Transparent and flexible ZnO nanorods induced by thermal dissipation annealing without polymer substrate deformation for next-generation wearable devices

The fabrication of a transparent and flexible ultraviolet photodetector based on hydrothermally grown ZnO nanorods requires an annealing step to render the sol–gel spin-coated ZnO seed layer crystalline. As high-temperature annealing deforms low-melting-point polymer substrates, we herein devised a thermal dissipation annealing (TDA) method in which heat transfer to ZnO thin films is synchronized with heat release from the polymer substrate to crystallize the ZnO seed layer without polymer substrate deformation and melting. ZnO nanorods (NRs) were hydrothermally grown on non-annealed and annealed ZnO seed layers, and NR density and diameter were shown to be higher in the latter case, as the crystallized ZnO seed layer provided heterogeneous nucleation sites for NR growth. In addition, the larger density and diameter of ZnO NRs grown on the annealed ZnO seed layer were confirmed by analysis of O 1s signal intensities. A transparent and flexible UV photodetector based on ZnO NRs grown on the annealed ZnO seed layer exhibited a higher photocurrent/dark current ratio, photosensitivity, and photoresponsivity than that fabricated using a non-annealed seed layer. Taken together, the above results suggest that the TDA method is an effective way of fabricating transparent and flexible UV photodetectors with high photosensitivity, photoresponsivity, and photocurrent stability and it means that the next generation wearable devices can be easily realized by using the TDA method.

A transparent and flexible ultraviolet (UV) photodetector based on ZnO nanorods grown onto the thermal dissipation annealed ZnO seed layer exhibited high photosensitivity, photoresponsivity, and photocurrent stability without substrate deformation.     

d-Glucose sensor based on ZnO·V2O5 NRs by an enzyme-free electrochemical approach

A simple wet-chemical technique was used to prepare zinc oxide-doped vanadium pentaoxide nanorods (ZnO·V₂O₅ NRs) in an alkaline environment. The synthesized ZnO·V₂O₅ NRs were characterized using typical methods, including UV-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD). The d-glucose (d-GLC) sensor was fabricated with modification of a slight coating of nanorods (NRs) onto a flat glassy carbon electrode (GCE). The analytical performances, such as the sensitivity, limit of quantification (LOQ), limit of detection (LOD), linear dynamic range (LDR), and durability, of the proposed d-GLC sensor were acquired by a dependable current–voltage (I–V) process. A calibration curve of the GCE/ZnO·V₂O₅ NRs/Nf sensor was plotted at +1.0 V over a broad range of d-GLC concentrations (100.0 pM–100.0 mM) and found to be linear (R² = 0.6974). The sensitivity (1.27 × 10⁻³ μA μM⁻¹ cm⁻²), LOQ (417.5 mM), and LOD (125 250 μM) were calculated from the calibration curve. The LDR (1.0 μM–1000 μM) was derived from the calibration plot and was also found to be linear (R² = 0.9492). The preparation of ZnO·V₂O₅ NRs by a wet-chemical technique is a good advancement for the expansion of nanomaterial-based sensors to support enzyme-free sensing of biomolecules in healthcare fields. This fabricated GCE/ZnO·V₂O₅ NRs/Nf sensor was used for the recognition of d-glucose in real samples (apple juice, human serum, and urine) and returned satisfactory and rational outcomes.

A simple wet-chemical technique was used to prepare zinc oxide-doped vanadium pentaoxide nanorods (ZnO·V₂O₅ NRs) in an alkaline environment.     

A novel n-type CdS nanorods/p-type LaFeO3 heterojunction nanocomposite with enhanced visible-light photocatalytic performance

