Growth of Hydrothermally Derived CdS-Based Nanostructures with Various Crystal Features and Photoactivated Properties

Nanoscale self-assembly: concepts, applications and challenges

Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapour or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.

Enhanced Sensing Ability of Brush-Like Fe2O3-ZnO Nanostructures towards NO2 Gas via Manipulating Material Synergistic Effect

Brush-like α-Fe2O3-ZnO heterostructures were synthesized through a sputtering ZnO seed-assisted hydrothermal growth method. The resulting heterostructures consisted of α-Fe2O3 rod templates and ZnO branched crystals with an average diameter of approximately 12 nm and length of 25 nm. The gas-sensing results demonstrated that the α-Fe2O3-ZnO heterostructure-based sensor exhibited excellent sensitivity, selectivity, and stability toward low-concentration NO2 gas at an optimal temperature of 300 °C. The α-Fe2O3-ZnO sensor, in particular, demonstrated substantially higher sensitivity compared with pristine α-Fe2O3, along with faster response and recovery speeds under similar test conditions. An appropriate material synergic effect accounts for the considerable enhancement in the NO2 gas-sensing performance of the α-Fe2O3-ZnO heterostructures.

Crystal Growth and Design of Disk/Filament ZnO-Decorated 1D TiO2 Composite Ceramics for Photoexcited Device Applications

Disk- and filament-like ZnO crystals were decorated on one-dimensional TiO2 nanostructures (TiO2-ZnO) through various integrated physical and chemical synthesis methods. The morphology of the ZnO crystals on TiO2 varied with the chemical synthesis method used. ZnO nanodisks decorated with TiO2 nanorods (TiO2-ZnO-C) were synthesized using the chemical bath deposition method, and ZnO filament-like crystals decorated with TiO2 nanorods (TiO2-ZnO-H) were synthesized through the hydrothermal method. Compared with the pristine TiO2 nanorods, the as-synthesized TiO2-ZnO composites exhibited enhanced photophysiochemical performance. Furthermore, because of their fast electron transportation and abundant surface active sites, the ZnO nanodisks in the TiO2-ZnO-C composite exhibited a higher photoactivity than those in the TiO2-ZnO-H composite. The morphology and crystal quality of the ZnO decoration layer were manipulated using different synthesis methods to realize disk- or filament-like ZnO-decorated TiO2 composites with various photoactive performance levels.

Improved photoelectrode performance of chemical solution-derived Bi2O3 crystals via manipulation of crystal characterization

Three-dimensional Bi2O3 crystals with various morphologies were successfully synthesized on F-doped tin oxide substrates with and without homoseed layers via chemical bath deposition (CBD) routes. The structural analysis reveals that control of the pH value of the reaction solution resulted in as-grown Bi2O3 crystals with nanosheet and plate morphologies. A lower pH value of the reaction solution engendered formation of a porous sheet-like morphology of Bi2O3; by contrast, a higher pH value of the reaction solution is favorable for formation of solid Bi2O3 plates on the substrates. Furthermore, a sputter coated Bi2O3 seed layer with dual α- and β-Bi2O3 phases plays an important role in the CBD-derived Bi2O3 crystallographic structures. The Bi2O3 crystals formed via CBD processes without a sputter coated Bi2O3 homoseed layer demonstrated a high purity in β-Bi2O3 phase; those grown with a homoseed layer exhibited a dual α/β phase. The photoactive performance results show that construction of an α/β-Bi2O3 homojunction in the CBD-derived Bi2O3 crystals substantially improved their photoactive performance. Comparatively, the porous Bi2O3 nanosheets with a dual α/β-Bi2O3 phase demonstrated the highest photoactive performance among various Bi2O3 crystals in this study. The superior photoactivity of the porous α/β-Bi2O3 nanosheets herein is attributed to their high light absorption capacity and photoinduced charge separation efficiency. The experimental results in this study provide a promising approach to design CBD-derived Bi2O3 crystals with desirable photoelectric conversion functions via facile morphology control and seed layer crystal engineering.

