Full solution-processed synthesis of all metal oxide-based tree-like heterostructures on fluorine-doped tin oxide for water splitting

Morphology-tunable synthesis of ZnO nanoforest and its photoelectrochemical performance

Understanding and manipulating synthesis reactions and crystal growth mechanisms are keys to designing and constructing the morphology and functional properties of advanced materials. Herein, the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoforests is reported via a facile hydrothermal route. Specifically, the respective and synergistic influence of polyethylenimine (PEI) and ammonia on tuning the architecture of ZnO nanoforests is systematically studied. An in-depth understanding of the mechanism of hydrothermal growth is vital for advancing this facile approach and incorporating special 3D nanostructures into versatile nanomanufacturing. More importantly, its unique architectural characteristics endow the willow-like ZnO nanoforest with prominent photoelectrochemical water splitting performance, including small charge transfer resistance, long photoelectron lifetime, a high photocurrent density of 0.919 mA cm(-2) at +1.2 V (vs. Ag/AgCl), and more important, a high photoconversion efficiency of 0.299% at 0.89 V (vs. RHE), which leads the realm of homogeneous ZnO nanostructures. In all, it is expected that this work will open up an unprecedented avenue to govern desirable 3D ZnO nanostructures and broaden the application potentials of 3D nanotechnology.

Branched WO3 nanosheet array with layered C3 N4 heterojunctions and CoOx nanoparticles as a flexible photoanode for efficient photoelectrochemical water oxidation

A hybrid WO3 /C3 N4 /CoOx system exhibits excellent photoelectrochemical activity for water oxidation. The system comprises a novel three-dimensionally branched WO3 nanosheet array coated with a layer of C3 N4 heterojunctions that are further decorated with CoOx nanoparticles. The photoelectrochemical activity arises from the effective light harvesting due to the 3D structure and "window effect," the excellent charge separation and transport in the heterojunction, and the fast interfacial charge collection and surface reactions due to the large surface area.

Optical properties of metal-molybdenum disulfide hybrid nanosheets and their application for enhanced photocatalytic hydrogen evolution

Limited control over charge recombination between photogenerated charge carriers largely hinders the progress in photocatalysis. Here, we introduce metal nanoparticles (Cr, Ag) to the surface of MoS2 nanosheets by simple synthetic means creating a hybrid metal-MoS2 nanosheet system with well-defined metal/semiconductor interfaces. We demonstrate that this hybrid nanosheet structure is capable of decoupling light absorption, primarily in MoS2, and carrier separation, across the metal-MoS2 heterostructure leading to drastic quenching of recombination between photogenerated carriers in MoS2, as proven by absorptance, photoluminescence, and ultrafast pump-probe spectroscopy. The photocatalytic activity in the hybrid system is also improved, which further shows excellent stability against photocorrosion.

Quantum dot-sensitized hierarchical micro/nanowire architecture for photoelectrochemical water splitting

We report the fabrication of quantum dot-sensitized hierarchical structure and the application of the structure as a photoanode for photoelectrochemical water splitting. The structure is synthesized by hydrothermally growing ZnO nanowires on silicon microwires grown with the vapor-liquid-solid method. Then the hierarchical structure is further sensitized with CdS and CdSe quantum dots and modified with IrOx quantum dots. As a result, the silicon microwires, ZnO nanowires, and the quantum dot/ZnO core/shell structure form a multiple-level hierarchical heterostructure, which is remarkably beneficial for light absorption and charge carrier separation. Our experimental results reveal that the photocurrent density of our multiple-level hierarchical structure achieves a surprising 171 times enhancement compared to that from simple ZnO nanowires on a planar substrate. In addition, the photoanode shows high stability during the water-splitting experiment. These results prove that the quantum dot-sensitized hierarchical structure is an ideal candidate for a photoanode in solar water splitting applications. Importantly, the modular design approach we take to produce the photoanode allows for the integration of future discoveries for further improvement of its performance.

CdS sensitized 3D hierarchical TiO2/ZnO heterostructure for efficient solar energy conversion

For conventional dye or quantum dot sensitized solar cells, which are fabricated using mesoporous films, the inefficient electron transport due to defects such as grain boundaries and surface traps is a major drawback. To simultaneously increase the carrier transport efficiency as well as the surface area, optimal-assembling of hierarchical nanostructures is an attractive approach. Here, a three dimensional (3D) hierarchical heterostructure, consisting of CdS sensitized one dimensional (1D) ZnO nanorods deposited on two dimensional (2D) TiO₂ (001) nanosheet, is prepared via a solution-process method. Such heterstructure exhibits significantly enhanced photoelectric and photocatalytic H₂ evolution performance compared with CdS sensitized 1D ZnO nanorods/1D TiO₂ nanorods photoanode, as a result of the more efficient light harvesting over the entire visible light spectrum and the effective electron transport through a highly connected 3D network.