In this work, a novel n-type CdS nanorods/p-type LaFeO₃ (CdS NRs/LFO) nanocomposite was prepared, for the first time, via a facile solvothermal method. The as-prepared n-CdS NRs/p-LFO nanocomposite was characterized by using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDX), UV-visible diffuse reflection spectroscopy (DRS), vibrating sample magnetometry (VSM), photoluminescence (PL) spectroscopy, and Brunauer–Emmett–Teller (BET) surface area analysis. All data revealed the attachment of the LFO nanoparticle on the surface of CdS NRs. This novel nanocomposite was applied as a novel visible light photocatalyst for the degradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) dyes under visible-light irradiation. Under optimized conditions, the degradation efficiency was 97.5% for MB, 80% for RhB and 85% for MO in the presence of H₂O₂ and over CdS NRs/LFO nanocomposite. The photocatalytic activity of CdS NRs/LFO was almost 16 and 8 times as high as those of the pristine CdS NRs and pure LFO, respectively. The photocatalytic activity was enhanced mainly due to the high efficiency in separation of electron–hole pairs induced by the remarkable synergistic effects of CdS and LFO semiconductors. After the photocatalytic reaction, the nanocomposite can be easily separated from the reaction solution and reused several times without loss of its photocatalytic activity. Trapping experiments indicated that ·OH radicals were the main reactive species for dye degradation in the present photocatalytic system. On the basis of the experimental results and estimated energy band positions, the mechanism for the enhanced photocatalytic activity was proposed.

A novel n–p CdS nanorods/LaFeO₃ (CdS NRs/LFO) heterojunction nanocomposite was prepared via a solvothermal route and applied as a visible-light photocatalyst for enhanced degradation of organic dye pollutants.     

Low temperature-processed ZnO thin films for p–n junction-based visible-blind ultraviolet photodetectors

Ultraviolet (UV) photodetectors have drawn extensive attention due to their numerous applications in both civilian and military areas including flame detection, UV sterilization, aerospace UV monitoring, missile early warning, and ultraviolet imaging. Zinc oxide (ZnO)-based UV detectors exhibit remarkable performance; however, many of them are not visible-blind, and the fabrication techniques involve a high-temperature annealing step. Here, we fabricated a p–n junction photodiode based on annealing-free ZnO thin films prepared from ZnO nanoparticles and N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB). NPB was chosen due to its transparent nature in the visible region and high hole mobility. The ZnO nanoparticles and thin films were characterized by UV-visible absorption spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic light scattering (DLS) particle size analysis, Fourier-transform infrared (FTIR) spectroscopy, photoluminescence spectroscopy, XRD and profilometry. The device exhibited responsivity of 0.037 A/W and an external quantum efficiency (EQE) of 12.86% at 5 V bias under 360 nm illumination. In addition, with no biasing, the device exhibited an on–off ratio of more than 10³ and a linear dynamic range (LDR) of 63 dB. A high built-in potential at the ZnO/NPB interface could be the reason for this performance at zero bias. The rise and fall times were 156 ms and 319 ms, respectively. The results suggest that a visible-blind UV photodetector with acceptable performance can be fabricated using annealing-free ZnO films, which may lead to the realization of flexible detectors due to the low-temperature processes involved.

Visible-blind ultraviolet photodetectors have been fabricated with a p–n junction based on ZnO and an organic hole transport layer.     

n–n ZnO–Ag2CrO4 heterojunction photoelectrodes with enhanced visible-light photoelectrochemical properties

In this study, ZnO nanorods (NRs) were hydrothermally grown on an Au-coated glass substrate at a relatively low temperature (90 °C), followed by the deposition of Ag₂CrO₄ particles via a successive ionic layer adsorption and reaction (SILAR) route. The content of the Ag₂CrO₄ particles on ZnO NRs was controlled by changing the number of SILAR cycles. The fabricated ZnO–Ag₂CrO₄ heterojunction photoelectrodes were subjected to morphological, structural, compositional, and optical property analyses; their photoelectrochemical (PEC) properties were investigated under simulated solar light illumination. The photocurrent responses confirmed that the ability of the ZnO–Ag₂CrO₄ heterojunction photoelectrodes to separate the photo-generated electron–hole pairs is stronger than that of bare ZnO NRs. Impressively, the maximum photocurrent density of about 2.51 mA cm⁻² at 1.23 V (vs. Ag/AgCl) was measured for the prepared ZnO–Ag₂CrO₄ photoelectrode with 8 SILAR cycles (denoted as ZnO–Ag₂CrO₄-8), which exhibited about 3-fold photo-enhancement in the current density as compared to bare ZnO NRs (0.87 mA cm⁻²) under similar conditions. The improvement in photoactivity was attributed to the ideal band gap and high absorption coefficient of the Ag₂CrO₄ particles, which resulted in improved solar light absorption properties. Furthermore, an appropriate annealing treatment was proven to be an efficient process to increase the crystallinity of Ag₂CrO₄ particles deposited on ZnO NRs, which improved the charge transport characteristics of the ZnO–Ag₂CrO₄-8 photoelectrode annealed at 200 °C and increased the performance of the photoelectrode. The results achieved in the present work present new insights for designing n–n heterojunction photoelectrodes for efficient and cost-effective PEC applications and solar-to-fuel energy conversions.