Thermal Annealing Induced Controllable Porosity and Photoactive Performance of 2D ZnO Sheets

Porous ZnO sheets containing various degrees of a nanoscaled pore were successfully synthesized using a simple hydrothermal method and various postannealing procedures. The porosity features of the ZnO sheets can be easily tuned by changing both the annealing temperature and annealing atmosphere. The dense porous nature of ZnO sheets is beneficial to enhance light absorption. Moreover, the substantially increased oxygen vacancies in the ZnO sheets were observed especially after the hydrogen treatment as revealed in the X-ray photoelectron spectroscope and photoluminescence analyses. The high density of surface crystal defect enhanced the photoinduced electron-hole separation rate of the ZnO sheets, which is crucial for an improved photoactivity. The porous ZnO sheets formed at a hydrogen atmosphere exhibited superior photoactive performance than the porous ZnO sheets formed at the high-temperature ambient air annealing. The dense pores and massive crystal defects formed by a hydrogen atmosphere annealing in the ZnO crystals might account for the observed photoactive behaviors in this study.

Coverage Layer Phase Composition-Dependent Photoactivity of One-Dimensional TiO2-Bi2O3 Composites

TiO₂–Bi₂O₃ composite rods were synthesized by combining hydrothermal growth of rutile TiO₂ rod templates and sputtering deposition of Bi₂O₃ thin films. The TiO₂–Bi₂O₃ composite rods with β-Bi₂O₃ phase and α/β-Bi₂O₃ dual-phase decoration layers were designed, respectively, via in situ radio-frequency magnetron sputtering growth and post-annealing procedures in ambient air. The crystal structure, surface morphology, and photo-absorption performances of the pristine TiO₂ rods decorated with various Bi₂O₃ phases were investigated. The crystal structure analysis reveals that the crystalline TiO₂–Bi₂O₃ rods contained β-Bi₂O₃ and α/β-Bi₂O₃ crystallites were separately formed on the TiO₂ rod templates with different synthesis approaches. The morphology analysis demonstrates that the β-Bi₂O₃ coverage layer on the crystalline rutile TiO₂ rods showed flat layer morphology; however, the surface morphology of the α/β-Bi₂O₃ dual-phase coverage layer on the TiO₂ rods exhibited a sheet-like feature. The results of photocatalytic decomposition towards methyl orange dyes show that the substantially improved photoactivity of the rutile TiO₂ rods was achieved by decorating a thin sheet-like α/β-Bi₂O₃ coverage layer. The effectively photoinduced charge separation efficiency in the stepped energy band configuration in the composite rods made from the TiO₂ and α/β-Bi₂O₃ explained their markedly improved photoactivity. The TiO₂-α/β-Bi₂O₃ composite rods are promising for use as photocatalysts and photoelectrodes.

Surface Morphology-Dependent Functionality of Titanium Dioxide-Nickel Oxide Nanocomposite Semiconductors

In this study, TiO₂–NiO heterostructures were synthesized by combining hydrothermal and chemical bath deposition methods. The post-annealing temperature was varied to control the surface features of the TiO₂–NiO heterostructures. TiO₂–NiO heterostructures annealed at 350 °C comprised NiO-nanosheet-decorated TiO₂ nanostructures (NST), whereas those annealed at 500 °C comprised NiO-nanoparticle-decorated TiO₂ nanostructures (NPT). The NPT exhibited higher photodegradation activity than the NST in terms of methylene blue (MB) degradation under irradiation. Structural analyses demonstrated that the NPT had a higher surface adsorption capability for MB dyes and superior light-harvesting ability; thus, they exhibited greater photodegradation ability toward MB dyes. In addition, the NST showed high gas-sensing responses compared with the NPT when exposed to acetone vapor. This result was attributable to the higher number of oxygen-deficient regions on the surfaces of the NST, which increased the amount of surface-chemisorbed oxygen species. This resulted in a relatively large resistance variation for the NST when exposed to acetone vapor.