Synthesis of porous ZnS:Ag2S nanosheets by ion exchange for photocatalytic H2 generation

ZnS:Ag2S porous nanostructures are prepared by a simple ion-exchange route using ZnS nanosheets as sacrificial templates. In solutions of different Ag ion concentrations, ZnS nanosheets are partially converted to Ag2S, resulting in porous ZnS:Ag2S nanosheet composites with different pore sizes. With the Ag2S nanocrystals playing the role of hole scavengers, the porous nanosheets exhibit a high photocatalytic H2 generation rate of 104.9 μmol/h/g without using any noble metal cocatalyst.

MoS2 nanoflower-decorated reduced graphene oxide paper for high-performance hydrogen evolution reaction

A facile, one-pot solvothermal method is developed to synthesize MoS2 nanoflowers (MoS2NFs) coated on reduced graphene oxide (rGO) paper. The resulting MoS2NF/rGO paper serves as a freestanding, flexible and durable working electrode for hydrogen evolution reaction (HER), exhibiting an overpotential lowered to -0.19 V with a Tafel slope of ∼95 mV per decade.

Programmable photo-electrochemical hydrogen evolution based on multi-segmented CdS-Au nanorod arrays

Programmable photocatalysts for hydrogen evolution have been fabricated based on multi-segmented CdS-Au nanorod arrays, which exhibited high-efficiency and programmability in hydrogen evolution as the photoanodes in the photoelectrochemical cell. Multiple different components each possess unique physical and chemical properties that provide these cascade nanostructures with multiformity, programmability, and adaptability. These advantages allow these nanostructures as promising candidates for high efficient harvesting and conversion of solar energy.

Direct evidence of plasmon enhancement on photocatalytic hydrogen generation over Au/Pt-decorated TiO2 nanofibers

Direct evidence of plasmon-enhanced H2 generation is observed in photocatalytic water reduction by using TiO2 electrospun nanofibers co-decorated with Au and Pt nanoparticles through dual-beam irradiation. The Au/Pt/TiO2 nanofibers exhibit certain activity for H2 generation under single irradiation at 420 nm that excites the defect/impurity states of TiO2. Significantly, when secondary irradiation at 550 nm is introduced to simultaneously excite Au SPR, we observed 2.5 times higher activity for H2 generation. Further investigation by finely controlling the irradiation wavelengths reveals that the enhancement factor on the photocatalytic activity for H2 generation is directly correlated with the plasmon absorption band of the Au nanoparticles in the Au/Pt/TiO2 nanofibers. The control experiments with different sacrificial agents suggest that the hot plasmonic electrons of Au are responsible for the enhanced photocatalytic activity that can be magnified when TiO2 is simultaneously excited.

Solution-processed flexible transparent conductors based on carbon nanotubes and silver grid hybrid films

In a simple, cost-effective, and solution-based process, a thin-film of single-walled carbon nanotubes is hybridized on a PET film which has been patterned with solution self-assembled Ag nanoparticles. Such a flexible and transparent electrode exhibits a sheet resistance down to ∼5.8 Ω sq(-1) at ∼83.7% optical transmittance. The hybrid films are stable under ambient conditions and offer excellent bendability.

Photoanode current of large-area MoS₂ ultrathin nanosheets with vertically mesh-shaped structure on indium tin oxide

A large area of hydrothermally grown MoS2 ultrathin nanosheets (NSs) with a vertically mesh-shaped structure on indium tin oxide (ITO) substrate was directly used as the photoanode of a potoelectrochemical (PEC) cell. The photoelectrocatalytic capacity of ultrathin MoS2 NSs was demonstrated, which was attributed not only to the excellent electrocatalytic activity originating from the exposed preferentially active edge sites but also to the superior photoelectric response resulting from the large light absorption of ultrathin MoS2 NSs and from the efficient separation of electron-hole pairs at the ITO/MoS2 interfaces. The significantly enhanced photocurrent indicates that the MoS2 ultrathin NSs can be a promising photoelectrocatalyst for PEC cells, unveiling the potential of MoS2-based PEC cells for solar energy absorption and conversion.

25th anniversary article: hybrid nanostructures based on two-dimensional nanomaterials

Two-dimensional (2D) nanomaterials, such as graphene and transition metal dichalcogenides (TMDs), receive a lot of attention, because of their intriguing properties and wide applications in catalysis, energy-storage devices, electronics, optoelectronics, and so on. To further enhance the performance of their application, these 2D nanomaterials are hybridized with other functional nanostructures. In this review, the latest studies of 2D nanomaterial-based hybrid nanostructures are discussed, focusing on their preparation methods, properties, and applications.