ZnO NRs hydrothermally grown on Au coated glass substrate, followed by deposition of Ag₂CrO₄ particles via SILAR route. The content of the Ag₂CrO₄ particles on the ZnO NRs were controlled by changing the number of SILAR cycles.     

Visible photodetector based on transition metal-doped ZnO NRs/PEDOT:PSS hybrid materials

A hybrid Cu-doped ZnO nanorods (ZnO:Cu NRs)/poly(3,4 ethylene dioxythiophene)-polystyrene sulfonate (PEDOT:PSS)-based photodetector was fabricated using a simple hydrothermal method with pre-patterned silver electrodes. In the hybrid structure, PEDOT:PSS with high mobility acts as a carrier transport layer, while ZnO:Cu NRs with high visible absorption works as an “antenna” material to generate electron–hole pairs under light illumination. As a result, the devices exhibits a high response in visible light at a wavelength of 395 nm. The responsivity and photoconductive gain of the hybrid photodetector reached 0.33 A W⁻¹ and 1.306, respectively, which is 1.36 times higher than those of Cu-doped ZnO NRs-based ones. The response and recovery times are improved, with values of 25.21 s and 42.01 s, respectively. The development of hybrid materials for visible photodetectors enables an innovative approach for future optoelectronic devices, especially optical sensors.

This study reports the fabrication of a hybrid photodetector based on Cu-doped ZnO NRs/PEDOT:PSS, which improves the device''s performance and applications.     

Hydrogen production from methanol aqueous solution by ZnO/Zn(OH)2 macrostructure photocatalysts

Photocatalytic H₂ generation was studied for a series of ZnO/Zn(OH)₂ macrostructure photocatalysts. Different ZnO/Zn(OH)₂ macrostructures were prepared through a one-step hydrothermal method by adjusting the pH values of the solution and the concentration of dodecyl sulfate. Three different morphologies of the ZnO/Zn(OH)₂ macrostructure were synthesized and studied using SEM and XRD. The reflectance spectra revealed that the cone shaped ZnO/Zn(OH)₂ macrostructure (ZnO-C) had the lowest reflectivity of UV light. It was found that the photoelectronic properties depend on the morphology of the ZnO/Zn(OH)₂ macrostructures. The photocatalytic activity of these ZnO/Zn(OH)₂ macrostructure hybrids (about 0.070 mmol g⁻¹ h⁻¹) were higher than that observed for ZnO nanorods (0.050 mmol g⁻¹ h⁻¹). These results suggest the substantial potential of metal oxide materials with macrostructures in photocatalytic water splitting applications.

ZnO/Zn(OH)₂ macrostructure photocatalysts prepared using a one-step hydrothermal method demonstrated highly efficient photocatalytic hydrogen production.     

Enhanced photoresponse of a high-performance self-powered UV photodetector based on ZnO nanorods and a novel electrolyte by the piezo-phototronic effect

A flexible self-powered ultraviolet (UV) photodetector based on ZnO nanorods (NRs) and a novel iodine-free quasi solid-state electrolyte was fabricated. The obtained device has a fast and high response to UV light illumination at zero bias and also shows long-term stability. The responsivity is 50.5 mA W⁻¹ and the response time is less than 0.2 s. Strain-induced piezo-phototronic potential within wurtzite-structured ZnO can optimize the performance of corresponding optoelectronic devices since it could effectively tune the charge carriers'' separation and transport. The photoresponse performances of the photodetector under different upward angles (tensile strain) and downward angles (compressive strain) at 0 V bias were studied in detail. A 163% change of responsivity was obtained when the downward angle reached 60°. The enhancement could be interpreted by the piezo-phototronic effect. The piezoelectric potential (piezopotential) at the ZnO NRs/electrolyte interface can expand the built-in field, and as a result, it is easier for charge carriers to separate and transport.

A flexible UV detector exhibits high performance. The photoresponse of the device under different upward angles (tensile strain) and downward angles (compressive strain) were studied. A 163% change in responsivity was obtained when the downward angle reached 60°.     

