Enhanced photoelectrochemical water-splitting effect with a bent ZnO nanorod photo anode decorated with Ag nanoparticles

Photoelectrochemical Green Hydrogen Production Utilizing ZnO Nanostructured Photoelectrodes

One of the emerging and environmentally friendly technologies is the photoelectrochemical generation of green hydrogen; however, the cheap cost of production and the need for customizing photoelectrode properties are thought to be the main obstacles to the widespread adoption of this technology. The primary players in hydrogen production by photoelectrochemical (PEC) water splitting, which is becoming more common on a worldwide basis, are solar renewable energy and widely available metal oxide based PEC electrodes. This study attempts to prepare nanoparticulate and nanorod-arrayed films to better understand how nanomorphology can impact structural, optical, and PEC hydrogen production efficiency, as well as electrode stability. Chemical bath deposition (CBD) and spray pyrolysis are used to create ZnO nanostructured photoelectrodes. Various characterization methods are used to investigate morphologies, structures, elemental analysis, and optical characteristics. The crystallite size of the wurtzite hexagonal nanorod arrayed film was 100.8 nm for the (002) orientation, while the crystallite size of nanoparticulate ZnO was 42.1 nm for the favored (101) orientation. The lowest dislocation values for (101) nanoparticulate orientation and (002) nanorod orientation are 5.6 × 10^-4 and 1.0 × 10^-4 dislocation/nm², respectively. By changing the surface morphology from nanoparticulate to hexagonal nanorod arrangement, the band gap is decreased to 2.99 eV. Under white and monochromatic light irradiation, the PEC generation of H₂ is investigated using the proposed photoelectrodes. The solar-to-hydrogen conversion rate of ZnO nanorod-arrayed electrodes was 3.72% and 3.12%, respectively, under 390 and 405 nm monochromatic light, which is higher than previously reported values for other ZnO nanostructures. The output H₂ generation rates for white light and 390 nm monochromatic illuminations were 28.43 and 26.11 mmol.h^-1cm^-2, respectively. The nanorod-arrayed photoelectrode retains 96.6% of its original photocurrent after 10 reusability cycles, compared to 87.4% for the nanoparticulate ZnO photoelectrode. The computation of conversion efficiencies, H₂ output rates, Tafel slope, and corrosion current, as well as the application of low-cost design methods for the photoelectrodes, show how the nanorod-arrayed morphology offers low-cost, high-quality PEC performance and durability.

Biosynthesis and chemical composition of nanomaterials in agricultural soil bioremediation: a review

Nanomaterials (NMs) are currently being used in agricultural soils as part of a new bioremediation (BR) process. In this study, we reviewed the biosynthesis of NMs, as well as their chemical composition and prospective strategies for helpful and sustainable agricultural soil bioremediation (BR). Different types of NMs, such as nanoparticles, nanocomposites, nanocrystals, nano-powders, and nanotubes, are used in agricultural soil reclamation, and they reflect the toxicity of NMs to microorganisms. Plants (Sargassum muticum, Dodonaea viscose, Aloe Vera, Rosemarinus officinalis, Azadirachta indica, Green tea, and so on) and microorganisms (Escherichia coli, Shewanella oneidensis, Pleurotus sp., Klebsiella oxytoca, Aspergillus clavatus, and so on) are the primary sources for the biosynthesis of NMs. By using the BR process, microorganisms, such as bacteria and plants, can immobilize metals and change both inorganic and organic contaminants in the soil. Combining NMs with bioremediation techniques for agricultural soil remediation will be a valuable long-term solution.

Recent advances on inactivation of waterborne pathogenic microorganisms by (photo) electrochemical oxidation processes: Design and application strategies

Compared with other conventional water disinfection processes, (photo) electrochemical oxidation (P/ECO) processes have the characteristics of environmental friendliness, convenient installation and operation, easy control and high efficiency of inactivating waterborne pathogenic microorganisms (PMs), so that more and more research work has been focused on this topic, but there is still a huge gap between the research and practical application. Here, the research network of inactivating PMs by P/ECO processes has been comprehensively summarized, and the electrode/reactor/process design strategies based on strengthening direct and indirect oxidation, enhancing mass transfer efficiency and electron transfer efficiency, and improving the effective dose of electrogenerated oxidants are discussed. Furthermore, the factors affecting the inactivation of PMs and the issues regarding to stability and lifetime of the electrode are discussed respectively. Finally, the important research priorities and possible research challenges of P/ECO processes are put forward to make significant progress of this technology.

