Crystalline Mesoporous F-Doped Tin Dioxide Nanomaterial Successfully Prepared via a One Pot Synthesis at Room Temperature and Ambient Pressure

We report the successful one pot synthesis of crystalline mesoporous tin dioxide powder doped with fluoride at ambient pressure and temperature. This material possesses a high surface area, narrow pore size distribution, small average crystallite sizes, and good opto-electrical properties. The existence of fluorine increased the opto-electronic activity of tin dioxide by 20 times, and conductivity by 100 times compared with pristine tin dioxide prepared via the same method. The conductivity of SnO2 in air at 25 °C is 5 × 10-5 S/m, whereas that of F-SnO2 is 4.8 × 10-3 S/m. The structures of these materials were characterized with powder X-ray diffraction, N2 sorption analysis, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and UV-visible spectroscopy. Fluorine occupies the framework of tin dioxide by replacing some of the oxygen atoms. The structure, conductance, and optical properties of these materials are discussed in this paper.

Two-Step Visible Light Photocatalytic Dye Degradation Phenomena in Ag2O-Impregnated ZnO Nanorods via Growth of Metallic Ag and Formation of ZnO/Ag0/Ag2O Heterojunction Structures

Visible light photocatalytic activity follows the single-slope pseudo-first-order reaction kinetics in pristine ZnO nanorods, while for pure Ag₂O, a two-slope paradigm is pursued with a higher slope at a later period. For the Ag₂O-impregnated ZnO heterostructured nanorod photocatalyst, the two-step photocatalysis phenomena proceed with dye degradation rate constants emerging higher than those of individual ZnO and Ag₂O, at both time zones. Improved performance of ZnO/Ag₂O heterostructures arises initially from the reduced e^-/h⁺ recombination rate by the synergistic effect between ZnO and Ag₂O. At a later phase, metallic Ag is produced, which traps the valence electrons of Ag₂O nanoparticles and advances the e^-/h⁺ separation across the ZnO/Ag⁰/Ag₂O heterojunction structures, rendering them promptly accessible for dye degradation. At an increased Ag₂O loading, the photodegradation rate constants boost up in both time zones, and the corresponding crossover time (t_C) between the two phases steadily diminishes, leading toward a unique photocatalytic phenomenon that prevails with a superior rate constant. The optimized ZAO₂₅ heterostructure photocatalyst demonstrates ∼96.24% photodegradation of methylene blue (MB) dye within 30 min of visible light exposure, and its degradation rate constant is ∼0.24848 min^-1, which is ∼26.75 times superior than that of pristine ZnO samples. The metal-induced biphasic photocatalysis phenomena have never been reported earlier.

Design and Synthesis of Novel 2D Porous Zinc Oxide-Nickel Oxide Composite Nanosheets for Detecting Ethanol Vapor

The porous zinc oxide-nickel oxide (ZnO-NiO) composite nanosheets were synthesized via sputtering deposition of NiO thin film on the porous ZnO nanosheet templates. Various NiO film coverage sizes on porous ZnO nanosheet templates were achieved by changing NiO sputtering duration in this study. The microstructures of the porous ZnO-NiO composite nanosheets were investigated herein. The rugged surface feature of the porous ZnO-NiO composite nanosheets were formed and thicker NiO coverage layer narrowed the pore size on the ZnO nanosheet template. The gas sensors based on the porous ZnO-NiO composite nanosheets displayed higher sensing responses to ethanol vapor in comparison with the pristine ZnO template at the given target gas concentrations. Furthermore, the porous ZnO-NiO composite nanosheets with the suitable NiO coverage content demonstrated superior gas-sensing performance towards 50-750 ppm ethanol vapor. The observed ethanol vapor-sensing performance might be attributed to suitable ZnO/NiO heterojunction numbers and unique porous nanosheet structure with a high specific surface area, providing abundant active sites on the surface and numerous gas diffusion channels for the ethanol vapor molecules. This study demonstrated that coating of NiO on the porous ZnO nanosheet template with a suitable coverage size via sputtering deposition is a promising route to fabricate porous ZnO-NiO composite nanosheets with a high ethanol vapor sensing ability.

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.

Design of Hydrothermally Derived Fe2O3 Rods with Enhanced Dual Functionality Via Sputtering Decoration of a Thin ZnO Coverage Layer

The Fe2O3-ZnO composite rods were successfully synthesized by combining hydrothermal growth of Fe2O3 rods and sputtering deposition of a thin ZnO coverage layer. Two types of the Fe2O3 rods with round and rectangular cross-sectional morphologies grown via control of the urea content in hydrothermal growth processes were used as rod templates to fabricate the Fe2O3-ZnO composite rods. The Fe2O3-ZnO composite rods exhibited an improved photoelectric conversion efficiency in the Fe2O3 rods via a construction of a heterogeneous structure. The photocatalytic degradation performance of rhodamine B dyes with Fe2O3 rods was substantially increased via sputtering decoration of a thin ZnO coverage layer on the Fe2O3 rods. Moreover, the Fe2O3-ZnO composite rods exhibited superior acetone vapor-sensing responses than the pristine Fe2O3 rods herein. The extended optical absorption ability together with the enhanced photoinduced charge separation efficiency via construction of the Fe2O3-ZnO heterogeneous system explained the improved photoactivity of the composite rods. Furthermore, the formation of a heterojunction between the Fe2O3 and ZnO increased the interfacial potential barrier height and enhanced the sensor resistance variation size upon exposure to the acetone vapor. This accounted for the improved gas-sensing performance of the Fe2O3-ZnO composite rods. The experimental results herein provide a promising approach to design Fe2O3-based composite rods with desirable photocatalytic and gas-sensing functionalities.

