Oriented assembled TiO2 hierarchical nanowire arrays with fast electron transport properties

Facile Fabrication of Lilac-Like Multiple Self-Supporting WO3 Nanoneedle Arrays with Cubic/Hexagonal Phase Junctions for Highly Sensitive Ethylene Glycol Gas Sensors

Metal oxides with nanoarray structures have been demonstrated to be prospective materials for the design of gas sensors with high sensitivity. In this work, the WO3 nanoneedle array structures were synthesized by a one-step hydrothermal method and subsequent calcination. It was demonstrated that the calcination of the sample at 400 °C facilitated the construction of lilac-like multiple self-supporting WO3 arrays, with appropriate c/h-WO3 heterophase junction and highly oriented nanoneedles. Sensors with this structure exhibited the highest sensitivity (2305) to 100 ppm ethylene glycol at 160 °C and outstanding selectivity. The enhanced ethylene glycol gas sensing can be attributed to the abundant transport channels and active sites provided by this unique structure. In addition, the more oxygen adsorption caused by the heterophase junction and the aggregation of reaction medium induced by tip effect are both in favor of the improvement on the gas sensing performance.

Nanostructure Control in Zinc Oxide Films and Microfibers through Bioinspired Synthesis of Liquid-Crystalline Zinc Hydroxide Carbonate; Formation of Free-Standing Materials in Centimeter-Level Lengths

Free-standing zinc oxide in the forms of films and fibrous materials are expected to be used as functional devices such as piezoelectric devices and catalyst filters without being limited by the growth substrate. Herein, a synthetic morphology-control method for 2D and 1D free-standing ZnO materials with ordered and nanoporous structures by conversion of liquid-crystalline (LC) zinc hydroxide carbonate (ZHC) nanoplates is reported. As a new colloidal liquid crystal, the LC ZHC nanoplate precursors are obtained by a biomineralization-inspired method. The approach is to control the morphology and crystallographic orientation of ZHC crystals by using acidic macromolecules. Their nano-scale and oriented structures are examined. The LC oriented ZHC nanoplates have led to the synthesis of free-standing films and microfibers of ZHC in centimeter-level lengths, with the successful thermal conversion into free-standing films and microfibers of ZnO. The resultant ZnO films and ZnO microfibers have nanoporous structures and preferential crystallographic orientations that preserve the alignment of ZHC nanoplates before conversion.

Solution chemistry quasi-epitaxial growth of atomic CaTiO3 perovskite layers to stabilize and passivate TiO2 photoelectrodes for efficient water splitting

Perovskite oxides with unique crystal structures and high defect tolerance are promising as atomic surface passivation layers for photoelectrodes for efficient and stable water splitting. However, controllably depositing and crystalizing perovskite-type metal oxides at the atomic level remains challenging, as they usually crystalize at higher temperatures than regular metal oxides. Here, we report a mild solution chemistry approach for the quasi-epitaxial growth of an atomic CaTiO3 perovskite layer on rutile TiO2 nanorod arrays. The high-temperature crystallization of CaTiO3 perovskite is overcome by a sequential hydrothermal conversion of the atomic amorphous TiOx layer to CaTiO3 perovskite. The atomic quasi-epitaxial CaTiO3 layer passivated TiO2 nanorod arrays exhibit more efficient interface charge transfer and high photoelectrochemical performance for water splitting. Such a mild solution-based approach for the quasi-epitaxial growth of atomic metal oxide perovskite layers could be a promising strategy for both fabricating atomic perovskite layers and improving their photoelectrochemical properties.

