensitized Solar Cell with Accelerated Charge Separation and Collection

Semiconductor-sensitized solar cell (SSSC) represents a new generation of device aiming to achieve easy fabrication and cost-effective performance. However, the power of the semiconductor sensitizers has not been fully demonstrated in SSSC, making it actually overshadowed by dye-sensitized solar cell (DSSC). At least part of the problem is related to the inefficient charge separation and severe recombination with the current technologies, which calls on rethinking about how to better engineer the semiconductor sensitizer structure in order to enhance the power conversion efficiency (PCE). Herein we report on using for the first time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20% PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only a quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core/shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed

Tailoring Metal–Oxygen Bonds Boosts Oxygen Reaction Kinetics for High-Performance Zinc–Air Batteries

Metal–oxygen bonds significantly affect the oxygen reaction kinetics of metal oxide-based catalysts but still face the bottlenecks of limited cognition and insufficient regulation. Herein, we develop a unique strategy to accurately tailor metal–oxygen bond structure via amorphous/crystalline heterojunction realized by ion-exchange. Compared with pristine amorphous CoSnO3–y, iron ion-exchange induced amorphous/crystalline structure strengthens the Sn–O bond, weakens the Co–O bond strength, and introduces additional Fe–O bond, accompanied by abundant cobalt defects and optimal oxygen defects with larger pore structure and specific surface area. The optimization of metal–oxygen bond structure is dominated by the introduction of crystal structure and further promoted by the introduction of Fe–O bond and rich Co defect. Remarkably, the Fe doped amorphous/crystalline catalyst (Co1–xSnO3–y-Fe0.021-A/C) demonstrates excellent oxygen evolution reaction and oxygen reduction reaction activities with a smaller potential gap (ΔE = 0.687 V), and the Zn–air battery based with Co1–xSnO3–y-Fe0.021-A/C exhibits excellent output power density, cycle performance, and flexibility

Exploratory Study of Zn_<i>x</i>PbO_<i>y</i> Photoelectrodes for Unassisted Overall Solar Water Splitting

A complete photoelectrochemical (PEC) water splitting system requires a photocathode and a photoanode to host water oxidation and reduction reactions, respectively. It is thus important to search for efficient photoelectrodes capable of full water splitting. Herein, we report on an exploratory study of a new photoelectrode family of Zn_<i>x</i>PbO_<i>y</i>ZnPbO₃ and Zn₂PbO₄similarly synthesized by a simple and economical method and shown to be a promising photocathode (p-type semiconductor) and photoanode (n-type semiconductor), respectively. From PEC measurements, the bare ZnPbO₃ photocathode achieved a photocurrent density of −0.94 mA/cm² at 0 V versus reversible hydrogen electrode (RHE), whereas the pristine Zn₂PbO₄ photoanode delivered a photocurrent density of 0.51 mA/cm² at 1.23 V versus RHE. By depositing suitable cocatalysts onto the photoelectrodes established above, we also demonstrated unassisted overall PEC water splitting, a rare case, if any, wherein a single material system is compositionally engineered for either of the photoelectrodes

Dithiafulvenyl Unit as a New Donor for High-Efficiency Dye-Sensitized Solar Cells: Synthesis and Demonstration of a Family of Metal-Free Organic Sensitizers

This work identifies the dithiafulvenyl unit as an excellent electron donor for constructing D−π–A-type metal-free organic sensitizers of dye-sensitized solar cells (DSCs). Synthesized and tested are three sensitizers all with this donor and a cyanoacrylic acid acceptor but differing in the phenyl (<b>DTF-C1</b>), biphenyl (<b>DTF-C2</b>), and phenyl–thiopheneyl–phenyl π-bridges (<b>DTF-C3</b>). Devices based on these dyes exhibit a dramatically improved performance with the increasing π-bridge length, culminating with DTF-C3 in η = 8.29% under standard global AM 1.5 illumination

In Situ Electrochemically Derived Nanoporous Oxides from Transition Metal Dichalcogenides for Active Oxygen Evolution Catalysts

