Hierarchical Double‐Shell Nanostructures of TiO2 Nanosheets on SnO2 Hollow Spheres for High‐Efficiency,Solid‐State,Dye‐Sensitized Solar Cells

A high‐energy conversion efficiency of 8.2% at 100 mW cm^?2 is reported, one of the highest values for N719‐based, solid‐state, dye‐sensitized solar cells (ssDSSCs). The solar cells are based on hierarchical double‐shell nanostructures consisting of inner SnO₂ hollow spheres (SHS) surrounded by outer TiO₂ nanosheets (TNSs). Deposition of the TNS on the SHS outer surface is performed via solvothermal reactions in order to generate a double‐shell SHS@TNS nanostructure that provides a large surface area and suppresses recombination of photogenerated electrons. An organized mesoporous (OM)‐TiO₂ film with high porosity, large pores, and good interconnectivity is also prepared via a sol‐gel process using a poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer template. This film is utilized as a matrix to disperse the double‐shell nanostructures. Such nanostructures provide good pore‐filling for solid polymer electrolytes, faster electron transfer, and enhanced light scattering, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and intensity‐modulated photocurrent spectroscopy (IMPS)/intensity‐modulated photovoltage spectroscopy (IMVS).     

Large‐scale Synthesis of Urchin‐like Mesoporous TiO2 Hollow Spheres by Targeted Etching and Their Photoelectrochemical Properties

A versatile targeted etching strategy is developed for the large‐scale synthesis of urchin‐like mesoporous TiO₂ hollow spheres (UMTHS) with tunable particle size. Its key feature is the use of a low‐temperature hydrothermal reaction of surface‐fluorinated, amorphous, hydrous TiO₂ solid spheres (AHTSS) under the protection of a polyvinylpyrrolidone (PVP) coating. With the confinement of PVP and water penetration, the highly porous AHTSS are selectively etched and hollowed by fluoride without destroying their spherical morphology. Meanwhile TiO₂ hydrates are gradually crystallized and their growth is preferentially along anatase (101) planes, reconstructing an urchin‐like shell consisting of numerous radially arranged single‐crystal anatase nanothorns. Complex hollow structures, such as core–shell and yolk–shell structures, can also be easily synthesized via additional protection of the interior by pre‐filling AHTSS with polyethylene glycol (PEG). The hollowing transformation is elucidated by the synergetic effect of etching, PVP coating, low hydrothermal reaction temperature, and the unique microstructure of AHTSS. The synthesized UMTHS with a large surface area of up to 128.6 m² g^‐1 show excellent light‐harvesting properties and present superior performances in photocatalytic removal of gaseous nitric oxide (NO) and photoelectrochemical solar energy conversion as photoanodes for dye‐sensitized mesoscopic solar cells.     

General Formation of MS (M = Ni,Cu, Mn) Box‐in‐Box Hollow Structures with Enhanced Pseudocapacitive Properties

Complex hollow structures of metal sulfides could be promising materials for energy storage devices such as supercapacitors and lithium‐ion batteries. However, it is still a great challenge to fabricate well‐defined metal sulfides hollow structures with multi‐shells, hierarchical architectures, and non‐spherical shape. In this work, a template‐engaged strategy is developed to synthesize hierarchical NiS box‐in‐box hollow structures with double‐shells. The NiS box‐in‐box hollow structures constructed by ultrathin nanosheets are evaluated as electrode materials for supercapacitors. As expected, the NiS box‐in‐box hollow structures exhibit excellent rate performance and impressive cycling stability due to their unique nano‐architecture. More importantly, the synthetic method can be easily extended to synthesize other transition metal sulfides box‐in‐box hollow structures. For example, we have also successfully synthesized similar CuS and MnS box‐in‐box hollow structures. The present work makes a significant contribution to the design and synthesis of transition metal sulfides hollow structures with non‐spherical shape and complex architecture, as well as their potential applications in electrochemical energy storage.     