Nanostructured TiO2 Arrays for Energy Storage

Because of their extensive specific surface area, excellent charge transfer rate, superior chemical stability, low cost, and Earth abundance, nanostructured titanium dioxide (TiO₂) arrays have been thoroughly explored during the past few decades. The synthesis methods for TiO₂ nanoarrays, which mainly include hydrothermal/solvothermal processes, vapor-based approaches, templated growth, and top-down fabrication techniques, are summarized, and the mechanisms are also discussed. In order to improve their electrochemical performance, several attempts have been conducted to produce TiO₂ nanoarrays with morphologies and sizes that show tremendous promise for energy storage. This paper provides an overview of current developments in the research of TiO₂ nanostructured arrays. Initially, the morphological engineering of TiO₂ materials is discussed, with an emphasis on the various synthetic techniques and associated chemical and physical characteristics. We then give a brief overview of the most recent uses of TiO₂ nanoarrays in the manufacture of batteries and supercapacitors. This paper also highlights the emerging tendencies and difficulties of TiO₂ nanoarrays in different applications.

Recent progress in one dimensional TiO2 nanomaterials as photoanodes in dye-sensitized solar cells

Exploiting the vast possibilities of crystal and electronic structural modifications in TiO2 based nanomaterials creatively attracted the scientific community to various energy applications. A dye sensitised solar cell, which converts photons into electricity, is considered a viable solution for the generation of electricity. TiO2 nanomaterials were well accepted as photoanode materials in dye-sensitized solar cells, and possess non-toxicity, high surface area, high electron transport rates, fine tuneable band gap, high resistance to photo corrosion and optimum pore size for better diffusion of dye and electrolyte. This review focuses on various aspects of TiO2 nanomaterials as photoanodes in dye-sensitized solar cells. TiO2 photoanode modification via doping and morphological variations were discussed in detail. The impact of various morphologies on the design of TiO2 photoanodes was particularly stressed.

Use of cubic structure with primitive nanochannels for fabrication of free standing 3D nanowire network of Pt withPm3msymmetry

In this work, we developed a lipid mixture based on phytantriol / polyoxyethylene surfactant (Brij-56) that forms aIm3msymmetry bicontinuous cubic phase based on the Schwartz primitive surface (Q_II^P), from which we templated highly ordered 3D nanoporous platinum with a novel 'single primitive' morphology (Pm3msymmetry). TheQ_II^Ptemplate phase is obtained by incorporation of 17.5% w/w Brij-56 (C₁₆EO₁₀) (a type-I surfactant) into phytantriol under excess hydration conditions. Phytantriol alone forms the double diamondQ_II^D(Pn3m) phase, and in previous studies incorporating Brij-56 at different compositions the cubic phase maintained this morphology, but increased its lattice parameter; mesoporous metals templated from theseQ_II^Dlipid templates all exhibited the 'single diamond' (Fd3m) morphology. In contrast, the current paper presents the availability of ourQ_II^Pcubic phases to template nanoporous materials of single primitivePm3mmorphology via chemical and electrochemical methods. To explore the structure porosity and morphological features of the templated Pt material, x-ray scattering and transmission electron microscopy are used. The resulting 3D nanoporous Pt materials are found to exhibit a regular network of Pt nanowires of ∼4 nm in diameter with a unit cell dimension of 14.8 ± 0.8 nm, reflecting the aqueous network within theQ_II^Ptemplate.

Recent Advances of Nanostructured Materials for Photoelectrochemical Bioanalysis

Nowadays, the emerging photoelectrochemical (PEC) bioanalysis has drawn intensive interest due to its numerous merits. As one of its core elements, functional nanostructured materials play a crucial role during the construction of PEC biosensors, which can not only be employed as transducers but also act as signal probes. Although both chemical composition and morphology control of nanostructured materials contribute to the excellent analytical performance of PEC bioassay, surveys addressing nanostructures with different dimensionality have rarely been reported. In this review, according to classification based on dimensionality, zero-dimensional, one-dimensional, two-dimensional, and three-dimensional nanostructures used in PEC bioanalysis are evaluated, with an emphasis on the effect of morphology on the detection performances. Furthermore, using the illustration of recent works, related novel PEC biosensing patterns with promising applications are also discussed. Finally, the current challenges and some future perspectives in this field are addressed based on our opinions.