Transition metal dichalcogenides have been widely studied as active electrocatalysts for hydrogen evolution reactions. However, their properties as oxygen evolution reaction catalysts have not been fully explored. In this study, we systematically investigate a family of transition metal dichalcogenides (MX, M = Co, Ni, Fe; X = S, Se, Te) as candidates for water oxidation. It reveals that the transition metal dichalcogenides are easily oxidized in strong alkaline media via an in situ electrochemical oxidation process, producing nanoporous transition metal oxides toward much enhanced water oxidation activity due to their increased surface area and more exposed electroactive sites. The optimal cobalt nickel iron oxides that derived from their sulfides and selenides demonstrate a low overpotential of 232 mV at current density of 10 mA cm^–2, a small Tafel slope of 35 mV per decade, and negligible degradation of electrochemical activity over 200 h of electrolysis. This study represents the discovery of nanoporous transition metal oxides deriving from their chalcogenides as outstanding electrocatalysts for water oxidation

Enhanced Charge Collection for Splitting of Water Enabled by an Engineered Three-Dimensional Nanospike Array

Photoelectrochemical (PEC) water splitting is a promising method of converting solar energy to hydrogen fuel from water using photocatalysts. Despite much effort in preparing mesoporous thin films on planar substrates, relatively little attention has been paid to their deposition on three-dimensional (3D) substrates, which could improve electron collection and enhance light-trapping. Here, we report the first synthesis of hierarchically branched anatase TiO₂ nanotetrapods, achieved by dissolution and nucleation processes on a ZnO nanotetrapods template. When used as a photoanode for efficient PEC water splitting, the unique branched anatase TiO₂ nanotetrapods yielded a photocurrent density of 0.54 mA cm^–2 at applied potential of 0.35 V vs RHE, much higher than that of commercial TiO₂ nanoparticles under otherwise identical conditions. Moreover, when the nanotetrapods were deposited on an ordered, purposely engineered 3D F-doped tin oxide (FTO) nanospike array, the photocurrent density was upgraded to 0.72 mA cm^–2. This large photocurrent enhancement can be attributed to the ultrahigh contact surface area with the electrolyte, which is bequeathed by the hierarchically branched TiO₂ nanotetrapods with a skin layer of vertically aligned ultrathin nanospines, as well as the short charge transport distance and enhanced light-trapping due to the peculiar 3D FTO nanospike array we have engineered by design

Boron Doping of Multiwalled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-Based Perovskite Solar Cells

Compared to the conventional perovskite solar cells (PSCs) containing hole-transport materials (HTM), carbon materials based HTM-free PSCs (C-PSCs) have often suffered from inferior power conversion efficiencies (PCEs) arising at least partially from the inefficient hole extraction at the perovskite–carbon interface. Here, we show that boron (B) doping of multiwalled carbon nanotubes (B-MWNTs) electrodes are superior in enabling enhanced hole extraction and transport by increasing work function, carrier concentration, and conductivity of MWNTs. The C-PSCs prepared using the B-MWNTs as the counter electrodes to extract and transport hole carriers have achieved remarkably higher performances than that with the undoped MWNTs, with the resulting PCE being considerably improved from 10.70% (average of 9.58%) to 14.60% (average of 13.70%). Significantly, these cells show negligible hysteretic behavior. Moreover, by coating a thin layer of insulating aluminum oxide (Al₂O₃) on the mesoporous TiO₂ film as a physical barrier to substantially reduce the charge losses, the PCE has been further pushed to 15.23% (average 14.20%). Finally, the impressive durability and stability of the prepared C-PSCs were also testified under various conditions, including long-term air exposure, heat treatment, and high humidity

Vertically Aligned Carbon Nanotubes on Carbon Nanofibers: A Hierarchical Three-Dimensional Carbon Nanostructure for High-Energy Flexible Supercapacitors

Hierarchical structures enable high-performance power sources. We report here the preparation of vertically aligned carbon nanotubes directly grown on carbon nanofibers (VACNTs/CNFs) by combining electrospinning with pyrolysis technologies. The structure and morphology of VACNTs/CNFs could be precisely tuned and controlled by adjusting the percentage of reactants. The desired VACNTs/CNFs could not only possess high electric conductivity for efficient charge transport but could also increase surface area for accessing more electrolyte ions. When using an ionic liquid electrolyte, VACNTs/CNFs-based electric double layer (EDL) flexible supercapacitors can deliver a high specific energy of 70.7 Wh/kg at a current density of 0.5 A/g and at 30 °C, and an ultrahigh-energy density of 98.8 Wh/kg at a current density of 1.0 A/g and at 60 °C. Even after 20 000 charging/discharging cycles, the EDL capacitor still retains 97.0% of the initial capacitance. The excellent performance highlights the important role of the branched VACNTs in storing and accumulating charge and the CNF backbone in transporting charge, thereby boosting both power density and energy density