Ultrafine Nickel‐Nanoparticle‐Enabled SiO2 Hierarchical Hollow Spheres for High‐Performance Lithium Storage

The high theoretical capacity and natural abundance of SiO₂ make it a promising high‐capacity anode material for lithium‐ion batteries. However, its widespread application is significantly hampered by the intrinsic poor electronic conductivity and drastic volume variation. Herein, a unique hollow structured Ni/SiO₂ nanocomposite constructed by ultrafine Ni nanoparticle (≈3 nm) functionalized SiO₂ nanosheets is designed. The Ni nanoparticles boost not only the electronic conductivity but also the electrochemical activity of SiO₂ effectively. Meanwhile, the hollow cavity provides sufficient free space to accommodate the volume change of SiO₂ during repeated lithiation/delithiation; the nanosheet building blocks reduce the diffusion lengths of lithium ions. Due to the synergistic effect between Ni and SiO₂, the Ni/SiO₂ composite delivers a high reversible capacity of 676 mA h g^?1 at 0.1 A g^?1. At a high current density of 10 A g^?1, a capacity of 337 mA h g^?1 can be retained after 1000 cycles.     

Multi‐Shell Porous TiO2 Hollow Nanoparticles for Enhanced Light Harvesting in Dye‐sensitized Solar Cells

An optimized configuration for nanomaterials in working electrodes is vital to the high performance of dye‐sensitized solar cells (DSSCs). Here, a fabrication method is introduced for multi‐shell TiO₂ hollow nanoparticles (MS‐TiO₂‐HNPs) via a sol–gel reaction, calcination, and an etching process. The prepared uniform MS‐HNPs have a high surface area (ca. 171 m² g^?1), multireflection, and facile electrolyte circulation and diffusion. During the MS‐HNP fabrication process, the amount of SiO₂ precursor and H₂O under reaction has a significant effect on aggregation and side reactions. The etching process to obtain pure TiO₂ is influenced by anatase crystallinity. Additionally, single‐shell (SS)‐TiO₂‐HNPs and double‐shell (DS)‐TiO₂‐HNPs are synthesized as a control. The MS‐TiO₂‐HNPs exhibit a high surface area and enhance light reflectance, compared with the SS‐ and DS‐TiO₂‐HNPs of the same size. The power conversion efficiency of the optimized MS‐TiO₂‐HNP‐based DSSCs is 9.4%, compared with the 8.0% efficiency demonstrated by SS‐TiO₂‐HNP‐DSSCs (a 17.5% improvement). These results enable the utilization of multifunctional MS‐HNPs in energy material applications, such as lithium ion batteries, photocatalysts, water‐splitting, and supercapacitors.     

Engineering Hierarchical Hollow Nickel Sulfide Spheres for High‐Performance Sodium Storage

Sodium‐ion batteries (SIBs) are considered as promising alternatives to lithium‐ion batteries (LIBs) for energy storage due to the abundance of sodium, especially for grid distribution systems. The practical implementation of SIBs, however, is severely hindered by their low energy density and poor cycling stability due to the poor electrochemical performance of the existing electrodes. Here, to achieve high‐capacity and durable sodium storage with good rate capability, hierarchical hollow NiS spheres with porous shells composed of nanoparticles are designed and synthesized by tuning the reaction parameters. The formation mechanism of this unique structure is systematically investigated, which is clearly revealed to be Ostwald ripening mechanism on the basis of the time‐dependent morphology evolution. The hierarchical hollow structure provides sufficient electrode/electrolyte contact, shortened Na⁺ diffusion pathways, and high strain‐tolerance capability. The hollow NiS spheres deliver high reversible capacity (683.8 mAh g^?1 at 0.1 A g^?1), excellent rate capability (337.4 mAh g^?1 at 5 A g^?1), and good cycling stability (499.9 mAh g^?1 with 73% retention after 50 cycles at 0.1 A g^?1).     