A photoanode with hierarchical nanoforest TiO2 structure and silver plasmonic nanoparticles for flexible dye sensitized solar cell

Due to unique photovoltaic properties, the nanostructured morphologies of TiO₂ on flexible substrate have been studied extensively in the recent years for applications in dye sensitized solar cells (DSSCs). Microstructured electrode materials with high surface area can facilitate rapid charge transport and thus improve the light-to-current conversion efficiency. Herein we present an improved photoanode with forest like photoactive TiO₂ hierarchical microstructure using a simple and facile hydrothermal route. To utilize the surface plasmon resonance (SPR) and hence increase the photon conversion efficiency, a plasmonic nanoparticle Ag has also been deposited using a very feasible photoreduction method. The branched structure of the photoanode increases the dye loading by filling the space between the nanowires, whereas Ag nanoparticles play the multiple roles of dye absorption and light scattering to increase the light-to-current conversion efficiency of the device. The branched structure provides a suitable matrix for the subsequent Ag deposition. They improve the charge collection efficiency by providing the preferential electron pathways. The high-density Ag nanoparticles deposited on the forest like structure also decrease the charge recombination and therefore improve the photovoltaic efficiency of the cells. As a result, the DSSC based on this novel photoanode shows remarkably higher photon conversion efficiency (η_max = 4.0% and η_opt = 3.15%) compared to the device based on pristine nanowire or forest-like TiO₂ structure. The flexibility of the device showed sustainable and efficient performance of the microcells.

Design and biosynthesis of functional protein nanostructures

Proteins are one of the major classes of biomolecules that execute biological functions for maintenance of life. Various kinds of nanostructures self-assembled from proteins have been created in nature over millions of years of evolution, including protein nanowires, layers and nanocages. These protein nanostructures can be reconstructed and equipped with desired new functions. Learning from and manipulating the self-assembly of protein nanostructures not only help to deepen our understanding of the nature of life but also offer new routes to fabricate novel nanomaterials for diverse applications. This review summarizes the recent research progress in this field, focusing on the characteristics, functionalization strategies, and applications of protein nanostructures.

Low-Temperature Solution-Processed Amorphous Titania Nanowire Thin Films for 1 cm2 Perovskite Solar Cells

The development of solution-processed inorganic amorphous electron-transporting layers (ETLs) is important for the future commercialization of perovskite solar cells (PSCs). The formation of such ETLs using low-temperature processing techniques will lower potential production costs and accommodate diverse substrate materials. Herein, a low-temperature (<150 °C) solution process forms amorphous titania nanowire (Am-TNW) thin films on fluorine-doped tin oxide conducting glass substrates. When applied as an ETL in PSCs, the Am-TNW layer achieves a higher average power conversion efficiency (18.3%) relative to that of a nanocrystalline anatase TNW (ATNW) layer obtained after high-temperature (500 °C) heating (16.7%). Compared to the ATNW counterparts, the Am-TNW-based PSCs exhibit inferior charge extraction across the TNW/CH₃NH₃PbI₃ interface but more effectively suppress interfacial charge recombination. The insertion of a fullerene layer between the Am-TNW and CH₃NH₃PbI₃ improves the charge extraction. The Am-TNW-based bilayer ETL gave optimal power conversion efficiencies of 20.3% and 19.0% for PSCs with 0.16 cm² and 1.00 cm² apertures, respectively. This is due to the concurrent advantages of enhanced light absorption, facilitated charge extraction, and reduced charge recombination. The use of the Am-TNW as an ETL in PSCs provides a facile, efficient way to increase the effectiveness of PSCs.