Morphology-dependent photocatalytic and gas-sensing functions of three-dimensional TiO2–ZnO nanoarchitectures

Nanocomposites consisting of three-dimensional ZnO nanorods-decorated TiO₂ nanorod templates (TiO₂–ZnO) have been prepared by combining sputtering and hydrothermal growth strategies.

Sputtering control of Ag2O decoration configurations on ZnO nanorods and their surface arrangement effects on photodegradation ability toward methyl orange

In this study, a combinational strategy for synthesizing ZnO nanorod arrays interlaced with Ag₂O particles was proposed. Hydrothermally derived ZnO nanorod templates were sputter coated with Ag₂O particles. The sputtered Ag₂O particles can be decorated on the surfaces of the ZnO nanorod arrays with a randomly dispersive or continuous coverage characteristic by controlling the sputtering duration. Structural analysis revealed the formation of satisfactory crystalline ZnO-Ag₂O composite nanorods through the hydrothermal and sputtering methods. The ZnO-Ag₂O composite nanorods exhibited a significantly enhanced photoactivity compared with that of pristine ZnO nanorods under light irradiation. Moreover, the Ag₂O content and the coverage feature of the ZnO-Ag₂O composite nanorods influence the photodegradation of methyl orange solution by the composite nanorods under light irradiation. The photodegradation efficiency of the ZnO nanorods was substantially enhanced when the Ag₂O particles were decorated on the surfaces in a dispersive manner. This can be attributed to the optimal content of Ag₂O particles and their randomly dispersive characteristic on the surface of the composite nanorods, which resulted in the efficient transfer of photocarriers and markedly suppressed the electron-hole recombination rate.

Heterostructural CuO-ZnO Nanocomposites: A Highly Selective Chemical and Electrochemical NO2 Sensor

A simple one-step chemical method is employed for the successful synthesis of CuO(50%)-ZnO(50%) nanocomposites (NCs) and investigation of their gas sensing properties. The X-ray diffraction studies revealed that these CuO-ZnO NCs display a hexagonal wurtzite-type crystal structure. The average width of 50-100 nm and length of 200-600 nm of the NCs were confirmed by transmission electron microscopic images, and the 1:1 proportion of Cu and Zn composition was confirmed by energy-dispersive spectra, i.e., CuO(50%)-ZnO(50%) NC studies. The CuO(50%)-ZnO(50%) NCs exhibit superior gas sensing performance with outstanding selectivity toward NO2 gas at a working temperature of 200 °C. Moreover, these NCs were used for the indirect evaluation of NO2 via electrochemical detection of NO2 - (as NO2 converts into NO2 - once it reacts with moisture, resulting into acid rain, i.e., indirect evaluation of NO2). As compared with other known modified electrodes, CuO(50%)-ZnO(50%) NCs show an apparent oxidation of NO2 - with a larger peak current for a wider linear range of nitrite concentration from 20 to 100 mM. We thus demonstrate that the as-synthesized CuO(50%)-ZnO(50%) NCs act as a promising low-cost NO2 sensor and further confirm their potential toward tunable gas sensors (electrochemical and solid state) (Scheme 1).

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.

Porous 3D flower-like CoAl-LDH nanocomposite with excellent performance for NO2 detection at room temperature

The 3D flower-like CoAl-layered double hydroxide (CoAl-LDH) was successfully prepared using the functional template agent of fluoride ions via a facile one-step hydrothermal route. Various techniques proved that all the samples presented 3D flower-like microstructural morphology. Representatively, the CA-2 sample, which was synthesized with the molar ratio of Co : Al of 3.65 : 1, had considerably abundant pores in its thin nanosheets. The average pore size was 2-4 nm, the specific surface area was equal to 49.45 m2 g-1, and the thickness of nanosheets was approximately 3.068 nm. The CA-2 sample showed an excellent response to 0.01-100 ppm NO2 with ultrafast response/recovery time at room temperature (RT). The detection limit of the sensor even reached 10 ppb. The superior gas sensing performance could be attributed to the synergistic effects of the functional template agent of fluoride ions and specific porous 3D flower-like nanostructure. The current study showed that the 3D flower-like CoAl-LDHs might a promising material in practical detection of NO2 at RT.