Enhanced performance of ZnO nanorod array/CuSCN ultraviolet photodetectors with functionalized graphene layers

Facile, convenient and low-cost processes, including a chemical hydrothermal method and impregnation technique, were demonstrated to fabricate a self-powered ZnO nanorod array/CuSCN/reduced graphene oxide (rGO) ultraviolet photodetector. ZnO nanorods (NRs) were fully filled and encased by the CuSCN layer, in which CuSCN acts as the primary hole-transport layer and an electron reflection layer, blocking the electron transfer towards the Au electrode and reducing the electron–hole pair recombination. After annealing, this encapsulated structure further reduces the surface state defects of ZnO NRs, which can isolate the electron exchange with oxygen in the air, dramatically reducing the rise and fall time; it also forms a p–n junction, providing a built-in electric field to improve the photoresponse without applying external power. The rGO layer was coated on the surface of CuSCN as the secondary hole-transport layer and then annealed, which could effectively block Au from entering CuSCN and contacting ZnO along cracks and holes during vapor deposition, avoiding the formation of leakage channels. Furthermore, due to the ultra-high carrier mobility and the increase in work function after Au doping, the functionalized graphene could reduce the valence band shift, which is beneficial to enhance hole transport. Meanwhile, rGO obstructs the undesired barrier formed by electrical potential-induced reaction of Au with thiocyanate anions. Finally, the ZnO NR/CuSCN/rGO ultraviolet photodetector exhibits a significant enhancement in device performance (responsivity: 18.65 mA W⁻¹ at 375 nm under 65 mW cm⁻² illumination, rectification ratio: 5690 at ±1 V), which is better that of than ZnO NR/CuSCN structure (10.88 mA W⁻¹, 10.22 at ±1 V) and maintains the 100 ms response time.

Facile, convenient and low-cost processes were used to fabricate self-powered ZnO nanorod array ultraviolet photodetectors with CuSCN/rGO hole-transport bilayers. The device performance with a functionalized graphene layer was greatly improved.     

Fragmented lignin-assisted synthesis of a hierarchical ZnO nanostructure for ammonia gas sensing

In the present study, we demonstrated the use of fragmented lignin in the synthesis of a hierarchical-type structure of ZnO nanorods. Lignin was isolated from bagasse by the microwave assisted method and its fragmentation was achieved in alkaline conditions along with hydrogen peroxide. Lignin and fragmented lignin were purified by crystallisation followed by column chromatography and characterized by UV-visible spectroscopy, Frontier infra-red spectroscopy (FTIR), ¹H-NMR and high resolution mass spectroscopy (HRMS). Fragmented lignin was utilized as a template for the synthesis of ZnO nanorods, which were characterized by powder XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-DRS for the determination of crystal structure, particle morphology and band gap. XRD of the ZnO samples revealed a hexagonal wurtzite structure. The morphology of ZnO without fragmented lignin showed agglomerated nanoparticles and with fragmented lignin, a self-assembled hierarchical nanostructure due to nanorods of 30 nm diameter and 200–500 nm length was observed. The fragmented lignin showed a pronounced effect on the particle size and morphology of ZnO nanoparticles. We measured the response of the hierarchical ZnO nanostructure (50 ppm) for sensing NH₃ in terms of change in voltage across known resistance. We observed the response and recovery upon introduction of the analyte ammonia gas at 175 °C.

Flow sheet for isolation of fragmented lignin.     

Synthesis of gemini basic ionic liquids and their application in anion exchange membranes

A gemini-type basic morpholine ionic liquid ([Nbmd][OH]) was synthesized via a two-step method with morpholine, bromododecane and 1,4-dibromobutane as raw materials, and its structure was characterized by ¹H NMR and FT-IR spectroscopy. Meanwhile, a series of anion exchange membranes ([Nbmd][OH]_x–QCS) were prepared with quaternized chitosan (QCS) as the polymer matrix and [Nbmd][OH] as the dopant owing to its strong alkalinity and good solubility. The structures of the [Nbmd][OH]_x–QCS composite membranes were characterized in detail by FT-IR spectroscopy, the OH⁻ conductivity by AC impedance spectroscopy, and the morphological features by scanning electron microscopy (SEM), thermal gravity analysis (TGA), etc. The results show that the [Nbmd][OH]_x–QCS composite membranes have uniform surfaces and cross-section morphology. Increasing the content of [Nbmd][OH] not only enhances the thermal stability but also increases the OH⁻ conductivity; the thermal decomposition temperature of the [Nbmd][OH]₄₀–QCS membrane is nearly 20 °C higher than that of the pristine QCS membrane, and the maximum OH⁻ conductivity is approximately 1.37 × 10⁻² S cm⁻² at 70 °C. The methanol permeability of the [Nbmd][OH]₄₀–QCS membrane in 1 M methanol at room temperature is 2.21 × 10⁻⁶ cm⁻² s⁻¹, which is lower than that of Nafion®115, indicating a promising potential use in alkaline direct methanol fuel cells. Moreover, the [Nbmd][OH]₄₀–QCS membrane exhibits the best alkaline stability of all the membranes prepared in this work, retaining approximately 81% of its initial conductivity after immersion in 3 M KOH solution for 120 h at 70 °C.