Enhanced catalytic reaction at an air-liquid-solid triphase interface

Gaseous reactant involved heterogeneous catalysis is critical to the development of clean energy, environmental management, health monitoring, and chemical synthesis. However, in traditional heterogeneous catalysis with liquid–solid diphase reaction interfaces, the low solubility and slow transport of gaseous reactants strongly restrict the reaction efficiency. In this minireview, we summarize recent advances in tackling these drawbacks by designing catalytic systems with an air–liquid–solid triphase joint interface. At the triphase interface, abundant gaseous reactants can directly transport from the air phase to the reaction centre to overcome the limitations of low solubility and slow transport of the dissolved gas in liquid–solid diphase reaction systems. By constructing a triphase interface, the efficiency and/or selectivity of photocatalytic reactions, enzymatic reactions, and (photo)electrochemical reactions with consumption of gaseous reactants oxygen, carbon dioxide, and nitrogen are significantly improved.

Gaseous reactant involved liquid–solid diphase interface reactions can be significantly enhanced using rationally designed and constructed air–liquid–solid triphase systems.

An Efficient Strategy for the Fabrication of CuS as a Highly Excellent and Recyclable Photocatalyst for the Degradation of Organic Dyes

An effective and practical in situ sulfuration approach has been developed in this work, for the fabrication of CuS with a 3D hierarchical network structure under mild preparation conditions. The prepared CuS consists of a primary structure of the multi-structure interchange copper foam precursor, and a secondary structure of nanoplates. The structural characteristics, morphologies, and photocatalytic performances of the prepared photocatalyst were investigated systematically. To evaluate the photocatalytic performance of the prepared CuS samples, we investigated the degradation of MB (methylene blue), RhB (Rhodamine B), and MB/RhB dye solutions over the samples under the irradiation of simulated solar light. Specifically, the degradation of RhB rapidly reached ≈100.0% after simulated solar light irradiation for 25 min, which is higher than those of P25 (83.0%) and bulk CuS (54.8%). For the mixed systems of MB/RhB, both the degradations of MB and RhB reached up to ≈99.0% after simulated solar light irradiation for 25 min. The superior photocatalytic performances of the prepared samples are attributed to the synergistic effects of high optical absorption, large specific surface area, and abundant active sites. The prepared catalysts can retain the photocatalytic activities during the entire reaction process without significant loss after four catalytic cycles, which reveals that the CuS with a stable 3D hierarchical network structure has a promising prospect as an ideal recyclable catalyst.

Rational Design of Photoelectrodes with Rapid Charge Transport for Photoelectrochemical Applications

Photoelectrode materials are the heart of photoelectrochemical (PEC) cells, which hold great promise to address global energy and environmental issues by converting solar energy into electricity or chemical fuels. In recent decades, significant research efforts have been devoted to the design and construction of photoelectrodes for the efficient generation and utilization of charge carriers to boost PEC performance. Herein, insights from a literature study on the relationship between the architecture and charge dynamics of photoelectrodes are presented. After briefly introducing the fundamental theories of charge dynamics in nanostructured photoelectrodes, the development of photoelectrode design in 1D polycrystalline nanotube arrays, 1D single-crystalline nanowire arrays, and hierarchical and mesoporous nanowire arrays is reviewed with a focus on the interplay between architecture and charge transport properties. For each design, commonly used synthetic approaches and the corresponding charge transport properties are discussed. Subsequently, the applications of these photoelectrodes in PEC systems are summarized. In conclusion, future challenges in the rational design of photoelectrode architecture are presented. The basic relationships between the architectures and charge dynamics of photoelectrode materials discussed here are expected to provide pertinent guidance and a reference for future advanced material design targeting improved light energy conversion systems.

Brand new 1D branched CuO nanowire arrays for efficient photoelectrochemical water reduction

1D branched CuO nanowire arrays, with large surface area and efficient charge transfer, are reported as photocathodes for photoelectrochemical water reduction.

