The humidity sensing characteristics of different sensing materials are important properties in order to monitor different products or events in a wide range of industrial sectors, research and development laboratories as well as daily life. The primary aim of this study is to compare the sensing characteristics, including impedance or resistance, capacitance, hysteresis, recovery and response times, and stability with respect to relative humidity, frequency, and temperature, of different materials. Various materials, including ceramics, semiconductors, and polymers, used for sensing relative humidity have been reviewed. Correlations of the different electrical characteristics of different doped sensor materials as the most unique feature of a material have been noted. The electrical properties of different sensor materials are found to change significantly with the morphological changes, doping concentration of different materials and film thickness of the substrate. Various applications and scopes are pointed out in the review article. We extensively reviewed almost all main kinds of relative humidity sensors and how their electrical characteristics vary with different doping concentrations, film thickness and basic sensing materials. Based on statistical tests, the zinc oxide-based sensing material is best for humidity sensor design since it shows extremely low hysteresis loss, minimum response and recovery times and excellent stability.
The concentration of acceptor carriers, depletion width, magnitude of donor level movement as well as the sensitivity factor are determined from the UV response of a heterojunction consisting of ZnO on type IIb diamond. From the comparison of the I-V measurements in dark condition and under UV illumination we show that the acceptor concentration (∼10(17) cm(-3)) can be estimated from p-n junction properties. The depletion width of the heterojunction is calculated and is shown to extend farther into the ZnO region in dark condition. Under UV illumination, the depletion width shrinks but penetrates both materials equally. The ultraviolet illumination causes the donor level to move closer to the conduction band by about 50 meV suggesting that band bending is reduced to allow more electrons to flow from the intrinsically n-type ZnO. The sensitivity factor of the device calculated from the change of threshold voltages, the ratio of dark and photocurrents and identity factor is consistent with experimental data.
The Burstein-Moss shift and band gap narrowing of sputtered indium-doped zinc oxide (IZO) thin films are investigated as a function of carrier concentrations. The optical band gap shifts below the carrier concentration of 5.61 × 1019 cm-3 are well-described by the Burstein-Moss model. For carrier concentrations higher than 8.71 × 1019 cm-3 the shift decreases, indicating that band gap narrowing mechanisms are increasingly significant and are competing with the Burstein-Moss effect. The incorporation of In causes the resistivity to decrease three orders of magnitude. As the mean-free path of carriers is less than the crystallite size, the resistivity is probably affected by ionized impurities as well as defect scattering mechanisms, but not grain boundary scattering. The c lattice constant as well as film stress is observed to increase in stages with increasing carrier concentration. The asymmetric XPS Zn 2p3/2 peak in the film with the highest carrier concentration of 7.02 × 1020 cm-3 suggests the presence of stacking defects in the ZnO lattice. The Raman peak at 274 cm-1 is attributed to lattice defects introduced by In dopants.
The performance of sensing surfaces highly relies on nanostructures to enhance their sensitivity and specificity. Herein, nanostructured zinc oxide (ZnO) thin films of various thicknesses were coated on glass and p-type silicon substrates using a sol-gel spin-coating technique. The deposited films were characterized for morphological, structural, and optoelectronic properties by high-resolution measurements. X-ray diffraction analyses revealed that the deposited films have a c-axis orientation and display peaks that refer to ZnO, which exhibits a hexagonal structure with a preferable plane orientation (002). The thicknesses of ZnO thin films prepared using 1, 3, 5, and 7 cycles were measured to be 40, 60, 100, and 200 nm, respectively. The increment in grain size of the thin film from 21 to 52 nm was noticed, when its thickness was increased from 40 to 200 nm, whereas the band gap value decreased from 3.282 to 3.268 eV. Band gap value of ZnO thin film with thickness of 200 nm at pH ranging from 2 to 10 reduces from 3.263eV to 3.200 eV. Furthermore, to evaluate the transducing capacity of the ZnO nanostructure, the refractive index, optoelectric constant, and bulk modulus were analyzed and correlated. The highest thickness (200 nm) of ZnO film, embedded with an interdigitated electrode that behaves as a pH-sensing electrode, could sense pH variations in the range of 2-10. It showed a highly sensitive response of 444 μAmM-1cm-2 with a linear regression of R2 =0.9304. The measured sensitivity of the developed device for pH per unit is 3.72μA/pH.