A gemini-type basic morpholine ionic liquid ([Nbmd][OH]) was synthesized via a two-step method with morpholine, bromododecane and 1,4-dibromobutane as raw materials, and its structure was characterized by ¹H NMR and FT-IR spectroscopy.     

Growth and properties of ZnO nanorods by RF-sputtering for detection of toxic gases

There is a strong demand for nanostructured materials prepared by an industrially-scalable technique. The current work is devoted to the preparation of ZnO polycrystalline nanorods using RF sputtering at 400 °C and Sn droplets as a catalyzer layer, for highly sensitive gas sensors. Nanorods with diameters ranging from 100 to 200 nm can be tailored by changing the RF power and the deposition time. Raman and PL spectroscopy indicate that the material obtained is ZnO, with a characteristic emission spectrum in the UV region and in the visible. The functional properties of the ZnO nanorods were investigated by studying the response to CBRN (acetonitrile and DMMP), explosive (H₂), and pollutant gases (H₂S, acetone, and NO₂) in the temperature range 200–500 °C. The sensors showed good response to reducing gases at higher temperatures (500 °C) and to NO₂ at lower temperature (200 °C).

ZnO polycrystalline nanorods were easily prepared via RF sputtering and proved excellent sensors for H₂S and other toxic/explosive gases.     

Temperature-dependent transport properties of CVD-fabricated n-GaN nanorods/p-Si heterojunction devices

We report on the structural, electrical, and transport properties of high quality CVD-fabricated n-GaN nanorods (NRs)/p-Si heterojunction diodes. The X-ray diffraction (XRD) studies reveal the growth of hexagonal wurtzite GaN structure. The current–voltage (I–V) characteristics of the n-GaN NRs/p-Si heterojunction were measured in the temperature range of 300–475 K. The ideality factor (n) and zero-bias barrier height (ϕ_B0) are found to be strongly temperature-dependent. The calculated values of ϕ_B0 are 0.95 and 0.99 eV according to Gaussian distributions (GD) and modified Richardson for GD, respectively, which are in good agreement with the band offset of GaN/Si (0.95 eV). A Richardson constant of 37 cm⁻² K⁻² was obtained from the modified Richardson plot, which is close to the theoretical value for p-Si (32 cm⁻² K⁻²). The Gaussian distributions (GD) of inhomogeneous barrier height (BHs) and modified Richardson for GD of BHs with TE have also been used to explain the obtained transport properties.

We report on the structural, electrical, and transport properties of high quality CVD-fabricated n-GaN nanorods (NRs)/p-Si heterojunction diodes.     

Facile construction of a polypyrrole–cobalt sulfide counter electrode for low-cost dye-sensitized solar cells

A polypyrrole–cobalt sulfide composite counter electrode (CE) was prepared in this work. Firstly, polypyrrole (PPy) nanorods were prepared by an in situ polymerization method on FTO, then cobalt sulfide (CoS) nanoparticles were coated on PPy nanorods by the electrodeposition method. The DSSC with PPy–CoS CE exhibits superior photoelectric conversion efficiency than that based on platinum (Pt, one of common counter electrodes), which is 7.52%, improving more than 20% compared to Pt CE (6.19%). In addition, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements demonstrated that the PPy–CoS CE exhibited excellent catalytic performance for I₃⁻/I⁻ solution.

A facile two-step strategy to construct a polypyrrole–cobalt sulfide counter electrode for low-cost dye-sensitized solar cells.     