In Situ Fabrication of Hierarchically Branched TiO2 Nanostructures: Enhanced Performance in Photocatalytic H2 Evolution and Li-Ion Batteries

1D branched TiO₂ nanomaterials play a significant role in efficient photocatalysis and high-performance lithium ion batteries. In contrast to the typical methods which generally have to employ epitaxial growth, the direct in situ growth of hierarchically branched TiO₂ nanofibers by a combination of the electrospinning technique and the alkali-hydrothermal process is presented in this work. Such the branched nanofibers exhibit improvement in terms of photocatalytic hydrogen evolution (0.41 mmol g^-1 h^-1 ), in comparison to the conventional TiO₂ nanofibers (0.11 mmol g^-1 h^-1 ) and P25 (0.082 mmol g^-1 h^-1 ). Furthermore, these nanofibers also deliver higher lithium specific capacity at different current densities, and the specific capacity at the rate of 2 C is as high as 201. 0 mAh g^-1 , roughly two times higher than that of the pristine TiO₂ nanofibers.

High-Performance Photoelectronic Sensor Using Mesostructured ZnO Nanowires

Semiconductor photoelectrodes that simultaneously possess rapid charge transport and high surface area are highly desirable for efficient charge generation and collection in photoelectrochemical devices. Herein, we report mesostructured ZnO nanowires (NWs) that not only demonstrate a surface area as high as 50.7 m²/g, comparable to that of conventional nanoparticles (NPs), but also exhibit a 100 times faster electron transport rate than that in NP films. Moreover, using the comparison between NWs and NPs as an exploratory platform, we show that the synergistic effect between rapid charge transport and high surface area leads to a high performance photoelectronic formaldehyde sensor that exhibits a detection limit of as low as 5 ppb and a response of 1223% (at 10 ppm), which are, respectively, over 100 times lower and 20 times higher than those of conventional NPs-based device. Our work establishes a foundational pathway toward a better photoelectronic system by materials design.

Balsam-pear-like rutile/anatase core/shell titania nanorod arrays for photoelectrochemical water splitting

In this work, a solution combustion followed by dissolution in hydrogen peroxide is adopted to achieve a precursor for decorating anatase TiO₂ nanosheets along single-crystalline rutile TiO₂ nanorods, which achieves balsam-pear-like core/shell nanorod arrays with enhanced photoelectrochemical water splitting. The enhanced photoelectrochemical performance is attributed to the novel nanoarchitecture, which can simultaneously offer a high surface area, enhanced light-harvesting, a rutile/anatase junction for charge carrier separation and a conductive pathway for charge carrier collection. The photoanode design can also give hints to other functional materials.

Photo-electrochemical properties of graphene wrapped hierarchically branched nanostructures obtained through hydrothermally transformed TiO2 nanotubes

Hierarchically structured nanomaterials play an important role in both light absorption and separation of photo-generated charges. In the present study, hierarchically branched TiO₂ nanostructures (HB-MLNTs) are obtained through hydrothermal transformation of electrochemically anodized TiO₂ multi-leg nanotubes (MLNT) arrays. Photo-anodes based on HB-MLNTs demonstrated 5 fold increase in applied bias to photo-conversion efficiency (%ABPE) over that of TiO₂ MLNTs without branches. Further, such nanostructures are wrapped with reduced graphene oxide (rGO) films to enhance the charge separation, which resulted in ∼6.5 times enhancement in %ABPE over that of bare MLNTs. We estimated charge transport (η _tr) and charge transfer (η _ct) efficiencies by analyzing the photo-current data. The ultra-fine nano branches grown on the MLNTs are effective in increasing light absorption through multiple scattering and improving charge transport/transfer efficiencies by enlarging semiconductor/electrolyte interface area. The charge transfer resistance, interfacial capacitance and electron decay time have been estimated through electrochemical impedance measurements which correlate with the results obtained from photocurrent measurements.