Enhancement of Acetone Gas-Sensing Responses of Tapered WO3 Nanorods through Sputtering Coating with a Thin SnO2 Coverage Layer

WO3-SnO2 composite nanorods were synthesized by combining hydrothermal growth of tapered tungsten trioxide (WO3) nanorods and sputter deposition of thin SnO2 layers. Crystalline SnO2 coverage layers with thicknesses in the range of 13-34 nm were sputter coated onto WO3 nanorods by controlling the sputtering duration of the SnO2. The X-ray diffraction (XRD) analysis results demonstrated that crystalline hexagonal WO3-tetragonal SnO2 composite nanorods were formed. The microstructural analysis revealed that the SnO2 coverage layers were in a polycrystalline feature. The elemental distribution analysis revealed that the SnO2 thin layers homogeneously covered the surfaces of the hexagonally structured WO3 nanorods. The WO3-SnO2 composite nanorods with the thinnest SnO2 coverage layer showed superior gas-sensing response to 100-1000 ppm acetone vapor compared to other composite nanorods investigated in this study. The substantially improved gas-sensing responses to acetone vapor of the hexagonally structured WO3 nanorods coated with the SnO2 coverage layers are discussed in relation to the thickness of SnO2 coverage layers and the core-shell configuration of the WO3-SnO2 composite nanorods.

Improvement of Ethanol Gas-Sensing Responses of ZnO⁻WO3 Composite Nanorods through Annealing Induced Local Phase Transformation

In this study, ZnO-WO3 composite nanorods were synthesized through a combination of hydrothermal growth and sputtering method. The structural analysis results revealed that the as-synthesized composite nanorods had a homogeneous coverage of WO3 crystallite layer. Moreover, the ZnO-WO3 composite nanorods were in a good crystallinity. Further post-annealed the composite nanorods in a hydrogen-containing atmosphere at 400 °C induced the local phase transformation between the ZnO and WO3. The ZnO-WO3 composite nanorods after annealing engendered the coexistence of ZnWO4 and WO3 phase in the shell layer which increased the potential barrier number at the interfacial contact region with ZnO. This further enhanced the ethanol gas-sensing response of the pristine ZnO-WO3 composite nanorods. The experimental results herein demonstrated a proper thermal annealing procedure of the binary composite nanorods is a promising approach to modulate the gas-sensing behavior the binary oxide composite nanorods.

Crystal phase content-dependent functionality of dual phase SnO2-WO3 nanocomposite films via cosputtering crystal growth

In this study, crystalline SnO2-WO3 nanocomposite thin films were grown through radio-frequency cosputtering of metallic Sn and ceramic WO3 targets. The W content in the SnO2 matrix was varied from 5.4 at% to 12.3 at% by changing the WO3 sputtering power during thin-film growth. Structural analyses showed that increased WO3 phase content in the nanocomposite films reduced the degree of crystallization of the SnO2 matrix. Moreover, the size of the composite films' surface crystallites increased with WO3 phase content, and the large surface crystallites were composed of numerous nanograins. Addition of WO3 crystals to the SnO2 matrix to form a composite film improved its light harvesting ability. The SnO2-WO3 nanocomposite films exhibited improved photodegradation ability for Rhodamine B dyes compared with their individual constituents (i.e., SnO2 and WO3 thin films), which is attributable to the suitable type II band alignment between the SnO2 and WO3. Moreover, an optimal WO3 phase content (W content: 5.4 at%) in the SnO2 matrix substantially enhanced the ethanol gas-sensing response of the SnO2 thin film. This suggested that the heterojunctions at the SnO2/WO3 interface regions in the nanocomposite film considerably affected its ethanol gas-sensing behavior.

Microstructures and Photodegradation Performance toward Methylene Orange of Sputtering-Assisted Decoration of ZnFe₂O₄ Crystallites onto TiO₂ Nanorods

In this study, TiO₂⁻ZnFe₂O₄ (ZFO) core-shell nanorods with various ZFO crystallite thicknesses were synthesized through sputtering-deposited ZFO thin films onto the surfaces of TiO₂ nanorods. By coupling the ZFO narrow bandgap oxide with TiO₂, an enhanced photodegradation efficiency of methylene orange under irradiation was achieved. Structural analyses revealed that ZFO crystallites fully covered the surfaces of the TiO₂ nanorods. The sputtering-deposited ZFO crystallites on the head region of the composite nanorods were markedly thicker than those covering the lateral region of the composite nanorods. The coverage of ZFO crystallites on the TiO₂ nanorods led to an improved light harvesting, a decrease in the hole⁻electron recombination rate, as well as the enhanced photodegradation activity of the TiO₂⁻ZFO heterostructures under irradiation. The optimized ZFO thickness on the head region of the composite nanorods was approximately 43 nm on average and that at the lateral region of the composite nanorods was 15 nm, which exhibited superior photodegradation ability to methylene orange and retained a stable photodegradation efficiency of approximately 97% after cycling tests. The results herein demonstrate that sputtering deposition of ZFO crystallite with tunable thickness is a promising approach to designing TiO₂⁻ZFO composite nanorods with various ZFO coverage sizes and to adjust their photodegradation ability toward organic dyes.

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.