A separation-free 3D network ZnO/rGO–rGH hydrogel: adsorption enriched photocatalysis for environmental applications

This study describes the encapsulation of ZnO by reduced graphene oxide to form a composite (ZnO/rGO) that can be incorporated into graphene to form hydrogels (ZnO/rGO–rGH) with three-dimensional (3D) network structures. The unique surface adsorption characteristics of graphene make ZnO/rGO–rGH materials have the ability of fast adsorption and desorption. Meanwhile, the combination of graphene and ZnO nanoparticles can promote the separation efficiency of electrons and holes and improve the photocatalytic activity. The sample showed the highest adsorption-photocatalysis synergistic activity and removed 100% of the BPA (10 mg L⁻¹) within 20 min under UV irradiation. The purification efficiency of ZnO/rGO–rGH can reach more than 90% after 5 rounds of repeated use. We also measured the performance of ZnO/rGO–rGH in removing BPA under flow conditions, and the results showed that this approach with ZnO/rGO–rGH removed 100% of the BPA in 16 h.

Proposed mechanism for photocatalytic BPA degradation by ZnO/rGO–rGH under ultraviolet light illumination.     

Synthesis of Ni–Ag–ZnO solid solution nanoparticles for photoreduction and antimicrobial applications

ZnO is one of the most promising and efficient semiconductor materials for various light-harvesting applications. Herein, we reported the tuning of optical properties of ZnO nanoparticles (NPs) by co-incorporation of Ni and Ag ions in the ZnO lattice. A sonochemical approach was used to synthesize pure ZnO NPs, Ni–ZnO, Ag–ZnO and Ag/Ni–ZnO with different concentrations of Ni and Ag (0.5%, 2%, 4%, 8%, and 15%) and Ni doped Ag–ZnO solid solutions with 0.25%, 0.5%, and 5% Ni ions. The as-synthesized Ni–Ag–ZnO solid solution NPs were characterized by powdered X-ray diffraction (pXRD), FT-IR spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), UV-vis (UV) spectroscopy, and photoluminescence (PL) spectroscopy. Ni–Ag co-incorporation into a ZnO lattice reduces charge recombination by inducing charge trap states between the valence and conduction bands of ZnO and interfacial transfer of electrons. The Ni doped Ag–ZnO solid solution NPs have shown superior 4-nitrophenol reduction compared to pure ZnO NPs which do not show this reaction. Furthermore, a methylene blue (MB) clock reaction was also performed. Antibacterial activity against E. coli and S. aureus has inhibited the growth pattern of both strains depending on the concentration of catalysts.

The synergic effect of Ni and Ag in Ni–Ag–ZnO solid solutions has tuned the optoelectronic properties of ZnO for photoreduction reactions.     

Synthesis and photoluminescence properties of hybrid 1D core–shell structured nanocomposites based on ZnO/polydopamine

In the present work, we report on the modelling of processes at the zinc oxide and polydopamine (ZnO/PDA) interface. The PDA layer was deposited onto ZnO nanorods (NRs) via chemical bath deposition. The defect concentrations in ZnO before and after PDA deposition were calculated and analysed. The ZnONRs/PDA core–shell nanostructures were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Fourier-transform infrared (FTIR) spectroscopy, photoluminescence (PL) measurements, and diffuse reflectance spectroscopy. The TEM and electron energy loss spectroscopy (EELS) measurements confirmed the conformal coating of PDA, while the PL emission from ZnO and ZnONRs/PDA samples showed a reduction of intensity after the PDA deposition. The decrease of defect concentration participating in PL and quantum efficiency explains the PL reduction. Finally, the observed decrease of activation energies and a shift of the PL peaks are attributed to the formation of an additional local electrical field between the PDA and ZnO nanostructures.

The results shown in this study provide a unique insight into the optical and electronic processes of the ZnO/PDA interface.     