Effect of Au Plasmonic Material on Poly M-Toluidine for Photoelectrochemical Hydrogen Generation from Sewage Water

This study provides H2 gas as a renewable energy source from sewage water splitting reaction using a PMT/Au photocathode. So, this study has a dual benefit for hydrogen generation; at the same time, it removes the contaminations of sewage water. The preparation of the PMT is carried out through the polymerization process from an acid medium. Then, the Au sputter was carried out using the sputter device under different times (1 and 2 min) for PMT/Au-1 min and PMT/Au-2min, respectively. The complete analyses confirm the chemical structure, such as XRD, FTIR, HNMR, SEM, and Vis-UV optical analyses. The prepared electrode PMT/Au is used for the hydrogen generation reaction using Na2S2O3 or sewage water as an electrolyte. The PMT crystalline size is 15 nm. The incident photon to current efficiency (IPCE) efficiency increases from 2.3 to 3.6% (at 390 nm), and the number of H2 moles increases from 8.4 to 33.1 mmol h−1 cm−2 for using Na2S2O3 and sewage water as electrolyte, respectively. Moreover, all the thermodynamic parameters, such as activation energy (Ea), enthalpy (ΔH*), and entropy (ΔS*), were calculated; additionally, a simple mechanism is mentioned for the water-splitting reaction.

Enhanced Performances of n-ZnO Nanowires/p-Si Heterojunctioned Pyroelectric Near-Infrared Photodetectors via the Plasmonic Effect

Although pyroelectric photodetectors have been intensively studied, the transient temperature change rate of pyroelectric materials is a main restrictive factor for improving the performance. In this work, we fabricate an ultrafast response self-powered near-infrared (NIR) photodetector (PD) based on Au nanoparticles (NPs) coated an n-ZnO nanowires (NWs)/p-Si heterojunction. The local surface plasmon resonance (LSPR) effect generated at the local contacts of Au NPs/ZnO NWs can significantly enhance the transient temperature change rate of the ZnO material to improve the photoresponse performances of the NIR PD. Compared with that in the pristine ZnO-based PD, the response time of the Au-coated NIR PD is decreased from 113 to 50 μs at the rising edge and 200 to 70 μs at the falling edge. Optical responsivity and detectivity of the Au-coated ZnO-based PD are increased by 212 and 266%, respectively. The pyroelectric current gain is produced by injecting hot electrons from the LSPR effect of Au NPs into the ZnO material and the thermal energy transfer caused by the photothermal effect of plasmonic Au nanostructure. This work provides an in-depth understanding of plasmonic effect-enhanced pyroelectric effect and presents a unique strategy for developing high-performance NIR photodetectors.

Bioinspired Synthesis of ZnO@polydopamine/Au for Label-Free Photoelectrochemical Immunoassay of Amyloid-β Protein

Amyloid-β protein (Aβ) is an important biomarker and plays a key role in the early stage of Alzheimer's disease (AD). Here, an ultrasensitive photoelectrochemical (PEC) sensor based on ZnO@polydopamine/Au nanocomposites was constructed for quantitative detection of Aβ. In this sensing system, the ZnO nanorod array decorated with PDA films and gold nanoparticles (Au NPs) have excellent visible-light activity. The PDA film was used as a sensitizer for charge separation, and it also was used for antibody binding. Moreover, Au NPs were loaded on the surface of PDA film by in situ deposition, which further improved the charge transfer efficiency and the PEC activity in visible light due to the localized surface plasmon resonance effect of Au NPs. Therefore, in ZnO@polydopamine/Au nanocomposites, a significantly enhanced photocurrent response was obtained on this photoelectrode, which provides a good and reliable signal for early detection of AD. Under the optimized conditions, the PEC immunosensor displayed a wide linear range from 1 pg/mL to 100 ng/mL and a low detection limit of 0.26 pg/mL. In addition, this PEC immunosensor also presented good selectivity, stability, and reproducibility. This work may provide a promising point-of-care testing method toward advanced PEC immunoassays for AD biomarkers.