To investigate the interaction of zinc oxide nanoparticles (ZnO NPs) with fly ash soil (FAS) for the reduction of metals from FAS by Parthenium hysterophorus were studied. The average accumulation of metals by P. hysterophorus stem were Fe 79.6%; Zn 88.5%; Cu 67.5%; Pb 93.6%; Ni 43.5% and Hg 39.4% at 5.5 g ZnO NP. The concentration of ZnO NP at 1.5 g did not affect the metals accumulation, however at 5.5 g ZnO NP showed highest metal reduction was 96.7% and at 10.5-15.5 g ZnO NP of 19.8%. The metal reduction rate was R max for Fe 16.4; Zn 21.1; Pb 41.9; Hg 19.1 was higher than Ni 6.4 and Cu 11.3 from the FAS at 5.5 g ZnO NP whereas, the reduction rate of Pb showed highest. With doses of 5.5 g ZnO NP the biomass increased upto 78%; the metal reduced upto 98.7% with the share of 100% ZnO NP from FAS. Further investigation with phytotoxicity the plant reactive oxygen species (ROS) production were affected due was mainly due to the recovery of metals from FAS (R2 = 0.99).
The enhancement of microwave absorbing properties in nickel zinc ferrite (Ni0.5Zn0.5Fe2O4) via multiwall carbon nanotubes (MWCNT) growth is studied in this research work. Ni0.5Zn0.5Fe2O4 was initially synthesized by mechanical alloying followed by sintering at 1200 °C and the microstructural, electromagnetic and microwave characteristics have been scrutinized thoroughly. The sintered powder was then used as a catalyst to grow MWCNT derived from chemical vapor deposition (CVD) method. The sample was mixed with epoxy resin and a hardener for preparation of composites. The composite of multi-walled carbon nanotubes/Ni0.5Zn0.5Fe2O4 shown a maximum reflection loss (RL) of -19.34 dB at the frequency and bandwidth of 8.46 GHz and 1.24 GHz for an absorber thickness of 3 mm for losses less than -10 dB. This acquired result indicates that multi-walled carbon nanotubes/Ni0.5Zn0.5Fe2O4 could be used as a microwave absorber application in X-band.
Arecanut husk (AH) was selected as a material for silica replacement in the synthesis process of glass-ceramics zinc silicate and also the fact that it has no traditional use and often being dumped and results in environmental issues. The process of pyrolysis was carried out at temperature 700 °C and above based on thermogravimetric analysis to produce arecanut husk ash (AHA). The average purity of the silica content in AHA ranged from 29.17% to 45.43%. Furthermore, zinc oxide was introduced to AHA and zinc silicate started to form at sintering temperature 700 °C and showed increased diffraction intensity upon higher sintering temperature of 600 °C to 1000 °C based on X-ray diffraction (XRD) analysis. The grain sizes of the zinc silicate increased from 1011 nm to 3518 nm based on the morphological studies carried out by field emission scanning electron microscopy (FESEM). In addition, the optical band gap of the sample was measured to be in the range from 2.410 eV to 2.697 eV after sintering temperature. From the data, it is believed that a cleaner production of low-cost zinc silicate can be achieved by using arecanut husk and have the potential to be used as phosphors materials.
Utilization of metal-oxide nanoparticles (NPs) in enhanced oil recovery (EOR) has generated substantial recent research interest in this area. Among these NPs, zinc oxide nanoparticles (ZnO-NPs) have demonstrated promising results in improving oil recovery due to their prominent thermal properties. These nanoparticles can also be polarized by electromagnetic (EM) field, which offers a unique Nano-EOR approach called EM-assisted Nano-EOR. However, the impact of NPs concentrations on oil recovery mechanism under EM field has not been well established. For this purpose, ZnO nanofluids (ZnO-NFs) of two different particle sizes (55.7 and 117.1 nm) were formed by dispersing NPs between 0.01 wt.% to 0.1 wt.% in a basefluid of sodium dodecylbenzenesulfonate (SDBS) and NaCl to study their effect on oil recovery mechanism under the electromagnetic field. This mechanism involved parameters, including mobility ratio, interfacial tension (IFT) and wettability. The displacement tests were conducted in water-wet sandpacks at 95˚C, by employing crude oil from Tapis. Three tertiary recovery scenarios have been performed, including (i) SDBS surfactant flooding as a reference, (ii) ZnO-NFs flooding, and (iii) EM-assisted ZnO-NFs flooding. Compare with incremental oil recovery from surfactant flooding (2.1% original oil in place/OOIP), nanofluid flooding reaches up to 10.2% of OOIP at optimal 0.1 wt.% ZnO (55.7 nm). Meanwhile, EM-assisted nanofluid flooding at 0.1 wt.% ZnO provides a maximum oil recovery of 10.39% and 13.08% of OOIP under EM frequency of 18.8 and 167 MHz, respectively. By assessing the IFT/contact angle and mobility ratio, the optimal NPs concentration to achieve a favorable ER effect and interfacial disturbance is determined, correlated to smaller hydrodynamic-sized nanoparticles that cause strong electrostatic repulsion between particles.