High‐Performance Metal‐Free Solar Cells Using Stamp Transfer Printed Vapor Phase Polymerized Poly(3,4‐Ethylenedioxythiophene) Top Anodes

The use of vapor phase polymerized poly(3,4‐ethylenedioxythiophene) (VPP‐PEDOT) as a metal‐replacement top anode for inverted solar cells is reported. Devices with both i) standard bulk heterojunction blends of poly(3‐hexylthiophene) (P3HT) donor and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₀ (PCBM) soluble fullerene acceptor and ii) hybrid inorganic/organic TiO₂/P3HT acceptor/donor active layers are studied. Stamp transfer printing methods are used to deposit both the VPP‐PEDOT top anode and a work function enhancing PEDOT:polystyrenesulphonate (PEDOT:PSS) interlayer. The metal‐free devices perform comparably to conventional devices with an evaporated metal top anode, yielding power conversion efficiencies of 3% for bulk heterojunction blend and 0.6% for organic/inorganic hybrid structures. These encouraging results suggest that stamp transfer printed VPP‐PEDOT provides a useful addition to the electrode materials tool‐box available for low temperature and non‐vacuum solar cell fabrication.     

Syntheses and Energy Storage Applications of MxSy (M = Cu,Ag, Au) and Their Composites: Rechargeable Batteries and Supercapacitors

The development of novel materials to improve energy storage efficiencies is essential to satisfy ever‐increasing energy demands. M_xS_y (M = Cu, Ag, Au) and their composites offer opportunities and enormous prospects in energy storage due to their extraordinary electrochemical properties, which promote promising energy storage characteristics in terms of stability, energy and power density, lifetime, etc. Recent developments of M_xS_y (M = Cu, Ag, Au) and their composites with various morphologies have received considerable attention. Multidimensional morphologies of M_xS_y (M = Cu, Ag, Au) and their composites have enriched charge‐storage and electron‐transport abilities. This review provides a detailed account of the synthetic strategies based on sulfur sources (i.e., inorganic sulfur sources, organosulfur sources, and other sulfur sources), which dictate the morphologies of nanosized M_xS_y (M = Cu, Ag, Au) and their composites. Notably, nanostructured silver sulfide can be prepared from the bulk to nanoscale. Moreover, the electrochemical applications of these materials for lithium‐ion batteries, sodium‐ion batteries, lithium‐sulfur batteries, and supercapacitors are summarized. Finally, future perspectives on the development challenges and major opportunities for M_xS_y (M = Cu, Ag, Au) and their composites, which must be overcome to achieve further improvements in electrochemical performance are outlined.     

Monodisperse Metallic NiCoSe2 Hollow Sub‐Microspheres: Formation Process,Intrinsic Charge‐Storage Mechanism,and Appealing Pseudocapacitance as Highly Conductive Electrode for Electrochemical Supercapacitors

Highly conductive metal selenides are gaining prominence as promising electrode materials in electrochemical energy‐storage fields. However, phase‐pure bimetallic selenides are scarcely retrieved, and their underlying charge‐storage mechanisms are still far from clear. Here, first a solvothermal strategy is devised to purposefully fabricate monodisperse hollow NiCoSe₂ (H‐NiCoSe₂) sub‐microspheres. Inherent formation of metallic H‐NiCoSe₂ is tentatively put forward with comparative structure‐evolution investigations. Interestingly, the fresh H‐NiCoSe₂ does not demonstrate striking supercapacitive behaviors when evaluated for electrochemical supercapacitors (ESs). But it exhibits competitive pseudocapacitance of ≈750 F g^?1 at a rate of 3 A g^?1 with a high loading of 7 mg cm^?2 after ≈100 cyclic voltammetry (CV) cycles. With systematic physicochemical/electrochemical analyses, intrinsic energy‐storage mechanism of the H‐NiCoSe₂ is convincingly revealed that the electrooxidation‐generated biactive CoOOH/NiOOH phases in aqueous KOH over CV scanning, rather than the H‐NiCoSe₂ itself, account for the remarkable pesudocapacitance observed after cycling. An assembled H‐NiCoSe₂‐based asymmetric device has delivered an energy density of ≈25.5 Wh kg^?1 with a power rate of ≈3.75 kW kg^?1, and long‐span cycle life. More significantly, the electrode design and new perspectives here hold profound promise in enriching material synthesis methodologies and in‐depth understanding of the complex charge‐storage process of newly emerging pseudocapacitive materials for next‐generation ESs.     