Oriented assembled TiO2 hierarchical nanowire arrays with fast electron transport properties

Developing high surface area nanostructured electrodes with rapid charge transport is essential for artificial photosynthesis, solar cells, photocatalysis, and energy storage devices. Substantial research efforts have been recently focused on building one-dimensional (1D) nanoblocks with fast charge transport into three-dimensional (3D) hierarchical architectures. However, except for the enlargement in surface area, there is little experimental evidence of fast electron transport in these 3D nanostructure-based solar cells. In this communication, we report single-crystal-like 3D TiO2 branched nanowire arrays consisting of 1D branch epitaxially grown from the primary trunk. These 3D branched nanoarrays not only demonstrate 71% enlargement in large surface area (compared with 1D nanowire arrays) but also exhibit fast charge transport property (comparable to that in 1D single crystal nanoarrays), leading to 52% improvement in solar conversion efficiency. The orientated 3D assembly strategy reported here can be extended to assemble other metal oxides with one or multiple components and thus represents a critical avenue toward high-performance optoelectronics.

Nanocrystalline TiO₂-catalyzed photoreversible color switching

We report a novel photoreversible color switching system based on the photocatalytic activity of TiO2 nanocrystals and the redox-driven color switching property of methylene blue (MB). This system rapidly changes from blue to colorless under UV irradiation and recovers its original blue color under visible light irradiation. We have identified four major competing reactions that contribute to the photoreversible switching, among which two are dominant: the decoloration process is mainly driven by the reduction of MB to leuco MB by photogenerated electrons from TiO2 nanocrystals under UV irradiation, and the recoloration process operates by the TiO2-induced self-catalyzed oxidation of LMB under visible irradiation. Compared with the conventional color switching systems based on photoisomerization of chromophores, our system has not only low toxicity but also significantly improved switching rate and cycling performance. It is envisioned that this photoreversible system may promise unique opportunities for many light-driven actuating or color switching applications.

Template-free synthesis of hematite photoanodes with nanostructured ATO conductive underlayer for PEC water splitting

Hematite is a promising semiconductor candidate for PEC water splitting. However, hematite is far well short of the theoretical value of solar-to-fuel conversion efficiency because of the fast recombination of photogenerated carriers. To address this limitation, a facile template-free preparation of hematite photoanode with nanostructured ATO (antimony-doped tin oxide) conductive underlayer served as a scaffold to transport photogenerated electron was developed to decrease the recombination opportunities of the carriers. Furthermore, the constructed ATO scaffold could also increase the light absorption of hematite and the number of the carriers, resulting in better PEC performance of hematite.

Vertically aligned MoS2/MoOx heterojunction nanosheets for enhanced visible-light photocatalytic activity and photostability

Vertically aligned nanosheet heterostructures with partly reduced MoO₃ cores and adjustable MoS₂ shells were fabricated via two-step chemical vapor deposition (CVD).

ZnO/CuO heterojunction branched nanowires for photoelectrochemical hydrogen generation

We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

Hierarchical macroporous Zn(2)SnO(4)-ZnO nanorod composite photoelectrodes for efficient CdS/CdSe quantum dot co-sensitized solar cells

A hierarchical macroporous Zn2SnO4-ZnO nanorod composite film is prepared through a drop-casting process of PS@Zn2SnO4 and subsequent hydrothermal growth of ZnO nanorod. CdS/CdSe co-sensitized solar cells based on the macroporous Zn2SnO4-ZnO nanorod composite photoelectrode exhibits an enhancement of 34.4% in power conversion efficiency (1.68%) compared to the pristine macroporous Zn2SnO4 photoelectrode (1.25%). Especially worth noting is that the growth of ZnO nanorods contributes greatly to the enlargement of surface area and improvement of light scattering ability of the composite film, which dominates the increase of Jsc values and eventual power conversion efficiency. QDSSCs based on the optimized 9 μm thick composite photoanode film exhibits a power conversion efficiency of 2.08%, which is the highest value for the reported QDs sensitized solar cells based on the Zn2SnO4 photoelectrode.

Visible light driven type II heterostructures and their enhanced photocatalysis properties: a review

Considerable efforts have been devoted to enhancing the photocatalytic activity and solar energy utilization of photocatalysts. The fabrication of type II heterostructures plays an important role in photocatalysts modification and has been extensively studied. In this review, we briefly trace the application of type II heterostructured semiconductors in the area of environmental remediation and water splitting, summarize major fabrication methods, describe some of the progress and resulting achievements, and discuss the future prospects. The scope of this review covers a variety of type II heterostructures, focusing particularly on TiO2 and ZnO based visible light driven type II 0D and 1D heterostructured photocatalysts. Some other low dimensional nanomaterials which have shown high-performance photocatalysis are also presented. We expect this review to provide a guideline for readers to gain a clear picture of fabrication and application of type II heterostructures.