ITO/Poly-3-Methylaniline/Au Electrode for Electrochemical Water Splitting and Dye Removal

Application of aniline derivative semiconductor nanopolymer and its Au composite for H2 generation and dye removal were investigated. Electrochemical polymerization of poly-3-methylaniline (P3MA) on ITO glass was carried out for acid medium. Au nanoparticles with crystal sizes of 15 and 30 nm were sputter coated on the surface. Chemical structure of the polymer and its composite was characterized using FTIR, XRD, 1HNMR, SEM, and UV-Vis. All function groups were confirmed using FTIR analyses. XRD confirmed the formation of nanopolymer with a crystal size of ∼15 nm. SEM confirmed the formation of smooth lamellar surface feature with a <20 nm nanoporous structure. Porosity and particle sizes increases with Au coating, confirmed using the modeling Image J program. Optical analysis also demonstrated that the strength of P3MA absorption peaks increases with rising Au coating time, in which the bandgap values changed from 1.64 to 1.63 eV for 15 and 30 nm Au, respectively. The photoelectrode ITO/PMT/30 nm Au was applied for H2 generation and dye removal. The current density (J ph) values were −0.3 and −1.6 mA.cm−2 in the absence and presence of the Congo red dye, respectively. The incident photon-to-current conversion efficiency (IPCE%) for the electrode was 2.3 at 390 nm. The activation energy (E a ) was 31.49 KJ mol−1. The enthalpy (∆H * ) and entropy (∆S * ) values were 114.49 and 160.46 JK−1 mol−1, respectively. A simple mechanism for the H2 generation and dye removal is mentioned.

Electrospinning for building 3D structured photoactive biohybrid electrodes

We describe the development of biohybrid electrodes constructed via combination of electrospun (e-spun) 3D indium tin oxide (ITO) with the trimeric supercomplex photosystem I and the small electrochemically active protein cytochrome c (cyt c). The developed 3D surface of ITO has been created by electrospinning of a mixture of polyelthylene oxide (PEO) and ITO nanoparticles onto ITO glass slides followed by a subsequent elimination of PEO by sintering the composite. Whereas the photosystem I alone shows only small photocurrents at these 3D electrodes, the co-immobilization of cyt c to the e-spun 3D ITO results in well-defined photoelectrochemical signals. The scaling of thickness of the 3D ITO layers by controlling the time (10 min and 60 min) of electrospinning results in enhancement of the photocurrent. Several performance parameters of the electrode have been analyzed for different illumination intensities.

Effect of Morphology and Plasmonic on Au/ZnO Films for Efficient Photoelectrochemical Water Splitting

To improve photoelectrochemical (PEC) water splitting, various ZnO nanostructures (nanorods (NRs), nanodiscs (NDs), NRs/NDs, and ZnO NRs decorated with gold nanoparticles) have been manufactured. The pure ZnO nanostructures have been synthesized using the successive ionic-layer adsorption and reaction (SILAR) combined with the chemical bath deposition (CBD) process at various deposition times. The structural, chemical composition, nanomorphological, and optical characteristics have been examined by various techniques. The SEM analysis shows that by varying the deposition time of CBD from 2 to 12 h, the morphology of ZnO nanostructures changed from NRs to NDs. All samples exhibit hexagonal phase wurtzite ZnO with polycrystalline nature and preferred orientation alongside (002). The crystallite size along (002) decreased from approximately 79 to 77 nm as deposition time increased from 2 to 12 h. The bandgap of ZnO NRs was tuned from 3.19 to 2.07 eV after optimizing the DC sputtering time of gold to 4 min. Via regulated time-dependent ZnO growth and Au sputtering time, the PEC performance of the nanostructures was optimized. Among the studied ZnO nanostructures, the highest photocurrent density (Jph) was obtained for the 2 h ZnO NRs. As compared with ZnO NRs, the Jph (7.7 mA/cm2) of 4 min Au/ZnO NRs is around 50 times greater. The maximum values of both IPCE and ABPE are 14.2% and 2.05% at 490 nm, which is closed to surface plasmon absorption for Au NPs. There are several essential approaches to improve PEC efficiency by including Au NPs into ZnO NRs, including increasing visible light absorption and minority carrier absorption, boosting photochemical stability, and accelerating electron transport from ZnO NRs to electrolyte carriers.