TiO2 Microspheres with Controllable Surface Area and Porosity for Enhanced Light Harvesting and Electrolyte Diffusion in Dye‐Sensitized Solar Cells

An optimized configuration of TiO₂ microspheres in photoanodes is of great importance to prepare highly efficient dye‐sensitized solar cells (DSSCs). In this work, TiO₂ microspheres with tunable diameter, pore size, and porosity are synthesized by subtly adjusting the synthesizing conditions, including ratios of deionized water, ammonia, and ethanol, respectively. TiO₂ microspheres are obtained with large pore sizes and a high porosity without sacrificing specific surface areas. In addition, the effect of their porosity and pore size on the performance of DSSCs is investigated. As confirmed by the dye‐loading ability and electrolyte diffusion resistance, the large mesopores and the high porosity of the TiO₂ microspheres can improve dye adsorption and facilitate electrolyte diffusion, giving rise to a high light‐harvesting and electron collection efficiency. Consequently, the highest photocurrent of 19.21 mA cm^?2 and a power conversion efficiency of 9.98% are obtained by using the TiO₂ microspheres with the highest porosity, compared with a 9.29% efficiency demonstrated by the lowest porosity (an improvement of 7.4%). By modifying the interconnection and the external pores of the microspheres photoanode, a high efficiency of 11.67% is achieved for a DSSC based on the most potent TiO₂ microspheres.     

Nanosized Anatase TiO2 Single Crystals with Tunable Exposed (001) Facets for Enhanced Energy Conversion Efficiency of Dye‐Sensitized Solar Cells

This paper reports the synthesis of nanosized TiO₂ single crystals with different percentages of exposed (001) facets in the presence of HF solution. Various characterizations are conducted to understand the correlation between particle morphology, exposed (001) facets and photo‐conversion efficiency of the nanosized anatase TiO₂ single crystals. An enhancement in dye‐sensitized solar cells (DSSCs) overall conversion efficiency is observed for the photoanode consisting of nanosized TiO₂ single crystals with higher percentage of exposed (001) facets, increasing from 7.47%, 8.14% to 8.49% for the TiO₂ single crystals with ca. 10%, 38%, and 80% percentage of exposed (001) facets. Experimentally confirmed by dark current potential and open‐circuit voltage decay scans, such highly exposed (001) facets are not only favorable for more dye adsorption but also effectively retard the charge recombination process in DSSCs.     

Compact Inverse‐Opal Electrode Using Non‐Aggregated TiO2 Nanoparticles for Dye‐Sensitized Solar Cells

Compact inverse‐opal structures are constructed using non‐aggregated TiO₂ nanoparticles in a three‐dimensional colloidal array template as the photoelectrode of a dye‐sensitized solar cell. Organic‐layer‐coated titania nanoparticles show an enhanced infiltration and a compact packing within the 3D array. Subsequent thermal decomposition to remove the organic template followed by impregnation with N‐719 dye results in excellent inverse‐opal photoelectrodes with a photo‐conversion efficiency as high as 3.47% under air mass 1.5 illumination. This colloidal‐template approach using non‐aggregated nanoparticles provides a simple and versatile way to produce efficient inverse‐opal structures with the ability to control parameters such as cavity diameter and film thickness.     

Efficient Dye‐Sensitized Solar Cells with Potential‐Tunable Organic Sulfide Mediators and Graphene‐Modified Carbon Counter Electrodes

A new class of organic sulfide mediators with programmable redox properties is designed via density functional theory calculations and synthesized for efficient dye‐sensitized solar cells (DSCs). Photophysical and electrochemical properties of these mediators derived from systematical functionalization of the framework with electron donating and withdrawing groups (MeO, Me, H, Cl, CF₃, and NO₂) are investigated. With this new class of organic mediators, the redox potential can be fine‐tuned over a 170 mV range, overlapping the conventional I^?/I₃^?couple. Due to the suitable interplay of physical properties and electrochemical characteristics of the mediator involving electron‐donating MeO group, the DSCs based on this mediator behave excellently in various kinetic processes such as dye regeneration, electron recombination, and mass transport. Thus, the MeO derivative of the mediator is identified as having the best performance of this series of redox shuttles. As inferred from electrochemical impedance spectroscopy and cyclic voltammetry measurements, the addition of graphene into the normal carbon counter electrode material dramatically improves the apparent catalytic activity of the counter electrode towards the MeO derivative of mediator, resulting in N719 based DSCs showing a promising conversion efficiency of 6.53% under 100 mW·cm^?2 simulated sunlight illumination.     