Photocapacitive CdS/WOx nanostructures for solar energy storage

Through a facile solvothermal procedure, a CdS/WO_x nanocomposite has been synthesised which exhibits photocapacitive behaviour under white light illumination at a radiant flux density of 99.3 mW cm⁻². Photoelectrochemical experiments were undertaken to examine the self-charging properties of the material and to develop an understanding of the underlying electronic band structure responsible for the phenomenon. By employing XPS, UPS and UV-Vis diffuse reflectance spectroscopy for further characterisation, the ability of the composite to generate current following the removal of incident light was related to the trapping of photoexcited electrons by the WO_x component. The presence of WO_x yielded an order of magnitude increase in the transient photocurrent response relative to CdS alone, an effect attributed to the suppression of electron-hole recombination in CdS due to hole transfer across the CdS/WO_x interface. Moreover, current discharge from the material persisted for more than twenty minutes after final illumination, an order of magnitude improvement over many existing binary composites. As a seminal investigation into the photocapacitive characteristics of CdS/WO_x composites, the work offers insight into how the constituent materials might be utilised as part of a future self-charging solar device.

Shell Layer Thickness-Dependent Photocatalytic Activity of Sputtering Synthesized Hexagonally Structured ZnO-ZnS Composite Nanorods

ZnO-ZnS core-shell nanorods are synthesized by combining the hydrothermal method and vacuum sputtering. The core-shell nanorods with variable ZnS shell thickness (7-46 nm) are synthesized by varying ZnS sputtering duration. Structural analyses demonstrated that the as-grown ZnS shell layers are well crystallized with preferring growth direction of ZnS (002). The sputtering-assisted synthesized ZnO-ZnS core-shell nanorods are in a wurtzite structure. Moreover, photoluminance spectral analysis indicated that the introduction of a ZnS shell layer improved the photoexcited electron and hole separation efficiency of the ZnO nanorods. A strong correlation between effective charge separation and the shell thickness aids the photocatalytic behavior of the nanorods and improves their photoresponsive nature. The results of comparative degradation efficiency toward methylene blue showed that the ZnO-ZnS nanorods with the shell thickness of approximately 17 nm have the highest photocatalytic performance than the ZnO-ZnS nanorods with other shell layer thicknesses. The highly reusable catalytic efficiency and superior photocatalytic performance of the ZnO-ZnS nanorods with 17 nm-thick ZnS shell layer supports their potential for environmental applications.

Surface crystal feature-dependent photoactivity of ZnO–ZnS composite rods via hydrothermal sulfidation

ZnO–ZnS core–shell composite rods were synthesized using a two-step facile hydrothermal methodology wherein different sulfidation durations were employed. The effects of sulfidation duration on the morphology and crystalline quality of ZnS shell layers on the surfaces of ZnO rods were investigated. A ZnS shell layer with visible granular features was obtained in the adequately controlled 3 h sulfidation process. A structural analysis demonstrated that the ZnS shell layers of ZnO–ZnS composite rods synthesized after 3 h sulfidation were in a well-defined crystalline cubic zinc blend phase. Moreover, optical properties revealed that these composite rods had a higher light-harvesting ability than those obtained after 1 and 2 h sulfidation. The density of surface crystal defects and the photoexcited charge separation efficiency of the composite rods were associated with changes in the microstructure of the synthesized ZnS shell layers. The optimal sulfidation duration of 3 h for the ZnO–ZnS composite rods resulted in the highest photocatalytic activity for the given photodegradation test conditions. The improved light harvesting and charge transport at the ZnO–ZnS heterointerface accounted for the enhanced photocatalytic activity of the ZnO–ZnS composite rods synthesized after 3 h sulfidation.

ZnO–ZnS core–shell composite rods were synthesized using a two-step facile hydrothermal methodology wherein different sulfidation durations were employed.