Photocatalytic activity and photoelectrochemical properties of Ag/ZnO core/shell nanorods under low-intensity white light irradiation

Zinc oxide (ZnO) nanorod thin films were prepared by CBD onto glass and FTO/glass substrates. Silver (Ag) nanoparticles were synthesized on the surface of the prepared ZnO nanorod thin films using electrochemical methods. The scanning electron microscopy images of the Ag/ZnO/glass core/shell nanostructure confirmed that the average particles size is 20 nm while it was 41 nm for Ag NPs that synthesized onto ZnO/FTO NRs. The photocatalytic activity of the prepared Ag/ZnO core/shell nanostructure was studied by analyzing the degradation of methylene blue (MB) dye under visible light. Various pH values (6 and 10) and exposure time (30-240) min were controlled to investigate the photocatalytic activity of as-prepared Ag/ZnO core/shell nanostructure and that annealed at 200 °C and 300 °C for 1 h. It was observed that when the pH was 6, the degradation rate increased with the annealing temperature and irradiation time reaching 51% at the annealing temperature of 300 °C and exposure time of 240 min. In other hands, when the pH was 10, and the sample was annealed at 200 °C, it showed a good degradation rate of 100% at the irradiation time of 90 min. By contrast, the sample annealed at 300 °C required 180 min to degrade the MB dye completely. The photoelectrochemical cell measurement based on photocurrent density revealed a slight response to light. Cycle voltammetry (CV) measurement was conducted, and the CV curves of the Ag/ZnO core/shell electrodes indicated nonfaradaic and pseudocapacitance behavior. The electrodes showed nearly rectangular CV curves, which indicated the dominance of the nonfaradaic capacitance behavior. The specific capacitance of the electrodes remained at approximately 99%. Mott-Schottky analysis revealed that the semiconductor was an n-type with dependence on flat band potential V _FB deviation in the negative direction.

Photo-electrocatalytic degradation of cyclic volatile methyl siloxane by ZnO-coated aluminum anode: Optimal parameters, kinetics, and reaction pathways

Cyclic volatile methylsiloxanes (cVMSs) are widely used in industrial processes and consumer products, which have been reported to be potentially toxic to human health due to their persistence and bioaccumulation. In this study, a novel photo-catalytic zinc oxide (ZnO)-coated aluminum (ZnO@Al) anode was prepared by a facile hydrothermal epitaxial process for the purpose of degrading cVMSs in practical wastewater. Morphological data and compositional analysis showed a compact coating layer that had the characteristic peaks of ZnO. To optimize the degradation process, central composite design combined with response surface methodology was applied to acquire the optimum parameters of cVMSs removal, and results indicated the cVMSs removal efficiency was approximately 63.3% at the conditions of current density = 17.3 mA/cm², initial pH of electrolyte = 7.8, plate distance = 18 mm, UV intensity = 90 W, and reaction time = 80 min. Furthermore, the photo-electrocatalytic degradation of cVMSs obeys the pseudo-first order kinetic reaction, and the anode exhibited high durability as the attenuation of cVMSs removal efficiency was <6% after four times reuse. It was also observed that with the application period of the anode was extended, the electroflocculation reaction gradually occurred. The FT-IR of the generated flocs and the total ion gas chromatograms mass spectrometer analysis unraveled the methyl groups in Si-CH₃ could be easily attacked by hydroxyl radicals to form the intermediates of monohydroxy substituted products (m/z = 298, 372, and 446) and eventually short-chain carboxylic acids, alkyl radical and silicate. The effective removal of cVMSs by photo-electrocatalytic process using ZnO@Al anode provide significant implication in treatment of practical wastewater.

Enhancing the quasi-theoretical photocurrent density of ZnO nanorods via a lukewarm hydrothermal method

A ∼10-μm-long one-dimensional (1D) ZnO nanorod array (NRA) vertically oriented on a fluorine-doped tin oxide (FTO) coated glass substrate is successfully fabricated via a lukewarm hydrothermal method. The reflection of light from the rough surface of this ultralong ZnO NRA, resulting from the variation in the characteristic length of individual ZnO NRs in a tapered geometry, is largely suppressed. This in turn favors the ZnO NRA as a photoelectrode effectively harnessing UV-light for solar water splitting, as evidently manifested in the quasi-theoretical photocurrent density that reached ∼0.9 mA cm^-2 at 1V_Ag/AgCl. A further contribution to such an outstanding performance stems from additional photocurrent generation by the ZnO NRA upon visible light illumination. This is attributed to a variety of native defects and the surface hydroxyl groups present in the ZnO NRA, giving rise to the mid-gap states that mediate the associated electronic transitions. Moreover, those lattice imperfections further boost the carrier concentration of the ZnO NRA to facilitate the carrier transport which in turn enhances the photoelectrochemical activity.