Hollow Spherical Nanoshell Arrays of 2D Layered Semiconductor for High‐Performance Photodetector Device

Well‐defined hollow spherical nanoshell arrays of 2D transitional metal dichalcogenide (TMDC) nanomaterials for MoSe₂ and MoS₂ are grown via chemical vapor deposition technique for the first time. The hollow sphere arrays display the uniform dimensions of ≈450 nm with the shell thickness of ≈10 nm. The unique hollow sphere architecture with increased active surface area is forecasted to supply more efficient route to improve light‐harvesting efficiency through repeated light reflection and scattering inside the hollow structure without decay of response and recovery speed, because exceptional “SP–SP” junction barriers conducting mechanism can facilitate carriers tunneling and transport during the electron transfer procedure within the present particular structure. The MoSe₂ hollow sphere photodetector exhibits an outstanding responsivity (8.9 A W^?1), which is tenfold higher than that for MoSe₂ compact film (0.9 A W^?1), fast response and recovery speed, and good durability under illumination wavelength of 365 nm. Meanwhile, MoSe₂ hollow sphere arrays on flexible polyethylene terephthalate substrates reveal excellent bending stability. Therefore, this research indicates that unique hollow sphere architecture of 2D TMDC materials will be an anticipated avenue for efficient photodetector devices with far‐ranging capability.     

Wood‐Inspired Morphologically Tunable Aligned Hydrogel for High‐Performance Flexible All‐Solid‐State Supercapacitors

Oriented microstructures are widely found in various biological systems for multiple functions. Such anisotropic structures provide low tortuosity and sufficient surface area, desirable for the design of high‐performance energy storage devices. Despite significant efforts to develop supercapacitors with aligned morphology, challenges remain due to the predefined pore sizes, limited mechanical flexibility, and low mass loading. Herein, a wood‐inspired flexible all‐solid‐state hydrogel supercapacitor is demonstrated by morphologically tuning the aligned hydrogel matrix toward high electrode‐materials loading and high areal capacitance. The highly aligned matrix exhibits broad morphological tunability (47–12 µm), mechanical flexibility (0°–180° bending), and uniform polypyrrole loading up to 7 mm thick matrix. After being assembled into a solid‐state supercapacitor, the areal capacitance reaches 831 mF cm^?2 for the 12 µm matrix, which is 259% times of the 47 µm matrix and 403% times of nonaligned matrix. The supercapacitor also exhibits a high energy density of 73.8 µWh cm^?2, power density of 4960 µW cm^?2, capacitance retention of 86.5% after 1000 cycles, and bending stability of 95% after 5000 cycles. The principle to structurally design the oriented matrices for high electrode material loading opens up the possibility for advanced energy storage applications.     

A Continuous Carbon Nitride Polyhedron Assembly for High‐Performance Flexible Supercapacitors

Flexible supercapacitors with high power density, flexibility, and durability have shown enormous potential for smart electronics. Here, a continuous graphitic carbon nitride polyhedron assembly for flexible supercapacitor that is prepared by pyrolysis of carbon nanotubes wired zeolitic imidazolate framework‐8 (ZIF‐8) composites under nitrogen is reported. It exhibits a high specific capacitance of 426 F g^?1 at current density of 1 A g^?1 in 1 m H₂SO₄ and excellent stability over 10 000 cycles. The remarkable performance results from the continuous hierarchical structure with average pore size of 2.5 nm, high nitrogen‐doping level (17.82%), and large specific surface area (920 m² g^?1). Furthermore, a flexible supercapacitor is developed by constructing the assembly with interpenetrating polymer network electrolyte. Stemming from the synergistic effect of high‐performance electrode and highly ion‐conductive electrolyte, superior energy density of 59.40 Wh kg^?1 at 1 A g^?1 is achieved. The device maintains a stable energy supply under cyclic deformations, showing wide application in flexible and even wearable conditions. The work paves a new way for designing pliable electrode with excellent electronic and mechanic property for long‐lived flexible energy storage devices.