One-Dimensional Electron Transport Layers for Perovskite Solar Cells

The electron diffusion length (Ln) is smaller than the hole diffusion length (Lp) in many halide perovskite semiconductors meaning that the use of ordered one-dimensional (1D) structures such as nanowires (NWs) and nanotubes (NTs) as electron transport layers (ETLs) is a promising method of achieving high performance halide perovskite solar cells (HPSCs). ETLs consisting of oriented and aligned NWs and NTs offer the potential not merely for improved directional charge transport but also for the enhanced absorption of incoming light and thermodynamically efficient management of photogenerated carrier populations. The ordered architecture of NW/NT arrays affords superior infiltration of a deposited material making them ideal for use in HPSCs. Photoconversion efficiencies (PCEs) as high as 18% have been demonstrated for HPSCs using 1D ETLs. Despite the advantages of 1D ETLs, there are still challenges that need to be overcome to achieve even higher PCEs, such as better methods to eliminate or passivate surface traps, improved understanding of the hetero-interface and optimization of the morphology (i.e., length, diameter, and spacing of NWs/NTs). This review introduces the general considerations of ETLs for HPSCs, deposition techniques used, and the current research and challenges in the field of 1D ETLs for perovskite solar cells.

Enhanced Self-Assembly of Crystalline, Large-Area, and Periodicity-Tunable TiO2 Nanotube Arrays on Various Substrates

Due to their superior physical properties, titanium dioxide (TiO₂) nanotube arrays are one of the most investigated nanostructure systems in materials science until now. However, it is still a great challenge to achieve damage-free techniques to realize controllable, cost-effective, and high-performance TiO₂ nanotube arrays on both rigid and flexible substrates for different technological applications. In this work, we demonstrate a unique strategy to achieve self-assemble crystalline, large-area, and regular TiO₂ nanotube arrays on various substrates via hybrid combination of conventional semiconductor processes. Besides the usual applications of TiO₂ as carrier transport layers in thin-film electronic devices, we demonstrate that the periodic TiO₂ nanotube arrays can show the effect of optical grating with large-area uniformity. Specifically, the fabricated nanotube geometries, such as the tube height, pitch, diameter, and wall thickness, as well as the crystallinity can be reliably controlled by varying the processing conditions. More importantly, utilizing these nanotube arrays in perovskite solar cells can further enhance the optical absorption, leading to improved power conversion efficiency. In contrast to other typical template-assisted fabrication approaches, we employ a soft template here, which would enable the construction of nanotube arrays without any significant damage associated with template removal. Furthermore, without the thermal restriction of underlying substrates, these crystalline nanotube arrays can be transferred to mechanically flexible substrates by a simple one-step method, which can expedite these nanotubes for potential utilization in other application domains.

Mesostructured perovskite solar cells based on highly ordered TiO2 network scaffold via anodization of Ti thin film

An anodized TiO₂ interconnected network was fabricated and utilized as a mesoporous scaffold and electron transporter in perovskite solar cells. By modifying the synthesis parameters, the morphological features of the interconnected TiO₂ nanostructures can be widely tuned and precisely controlled. The functional properties of the anodized TiO₂ network are found to be severely influenced by morphology as well as the extent of oxidation. The device with the optimized TiO₂ network exhibits superior electron extraction and transferability, resulting in conspicuous enhancement of the photocurrent and power conversion efficiency (PCE). This work proposes a promising and facile method for improving the performance of perovskite solar cells.

UV and visible light photocatalytic activity of Au/TiO2 nanoforests with Anatase/Rutile phase junctions and controlled Au locations

To magnify anatase/rutile phase junction effects through appropriate Au decorations, a facile solution-based approach was developed to synthesize Au/TiO2 nanoforests with controlled Au locations. The nanoforests cons®isted of anatase nanowires surrounded by radially grown rutile branches, on which Au nanoparticles were deposited with preferred locations controlled by simply altering the order of the fabrication step. The Au-decoration increased the photocatalytic activity under the illumination of either UV or visible light, because of the beneficial effects of either electron trapping or localized surface plasmon resonance (LSPR). Gold nanoparticles located preferably at the interface of anatase/rutile led to a further enhanced photocatalytic activity. The appropriate distributions of Au nanoparticles magnify the beneficial effects arising from the anatase/rutile phase junctions when illuminated by UV light. Under the visible light illumination, the LSPR effect followed by the consecutive electron transfer explains the enhanced photocatalysis. This study provides a facile route to control locations of gold nanoparticles in one-dimensional nanostructured arrays of multiple-phases semiconductors for achieving a further increased photocatalytic activity.