Effects of Au loading on the enhancement of photoelectrochemical activities of the Au@ZnO nano-heteroarchitecture

The nano-heteroarchitecture of Au@ZnO evidencing the surface attachment without chemical reaction at the interface delivered enhanced PEC activities by facilitating the injection of hot electrons from the SP state into the conduction band of ZnO.

Recent Progress and Approaches on Carbon-Free Energy from Water Splitting

Sunlight is the most abundant renewable energy resource, providing the earth with enough power that is capable of taking care of all of humanity's desires-a hundred times over. However, as it is at times diffuse and intermittent, it raises issues concerning how best to reap this energy and store it for times when the Sun is not shining. With increasing population in the world and modern economic development, there will be an additional increase in energy demand. Devices that use daylight to separate water into individual chemical elements may well be the answer to this issue, as water splitting produces an ideal fuel. If such devices that generate fuel were to become widely adopted, they must be low in cost, both for supplying and operation. Therefore, it is essential to research for cheap technologies for water ripping. This review summarizes the progress made toward such development, the open challenges existing, and the approaches undertaken to generate carbon-free energy through water splitting.

Current progress in developing metal oxide nanoarrays-based photoanodes for photoelectrochemical water splitting

Solar energy driven photoelectrochemical (PEC) water splitting is a clean and powerful approach for renewable hydrogen production. The design and construction of metal oxide based nanoarray photoanodes is one of the promising strategies to make the continuous breakthroughs in solar to hydrogen conversion efficiency of PEC cells owing to their owned several advantages including enhanced reactive surface at the electrode/electrolyte interface, improved light absorption capability, increased charge separation efficiency and direct electron transport pathways. In this Review, we first introduce the structure, work principle and their relevant efficiency calculations of a PEC cell. We then give a summary of the state-of the-art research in the preparation strategies and growth mechanism for the metal oxide based nanoarrays, and some details about the performances of metal oxide based nanoarray photoanodes for PEC water splitting. Finally, we discuss key aspects which should be addressed in continued work on realizing high-efficiency metal oxide based nanoarray photoanodes for PEC solar water splitting systems.

Surface plasmon assisted laser ablation of stainless steel

Colloidal Au nanoparticles (NPs) were decorated on stainless steel for surface plasmon enhanced laser ablation. A comparative study of the laser ablation efficiency was carried out on stainless steel samples with and without the Au NPs decoration at a variable pulsed laser fluence and laser pulse number. Higher ablation efficiency was clearly demonstrated in the former as illustrated from the larger diameter, maximum depth and the cross-sectional area of the crater generated by the laser ablation under the same conditions. Additionally, both the maximum depth and efficiency enhancement were found to depend on the laser fluence and pulse number. The maximum enhanced ablation efficiency of 36% based on the cross-sectional area of the crater was obtained at 1 pulse number of laser fluence 1.53 J cm^-2. The efficiency enhancement of laser ablation is attributed to the highly enhanced surface plasmon field at the interface between Au NPs and stainless steel.

Robust Photodetectable Paper from Chemically Exfoliated MoS2-MoO3 Multilayers

Photodetectors, which are capable of detecting light with varied wavelength, have nowadays been widely applied onto emerging fields such as security, entertainment, healthcare, environment, and so on. As the one with a two-dimensional layered structure, molybdenum disulfide (MoS₂) possesses striking optical and electrical properties that can be used in photodetecting, yet the challenges remain in terms of material processing, device fabrication simplicity, and enhancement of overall photodetection performance. In this work, a photodetectable paper based on a mixture of double-phased MoS₂ (1T and 2H) and MoO₃ was successfully fabricated through a straightforward route, that is, chemical exfoliation and deposition of MoS₂ powder on a flexible cellulose ester membrane, followed by inkjet-printed PEDOT:PSS as electrodes. The obtained device shows varied sensitivity to the light with different wavelengths. Compared with that under green and red lights, the prepared photodetector has the highest internal quantum efficiency (0.063%) and responsivity (0.134 mA W^-1), while having longest response/recovery time (17.5/15.3 s) when illuminated with purple light (405 nm). The achieved responsivity is much higher than other reported liquid exfoliation- and solution-derived MoS₂ photodetectors. This is ascribed to (1) the enhanced photoelectron generation caused by both MoS₂ and MoO₃ and (2) the good electric conductivity and efficient charge transport caused by the metallic 1T MoS₂. This work demonstrates the feasibility of fabricating the MoS₂-based photodetector with excellent performance through a simple exfoliation/filtration and inject printing route, and the detailed study on the response to light with different wavelengths unveils the interaction between the device and the incident light, further broadening the potential applications of such design.