Humidity sensor based on poly(lactic acid)/PANI–ZnO composite electrospun fibers

The electrospinning technique has been successfully used to prepared micro-fibers of the poly(lactic acid)/polyaniline–zinc oxide (PLA/PANI–ZnO) composite. The polyaniline–zinc oxide (PANI–ZnO) nanocomposites are synthesized by hydrothermal and in situ polymerization methods. X-ray diffraction techniques are used to study the structural properties of the PLA/PANI–ZnO composite fibers and the PANI–ZnO nanocomposite. The average crystallite size of the PANI–ZnO nanocomposite is found to be 36 nm. The morphology and diameter of the composite fibers are analyzed by scanning electron microscopy (SEM). The average fiber diameter of the pure poly(lactic acid) (PLA) fiber is around 2.5 μm and that of the PLA/PANI–ZnO composite fiber is around 1.4 μm. Differential scanning calorimetry (DSC) provides the thermal properties of the PLA/PANI–ZnO composite fibers. The melting temperature (T_m) for the pure PLA is observed at 149.3 °C, and it is shifted to 153.0 °C for the PLA/PANI–ZnO composite fibers. The enhanced thermal properties of the composite fibers are due to the interaction between the polymer and the nanoparticles. The water contact angle measurements probe the surface hydrophilicity of the PLA/PANI–ZnO composite fibers. The role of the PANI–ZnO nanocomposite on the sensing behavior of PLA fibers has also been investigated. The humidity sensing properties of the composite fiber based sensor are studied in the relative humidity (RH) range of 20–90% RH. The experimental results show that the composite fiber exhibited good response (85 s) and recovery (120 s) times. These results indicate that the one-dimensional (1D) fiber structure enhances the humidity sensing properties.

The electrospinning technique has been successfully used to prepared micro-fibers of the poly(lactic acid)/polyaniline–zinc oxide (PLA/PANI–ZnO) composite for humidity sensor application.     

Towards cancer diagnostics – an α-feto protein electrochemical immunosensor on a manganese(iv) oxide/gold nanocomposite immobilisation layer

A novel electrochemical immunosensor for the quantification of α-feto protein (AFP) using a nanocomposite of manganese(iv) oxide nanorods (MnO₂NRs) and gold nanoparticles (AuNPs) as the immobilisation layer is presented. The MnO₂NRs was synthesised using a hydrothermal method and AuNPs were electrodeposited on a glassy carbon electrode surface. The MnO₂NRs were characterised with scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and X-ray powder diffraction (XRD). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterise the immunosensor at each stage of the biosensor preparation. The MnO₂ nanorods and AuNPs were applied as the immobilisation layer to efficiently capture the antibodies and amplify the electrochemical signal. Under optimised conditions, the fabricated immunosensor was utilised for the quantification of AFP with a wide dynamic range of 0.005 to 500 ng mL⁻¹ and detection limits of 0.00276 ng mL⁻¹ and 0.00172 ng mL⁻¹ (S/N = 3) were obtained from square wave anodic stripping voltammetry and EIS respectively. The nanocomposite modifier enhanced the immunosensor performance. More so, this label-free immunosensor possesses good stability over a period of two weeks when stored at 4 °C and was selective in the presence of some interfering species.

A novel electrochemical immunosensor for the quantification of α-feto protein (AFP) using a nanocomposite of manganese(iv) oxide nanorods (MnO₂NRs) and gold nanoparticles (AuNPs) as the immobilisation layer is presented.     

Insights into non-noble metal based nanophotonics: exploration of Cr-coated ZnO nanorods for optoelectronic applications

Herein, the room temperature photoluminescence and Raman spectra of hydrothermally grown ZnO nanorods coated with Cr are investigated for optoelectronic applications. A thorough examination of the photoluminescence spectra of Cr coated ZnO nanorods showed the suppression of deep level emissions by more than twenty five times with Cr coating compared to that of pristine ZnO nanorods. Moreover, the underlying mechanism was proposed and can be attributed to the formation of Schottky contacts between Cr and ZnO resulting in defect passivation, weak exciton–plasmon coupling, enhanced electric field effect and formation of hot carriers due to interband transitions. Interestingly, with the increase in sputtering time, the ratio of the intensities corresponding to the band gap emission and deep level emission was observed to increase from 6.2 to 42.7, suggesting its application for UV only emission. Further, a planar photodetector was fabricated (Ag–ZnO–Ag planar configuration) and it was observed that the dark current value got reduced by more than ten times with Cr coating, thereby opening up its potential for transistor applications. Finally, Cr coated ZnO nanorods were employed for green light sensing. Our results demonstrated that ZnO nanorods decorated with Cr shed light on developing stable and high-efficiency non-noble metal based nanoplasmonic devices such as photodetectors, phototransistors and solar cells.

Herein, the room temperature photoluminescence and Raman spectra of hydrothermally grown ZnO nanorods coated with Cr are investigated for optoelectronic applications.