Synthesis of Monodisperse Mesoporous TiO2 Nanospheres from a Simple Double-Surfactant Assembly-Directed Method for Lithium Storage

Exploring facile and reproducible methods to prepare mesoporous TiO2 nanospheres is crucial for improving the performance of TiO2 materials for energy conversion and storage. Herein, we report a simple and reproducible double-surfactant assembly-directed method to prepare monodisperse mesoporous TiO2 nanospheres. A double-surfactant system of n-dodecylamine (DDA) and Pluronic F127 was adopted to control the hydrolysis and condensation rates of tetrabutyl titanate in a mixture of water and alcohol at room temperature. In this process, the diameter size of mesoporous TiO2 nanospheres can be simply tuned from ∼50 to 250 nm by varying the concentration of H2O and surfactants. The double-surfactant system of DDA and F127 plays an effective role in determining the size, morphology, and monodispersity of mesoporous TiO2 nanospheres to reduce agglomeration during the sol-gel process. The resultant mesoporous anatase TiO2 nanospheres after solvothermal treatment at 160 °C are built of interpenetrating nanocrystals with a size of ∼10 nm, which are arranged to obtain a large number of connecting mesopores. Mesoporous TiO2 nanospheres with a small diameter size of around 50 nm possess a high surface area (∼160 m(2)/g) and mesopores with sizes of 4-30 nm. The small diameter size, high crystallinity, and mesoporous structure of TiO2 nanospheres lead to excellent performance in cycling stability and rate capability for lithium-ion batteries. After 500 cycles, the monodisperse mesoporous TiO2 nanospheres exhibit a charge capacity as high as 156 mAhg(-1) without obvious fade, and the Coulombic efficiency can reach up to 100%.

Consecutive Reaction to Construct Hierarchical Nanocrystalline CuS "Branch" with Tunable Catalysis Properties

New CuS nanocrystals with a 3D hierarchical branched structure are successfully synthesized through in situ consecutive reaction method with copper foam as template. The formation mechanism of the 3D hierarchical branched structure obtained from the secondary reaction is investigated by adjusting the reaction time. The morphology of CuS nanosheet arrays with the 3D hierarchical branched structure is changed through Cu(2+) exchange. In this method, the copper foam reacted completely, and the as-synthesized CuS@Cu9S5 nanocrystals are firmly grown on the surface of the 3D framework. This tunable morphology significantly influence the physical and chemical properties, particularly catalytic performance, of the materials. The as-obtained material of Cu@CuS-2 with the 3D hierarchical branched structure as catalyst for methylene blue degradation exhibits good catalytic performance than that of the material of Cu@CuS with 2D nanosheets in dark environment. Furthermore, the cation exchange between Cu and Cu(2+) indicates that Cu(2+) in wastewater could be absorbed by Cu@CuS-2 with the 3D hierarchical branched structure. The exchanged resultant of CuS@Cu9S5 retains its capability to degrade organic dyes. This in situ consecutive reaction method may have a significant impact on controlling the crystal growth direction of inorganic material.

Fabrication of porous TiO2 nanorod array photoelectrodes with enhanced photoelectrochemical water splitting by helium ion implantation

Porous photoelectrodes show high efficiency in hydrogen production by water splitting. However, fabrication of porous nanorods is usually difficult. Here, we report a simple approach to fabricate a kind of novel porous rutile titanium dioxide nanorod array by an advanced ion implantation method using multiple-energy helium ion implantation and subsequent annealing. The porous nanostructure enhances the photoelectrochemical performance of the titanium dioxide nanorod array photoelectrodes under Uv-visible light illumination, where the highest photocurrent density was relatively about 10 times higher than that of the pristine titanium dioxide nanorod array. The formation of nanocavities mainly contributes to the enhancement of the photocurrent density by trapping holes inside to separate the charge carriers. The study demonstrates that ion implantation could be an effective approach to develop novel porous nanostructural photoelectrodes for the application of hydrogen production.