Photocatalytic generation of multiple ROS types using low-temperature crystallized anodic TiO₂ nanotube arrays

This study investigates the photocatalytic efficiency, type of reactive oxygen species (ROS) produced, and potential for structural and morphological modification of anodic TiO₂ nanotubes (NTs) synthesized using a novel, energy efficient, low temperature crystallization process. These TiO₂ NTs show greater photocatalytic efficiency than traditional high-temperature sintered NTs or supported Degussa P25 TiO₂, as measured by degradation of methyl orange, a model organic dye pollutant. EPR analysis shows that low-temperature crystallized TiO₂ NTs generate both hydroxyl radicals and singlet oxygen, while high-temperature sintered TiO₂ NTs generate primarily hydroxyl radicals but no singlet oxygen. This "cocktail" of reactive oxygen species, combined with an increased surface area, contributes to the increased efficiency of this photocatalytic material. Furthermore, variation of the NT crystallization parameters enables control of structural and morphological properties so that TiO₂-NTs can be optimized for scale-up and for specific treatment scenarios.

Facile fabrication of hierarchical TiO2 nanobelt/ZnO nanorod heterogeneous nanostructure: an efficient photoanode for water splitting

The TiO2 nanobelt/ZnO nanorod composite photoelectrodes with flower-like and/or grass-like microstructures have been fabricated via a facile solution growth routine, just by controlling the treatment of the TiO2 nanobelt substrate. For the flower-like composite, the ZnO nanorods disperse orientationally on TiO2 nanobelt films, while for the grass-like one, ZnO nanorods grow disorderly like grass on the TiO2 nanobelt film surface. Furthermore, quasi-Fermi energy level changes of both photoelectrodes have been quantitatively characterized by the surface photovoltage based on the Kelvin probe, which clearly reveals the efficiency of photogenerated electron-hole separation. Owing to the decrease of quasi-Fermi energy level, the flower-like TiO2 nanobelt/ZnO nanorod heterogeneous nanostructure presents a high efficiency of photogenerated electron-hole separation. Therefore, the flower-like TiO2 nanobelt/ZnO nanorod heterogeneous nanostructure photoelectrode has achieved a better performance of water splitting compared with the grass-like TiO2 nanobelt/ZnO nanorod one.

Controllable synthesis of recyclable core-shell γ-Fe2O3@SnO2 hollow nanoparticles with enhanced photocatalytic and gas sensing properties

Composite materials containing different components with well-defined structures may cooperatively enhance their performance and extend their applications. In this work, core-shell γ-Fe2O3@SnO2 hollow nanoparticles (NPs) were synthesized by a low-cost and environmentally friendly seed-mediated hydrothermal method. Firstly, the γ-Fe2O3 hollow NPs were synthesized by a template-free method. Then they were used as the cores for the growth of SnO2 shells. The thickness of the shell can be simply tailored by controlling the reaction time. Various techniques, including SEM, XRD, TEM and HRTEM, were employed to investigate the morphology, structure and formation process of the special core-shell hollow structures. The combination of magnetic semiconductor (γ-Fe2O3) and wide band-gap semiconductor (SnO2) endowed them with great potential to be used as recyclable photocatalysts. Experiments on photo-degradation of Rhodamin B (RhB) dye in the presence of the samples showed that the hybrid structures possessed higher photocatalytic activities than the monomer structures of SnO2 and γ-Fe2O3 materials indicating a strong coupling enhancement effect between the wide and narrow band-gap semiconductors. Moreover, the gas sensing tests of the γ-Fe2O3@SnO2 hollow NPs revealed that the samples exhibited fast response and recovery rates, which enable them to be promising materials for gas sensors.

Synthesis of few-layer MoS2 nanosheet-coated TiO2 nanobelt heterostructures for enhanced photocatalytic activities

MoS(2) nanosheet-coated TiO(2) nanobelt heterostructures--referred to as TiO(2)@MoS(2)--with a 3D hierarchical configuration are prepared via a hydrothermal reaction. The TiO(2) nanobelts used as a synthetic template inhibit the growth of MoS(2) crystals along the c-axis, resulting in a few-layer MoS(2) nanosheet coating on the TiO(2) nanobelts. The as-prepared TiO(2)@MoS(2) heterostructure shows a high photocatalytic hydrogen production even without the Pt co-catalyst. Importantly, the TiO(2)@MoS(2) heterostructure with 50 wt% of MoS(2) exhibits the highest hydrogen production rate of 1.6 mmol h(-1) g(-1). Moreover, such a heterostructure possesses a strong adsorption ability towards organic dyes and shows high performance in photocatalytic degradation of the dye molecules.