Microwave-Assisted Rapid Preparation of Nano-ZnO/Ag Composite Functionalized Polyester Nonwoven Membrane for Improving Its UV Shielding and Antibacterial Properties

The cost and efficiency of preparing ZnO/Ag composite functional polyester membrane affect their application, for which a rapid microwave-assisted method was studied for coating ZnO/Ag composite nanoparticles on polyester nonwoven. The surface morphology, crystalline structure, and surface chemistry of the uncoated and coated polyester nonwoven was investigated by X-ray diffractometer (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric (TG), respectively. Washing stability, ultraviolet properties, and antibacterial properties of before and after treatment polyester nonwoven were also investigated. The results indicated that Ag/ZnO composite nanoparticles were successfully deposited on polyester nonwoven surface. The amount of silver nitrate added in reaction has an important effect on the morphology and structure of Ag/ZnO composite on the surface of polyester fiber. The washing experiment results show that the ZnO/Ag composite functional polyester nonwoven fabric prepared by this method exhibits good washing durability after 90 min of washing. The results of UV transmission analysis showed that polyester nonwoven has an obvious increase in ultraviolet resistant properties after Ag/ZnO composite coating. When 0.2 g of silver nitrate was added into 100 mL of the reaction solution, the mean ultraviolet protection factor (UPF) of the treated polyester nonwoven reached a maximum of 219.8. The antibacterial results showed that the coated nonwoven against Escherichia coli and Staphylococcus aureus was about 94.5% and 96.6%, respectively, showing very good antibacterial properties.

A Synergistic Effect of Surfactant and ZrO2 Underlayer on Photocurrent Enhancement and Cathodic Shift of Nanoporous Fe2O3 Photoanode

Augmenting the donor density and nanostructure engineering are the crucial points to improve solar water oxidation performance of hematite (α-Fe2O3). This work addresses the sluggish water oxidation reaction associated with hematite photoanode by tweaking its internal porosity. The porous hematite photoanodes are fabricated by a novel synthetic strategy via pulse reverse electrodeposition (PRED) method that involves incorporation of a cationic CTAB surfactant in a sulfate electrolyte and spin-coated ZrO2 underlayer (UL) on FTO. CTAB is found to be beneficial in promoting the film growth rate during PRED. Incorporation of Zr(4+) ions from ZrO2 UL and Sn(4+) ions from FTO into the Fe2O3 lattice via solid-state diffusion reaction during pertinent annihilation of surfactant molecules at 800 °C produced internally porous hematite films with improved carrier concentration. The porous hematite demonstrated a sustained photocurrent enhancement and a significant cathodic shift of 130 mV relative to the planar hematite under standard illumination conditions (AM 1.5G) in 1 M NaOH electrolyte. The absorption, electrochemical impedance spectroscopy and Mott-Schottky analyses revealed that the ZrO2 UL and CTAB not only increased the carrier density and light harvesting but also accelerated the surface oxidation reaction kinetics, synergistically boosting the performance of internally porous hematite photoanodes.

In situ formation of a ZnO/ZnSe nanonail array as a photoelectrode for enhanced photoelectrochemical water oxidation performance

In this study, a ZnO/ZnSe nanonail array was prepared via a two-step sequential hydrothermal synthetic route. In this synthetic process, the ZnO nanorod array was first grown on a fluorine-doped tin oxide (FTO) substrate using a seed-mediated growth approach via the hydrothermal process. Then, the ZnO nanonail array was obtained via in situ growth of ZnSe nano caps onto the ZnO nanorod array via a hydrothermal process in the presence of a Se source. The surface morphology and amount of ZnSe grown on the surface of the ZnO nanorods can be regulated by varying the reaction time and reactant concentration. Compared with pure ZnO nanorods, this unique nanonail array heterostructure exhibits enhanced visible light absorption. The transient photocurrent condition, in combination with steady-state and time-resolved photoluminescence spectroscopy, reveals that the ZnO/ZnSe nanonail array electrode has the highest charge separation rate, highest electron injection efficiency, and highest chemical stability. The photocurrent density of the ZnO/ZnSe nanonail array heterostructure reaches 1.01 mA cm(-2) at an applied potential of 0.1 V (vs. Ag/AgCl), which is much higher than that of the ZnO/ZnSe nanorod array (0.71 mA cm(-2)), the pristine ZnO nanorod array (0.39 mA cm(-2)), and the ZnSe electrode (0.21 mA cm(-2)), indicating its significant visible light driven activities for photoelectrochemical water oxidation. This unique morphology of nail-capped nanorods might be important for providing better insight into the correlation between heterostructure and photoelectrochemical activity.