Hydrothermal synthesis of In2O3 nanoparticles hybrid twins hexagonal disk ZnO heterostructures for enhanced photocatalytic activities and stability

In₂O₃ nanoparticles hybrid twins hexagonal disk (THD) ZnO with different ratios were fabricated by a hydrothermal method. The as-obtained ZnO/In₂O₃ composites are constituted by hexagonal disks ZnO with diameters of about 1 μm and In₂O₃ nanoparticles with sizes of about 20-50 nm. With the increase of In₂O₃ content in ZnO/In₂O₃ composites, the absorption band edges of samples shifted from UV to visible light region. Compared with pure ZnO, the ZnO/In₂O₃ composites show enhanced photocatalytic activities for degradation of methyl orange (MO) and 4-nitrophenol (4-NP) under solar light irradiation. Due to suitable alignment of their energy band-gap structure of the In₂O₃ and ZnO, the formation of type п heterostructure can enhance efficient separation of photo-generate electro-hole pairs and provides convenient carrier transfer paths.

Photoexcited Properties of Tin Sulfide Nanosheet-Decorated ZnO Nanorod Heterostructures

In this study, ZnO-Sn₂S₃ core-shell nanorod heterostructures were synthesized by sputtering Sn₂S₃ shell layers onto ZnO rods. The Sn₂S₃ shell layers consisted of sheet-like crystallites. A structural analysis revealed that the ZnO-Sn₂S₃ core-shell nanorod heterostructures were highly crystalline. In comparison with ZnO nanorods, the ZnO-Sn₂S₃ nanorods exhibited a broadened optical absorption edge that extended to the visible light region. The ZnO-Sn₂S₃ nanorods exhibited substantial visible photodegradation efficiency of methylene blue organic dyes and high photoelectrochemical performance under light illumination. The unique three-dimensional sheet-like Sn₂S₃ crystallites resulted in the high light-harvesting efficiency of the nanorod heterostructures. Moreover, the efficient spatial separation of photoexcited carriers, attributable to the band alignment between ZnO and Sn₂S₃, accounted for the superior photocatalytic and photoelectrochemical properties of the ZnO-Sn₂S₃ core-shell nanorod heterostructures.

Fabrication of Nanosized Island-Like CdO Crystallites-Decorated TiO₂ Rod Nanocomposites via a Combinational Methodology and Their Low-Concentration NO₂ Gas-Sensing Behavior

TiO₂–CdO composite rods were synthesized through a hydrothermal method and sputtering thin-film deposition. The hydrothermally derived TiO₂ rods exhibited a rectangular cross-sectional crystal feature with a smooth surface, and the as-synthesized CdO thin film exhibited a rounded granular surface feature. Structural analyses revealed that the CdO thin film sputtered onto the surfaces of the TiO₂ rods formed a discontinuous shell layer comprising many island-like CdO crystallites. The TiO₂–CdO composite rods were highly crystalline, and their surfaces were rugged. A comparison of the NO₂ gas-sensing properties of the CdO thin film, TiO₂ rods, and TiO₂–CdO composite rods revealed that the composite rods exhibited superior gas-sensing responses to NO₂ gas than did the CdO thin film and TiO₂ rods, which can be attributed to the microstructural differences and the formation of heterojunctions between the TiO₂ core and CdO crystallites.

Microstructure-Dependent Visible-Light Driven Photoactivity of Sputtering-Assisted Synthesis of Sulfide-Based Visible-Light Sensitizer onto ZnO Nanorods

The ZnO-CdS core-shell composite nanorods with CdS shell layer thicknesses of 5 and 20 nm were synthesized by combining the hydrothermal growth of ZnO nanorods with the sputtering thin-film deposition of CdS crystallites. The microstructures and optical properties of the ZnO-CdS nanorods were associated with the CdS shell layer thickness. A thicker CdS shell layer resulted in a rougher surface morphology, more crystal defects, and a broader optical absorbance edge in the ZnO-CdS rods. The ZnO-CdS (20 nm) nanorods thus engaged in more photoactivity in this study. When they were further subjected to a postannealing procedure in ambient Ar/H₂, this resulted in the layer-like CdS shell layers being converted into the serrated CdS shell layers. By contrast, the ZnO-CdS nanorods conducted with the postannealing procedure exhibited superior photoactivity and photoelectrochemical performance; the substantial changes in the microstructures and optical properties of the composite nanorods following postannealing in this study might account for the observed results.