High Performance Perovskite Solar Cells

Perovskite solar cells fabricated from organometal halide light harvesters have captured significant attention due to their tremendously low device costs as well as unprecedented rapid progress on power conversion efficiency (PCE). A certified PCE of 20.1% was achieved in late 2014 following the first study of long-term stable all-solid-state perovskite solar cell with a PCE of 9.7% in 2012, showing their promising potential towards future cost-effective and high performance solar cells. Here, notable achievements of primary device configuration involving perovskite layer, hole-transporting materials (HTMs) and electron-transporting materials (ETMs) are reviewed. Numerous strategies for enhancing photovoltaic parameters of perovskite solar cells, including morphology and crystallization control of perovskite layer, HTMs design and ETMs modifications are discussed in detail. In addition, perovskite solar cells outside of HTMs and ETMs are mentioned as well, providing guidelines for further simplification of device processing and hence cost reduction.

Hierarchical nanostructures of metal oxides for enhancing charge separation and transport in photoelectrochemical solar energy conversion systems

Photoelectrochemical solar energy conversion systems, including photoelectrochemical water splitting and photoelectrochemical solar cells (dye-sensitized solar cells, DSSCs), are under intensive development aiming at efficiently harvesting and utilizing solar energy. Metal oxides carved into hierarchical nanostructures are thought to be promising for improving photoelectrochemical performance by enhancing charge separation and transport. Herein, we review the recent progress in the research on the design and applications of metal oxide hierarchical nanostructures in water splitting and DSSC systems with a view to understanding how they improve the device performance in terms of enhanced charge separation and transport properties. This review will end with a conclusion on metal oxide hierarchical nanostructures together with potential future research directions thereof.

A room temperature approach for the fabrication of aligned TiO2 nanotube arrays on transparent conductive substrates

A novel room-temperature solution-approach is reported for the fabrication of highly crystallized TiO₂ nanotube arrays on transparent conductive substrates.

Ordered RTiO2@ATiO2 architecture for dye-sensitized solar cell applications

R_TiO2@A_TiO2 architectures are constructed, in which 1D rutile TiO₂ (R_TiO2) arrays allow a fast electron transport and branched anatase TiO₂ (A_TiO2) particles benefit the dye harvesting.

In-situ microfluidic controlled, low temperature hydrothermal growth of nanoflakes for dye-sensitized solar cells

In this paper, an in-situ microfluidic control unit (MCU) was designed and applied in a hydrothermal synthesis process, which provides an easy way to localize liquid-phase reaction and realize selective synthesis and direct growth of nanostructures as well as their morphology, all in a low-temperature and atmospheric environment. The morphology was controlled through controlling the amount of additivities using the MCU. This achieved a facile fabrication of Al doped ZnO (AZO) nanoflakes vertically grown on flexible polymer substrates with enhanced light scattering and dye loading capabilities. Flexible DSSCs with a significant enhancement (410% compare to ZnO NRs based devices) in power conversion efficiency were obtained using AZO nanoflake photoanodes of 6 μm thick, due to the enhancement in electron mobility and reduction in recombination. This hydrothermal synthesis using the in-situ MCU provides an efficient and scalable technique to synthesize controllable nanostructures with characteristics of easy set-up, low energy consumption and low cost.

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.

Hyperbranched self-assembled photoanode for high efficiency dye-sensitized solar cells

The photoanode morphology is key for improving photovoltaic performance of dye-sensitized solar cells.

High aspect ratio TiO2 nanowires tailored in concentrated HCl hydrothermal condition for photoelectrochemical water splitting

Discrete high aspect ratio TiO₂ nanowire arrays were synthesized with its structure as an optimal structure for photoelectrochemical applications.