ZnO nanoflowers-based photoanodes: aqueous chemical synthesis, microstructure and optical properties

We have developed an efficient, low temperature, synthetic route for ZnO nanoflowers (NFs) as photoanode material. This alternative route yields small flowerlike nanostructures, built from densely self-assembled tip-ended rod structures. The obtained ZnO NFs possess a large bandgap of 3.27 - 3.39 eV, enabling the generation of an average open current voltage of 0.56 V. Additionally, they show a high internal light harvesting of 14.6•10-7A-mol-1. The growth mechanism and self-assembly of ZnO NFs were studied in detail by joint spectroscopic-TEM investigations. It is shown that the ZnO crystallite size increases with increasing annealing temperatures and that the stress and the improved crystallinity are induced by annealing and reduce the lattice strain and the dislocation density. The bandgaps of ZnO are affected by the lattice strain revealing an optimal region of lattice strain to gain high bandgap energies. The properties of the synthesized ZnO NFs are compared with other morphologies, i.e. ZnO spherical aggregates (SPs) and ZnO nanorods (NRs), and are tested as electrode materials in dye-sensitized solar cells.

Amorphous NiO electrocatalyst overcoated ZnO nanorod photoanodes for enhanced photoelectrochemical performance

This research demonstrated that introducing amorphous NiO electrocatalysts onto the surface of ZnO photoanodes can effectively facilitate their PEC performance.

Selective photochemical synthesis of Ag nanoparticles on position-controlled ZnO nanorods for the enhancement of yellow-green light emission

A novel technique for the selective photochemical synthesis of silver (Ag) nanoparticles (NPs) on ZnO nanorod arrays is established by combining ultraviolet-assisted nanoimprint lithography (UV-NIL) for the definition of growth sites, hydrothermal reaction for the position-controlled growth of ZnO nanorods, and photochemical reduction for the decoration of Ag NPs on the ZnO nanorods. During photochemical reduction, the size distribution and loading of Ag NPs on ZnO nanorods can be tuned by varying the UV-irradiation time. The photochemical reduction is hypothesized to facilitate the adsorbed citrate ions on the surface of ZnO, allowing Ag ions to preferentially form Ag NPs on ZnO nanorods. The ratio of visible emission to ultraviolet (UV) emission for the Ag NP-decorated ZnO nanorod arrays, synthesized for 30 min, is 20.5 times that for the ZnO nanorod arrays without Ag NPs. The enhancement of the visible emission is believed to associate with the surface plasmon (SP) effect of Ag NPs. The Ag NP-decorated ZnO nanorod arrays show significant SP-induced enhancement of yellow-green light emission, which could be useful in optoelectronic applications. The technique developed here requires low processing temperatures (120 °C and lower) and no high-vacuum deposition tools, suitable for applications such as flexible electronics.

Cicada-Wing-Inspired Self-Cleaning Antireflection Coatings on Polymer Substrates

The cicada has transparent wings with remarkable self-cleaning properties and high transmittance over the whole visible spectral range, which is derived from periodic conical structures covering the wing surface. Here we report a scalable self-assembly technique for fabricating multifunctional optical coatings that mimic cicada-wing structures. Spin-coated two-dimensional non-close-packed colloidal crystals are utilized as etching masks to pattern subwavelength-structured cone arrays directly on polymer substrates. The resulting gratings exhibit broadband antireflection performance and superhydrophobic properties after surface modification. The dependence of the cone shape and size on the antireflective and self-cleaning properties has also been investigated in this study.

Au nanoparticle sensitized ZnO nanopencil arrays for photoelectrochemical water splitting

This paper describes the synthesis of Au nanoparticle sensitized ZnO nanopencil arrays on F-doped SnO2 substrates by an aqueous chemical growth and subsequent photoreduction method. The Au-ZnO nanopencil arrays yield a photocurrent of ∼ 1.5 mA cm(-2) at 1 V versus Ag/AgCl. The enhanced photocurrent is attributed to the surface plasmon resonance effect of Au nanoparticles and the prolonged lifetime of the photo-generated electron-hole pairs. The improved stability of ZnO is due to the plasmon resonance energy transfer process enabled by the Au nanoparticles, which enhances the electric field intensity in a small, well-defined location of the ZnO semiconductor.

Modulating the interaction between gold and TiO2nanowires for enhanced solar driven photoelectrocatalytic hydrogen generation

The interaction strength of Au nanoparticles with pristine and nitrogen doped TiO₂nanowire surfaces was analysed using density functional theory and their significance in enhancing the solar driven photoelectrocatalytic properties was elucidated.

Silver thin film as a temporal template and dopant source for vertically aligned p-n homojunction of ZnO nanorods

A silver thin film is introduced as a temporal template and dopant source for vertically aligned p-n homojunction of ZnO nanorods. The growth of Ag-doped ZnO nanorods was observed over time and the doping effect was determined through various characterization methods. Furthermore, a p-n homojunction diode of ZnO nanorods was fabricated.

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.

Growth of rutile TiO₂ on the convex surface of nanocylinders: from nanoneedles to nanorods and their electrochemical properties

In this work, bundles of rutile TiO₂ nanoneedles/nanorods are hydrothermally grown on carbon nanofibers (CNFs), forming free-standing mats consisting of three dimensional hierarchical nanostructures (TiO₂-on-CNFs). Morphologies and structures of the TiO₂-on-CNFs are studied using a field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-ray diffractometer (XRD) and thermogravimetric analyzer (TGA). Their electrochemical properties as electrodes in lithium ion batteries (LIBs) are investigated and correlated with the morphologies and structures. It is shown that the lateral size of the TiO₂ nanoneedles/nanorods ranges from a few nanometers to tens of nanometers, and increases with the hydrothermal temperature. Small interspaces are observed between individual nanoneedles/nanorods, which are due to the diverging arrangement of nanoneedles/nanorods induced by growing on the convex surface of nanocylinders. It is found that the growth process can be divided into two stages: initial growth on the CNF surface and further growth upon re-nucleation on the TiO₂ bundles formed in the initial growth stage. In order to achieve good electrochemical performance in LIBs, the size of the TiO₂ nanostructures needs to be small enough to ensure complete alloying and fast charge transport, while the further growth stage has to be avoided to realize direct attachment of TiO₂ nanostructures on the CNFs, facilitating electron transport. The sample obtained after hydrothermal treatment at 130 °C for 2 h (TiO₂-130-2) shows the above features and hence exhibits the best cyclability and rate capacity among all samples; the cyclability and rate capacity of TiO₂-130-2 are also superior to those of other rutile TiO₂-based LIB electrodes.

Simultaneous glucose sensing and biohydrogen evolution from direct photoelectrocatalytic glucose oxidation on robust Cu2O–TiO2 electrodes

An efficient solar-driven biofuel hydrogen production from direct photoelectrocatalytic oxidation of glucose on a robust Cu₂O–TiO₂ photoelectrode was demonstrated.

Highly flexible solution processable heterostructured zinc oxide nanowires mesh for environmental clean-up applications

ZnO nanowires loaded with Pt, Ag and CuO nanoparticles were synthesized on a flexible mesh for environmental clean-up applications. The heterostructured nanowires mesh is robust with stable photoresponse under bending. The mesh shows photocatalytic degradation and oil–water separation capabilities.

Solution processed transparent nanoparticulate ZnO thin film electrode for photoelectrochemical water oxidation

A solution processed synthetic route has been developed for making transparent ZnO thin film having nanoparticle size of around 21 nm. Without additional modification ZnO films shows current density of 1 mA cm⁻². The films also exhibit the production of oxygen gas from photo electrochemical water oxidation.

Vertically aligned ZnO nanowire arrays tip-grafted with silver nanoparticles for photoelectrochemical applications

Herein, we demonstrate that uniform Ag nanoparticles could be directionally grafted on the tip of ZnO nanowire arrays by a simple photo-reduction method. Furthermore, the structure, position, and amount of Ag nanoparticles supported on ZnO nanowire arrays could be further rationally tailored by changing the reaction parameters such as the category, concentration of reagents, and annealing temperature. Moreover, their photoelectrochemical performances under both UV-vis and monochromatic light irradiation have been explored. Interestingly, the photocurrent density of Ag-ZnO heterostructures could reach up to 2.40 mA cm(-2), which is much higher than that of pure ZnO nanowire arrays. It has been proposed that the formation of ZnO nanowire arrays tip-grafted with Ag nanoparticles could promote the effective separation and directional transfer of photoexcited electron-hole pairs, and thus enhance the photoconversion properties.