Zinc oxide (ZnO) photocatalysts were successfully synthesized via chemical and green, environmentally-benign methods. The work highlights the valorization of banana peel (BP) waste extract as the reducing and capping agents to produce pure, low temperature, highly crystalline, and effective ZnO nanoparticles with superior photocatalytic activities for the removal of hazardous Basic Blue 9 (BB9), crystal violet (CV), and cresol red (CR) dyes in comparison to chemically synthesized ZnO. Their formation and morphologies were verified by various optical spectroscopic and electron microscopic techniques. XRD results revealed that the biosynthesized ZnO exhibited 15.3 nm crystallite size when determined by Scherrer equation, which was smaller than the chemically synthesized ZnO. The FTIR spectra confirmed the presence of biomolecules in the green-mediated catalyst. EDX and XPS analyses verified the purity and chemical composition of ZnO. Nitrogen sorption analysis affirmed the high surface area of bio-inspired ZnO. Maximum removal efficiencies were achieved with 30 mg green ZnO catalyst, 2.0 × 10-5 M BB9 solution, alkaline pH 12, and irradiation time 90 min. Green-mediated ZnO showed superior photodegradation efficiency and reusability than chemically synthesized ZnO. Therefore, this economical, environment-friendly photocatalyst is applicable for the removal of organic contaminants in wastewater treatment under visible light irradiation.
Magnetic nanoparticles in the hollow region of carbon nanotubes have attraction due to their changing physical electrical and magnetic properties. Nickel zinc ferrite plays an important role in many applications due to its superior magnetic properties. Ni0.8Zn0.2Fe2O4 single crystals were encapsulated in multiwall carbon nanotubes (MWCNTs). The magnetic nano crystals were prepared using a sol-gel self combustion method at the sintering temperature of 750 degrees C and were characterized by XRD, FESEM, TEM and VSM. Initial permeability, Q-factor and relative loss factor were measured by impedance vector network analyzer. XRD patterns were used for the phase identification. FESEM images show morphology and dimensions of the grains of Ni0.8Zn0.2Fe2O4 single crystals and Ni0.8Zn0.2Fe2O4 single crystals in MWCNTs. TEM images were used to investigate single crystal and encapsulation of Ni0.8Zn0.2Fe2O4 single crystals in the MWCNTs. VSM results confirmed super paramagnetic behaviour of encapsulated Ni0.8Zn0.2Fe2O4 single crystals. It was also attributed that encapsulated Ni0.8Zn0.2Fe2O4 single crystals in MWCNTs showed a higher initial permeability (51.608), Q-factor (67.069), and low loss factor (0.0002) as compared to Ni0.8Zn0.2Fe2O4 single crystals. The new encapsulated Ni0.8Zn0.2Fe2O4 single crystals in the MWCNTs may have potential applications in electronic and medical industries.
Textile industry is one of the most environmental unfriendly industrial processes due to the massive generation of colored wastewater contaminated with dyes and other chemical auxiliaries. These contaminants are known to have undesirable consequences to ecosystem. The present study investigated the best operating parameters for the removal of congo red (CR, as the model for dye wastewater) by orange peels extract biosynthesized zinc oxide nanoparticles (ZnO NPs) via photocatalysis in an aqueous solution. The response surface methodology (RSM) with ZnO NPs loadings (0.05-0.20 g), pH (3.00-11.00), and initial CR concentration (5-20 ppm) were used for the optimization process. The applicability of ZnO NPs in the dye wastewater treatment was evaluated based on the techno-economic analysis (TEA). ZnO NPs exhibited hexagonal wurtzite structure with = C-H, C-O, -C-O-C, CC, O-H as the main functional groups. The maximum degradation of CR was more than 96% with 0.171 g of ZnO NPs, at pH 6.43 and 5 ppm of CR and 90% of the R2 coefficient. The specific cost of ZnO NPs production is USD 20.25 per kg. These findings indicated that the biosynthesized ZnO NPs with orange peels extract provides alternative method for treating dye wastewater.
The effects of zeolite and zinc foliar applications on the biochemical characteristics of canola cultivars under different moisture regimes were investigated in a study conducted during the 2010 and 2011 growing seasons. The study was completed using a factorial split-plot experiment based on randomized complete block design (RCBD) with three replications at the Seed and Plant Improvement Institute (SPII), Karaj, Iran. The treatments were: irrigation (I): complete (I1) and restricted (I2); zeolite (Z): 0 (Z1) and 15 ton ha-1 (Z2) and Zn: 0, 0.1 and 0.2% concentrations of zinc sulfate (Zn1, Zn2, and Zn3) at the pod formation stage. These treatments were applied during the pod formation stage to the Licord, RGS003 and Opera cultivars. This study showed that although applying Z and Zn had positive effects on the quality of canola, the highest performance and the best results were obtained using a combination of Z and Zn. The combined application of Z and Zn decreased the proline and carbohydrate contents to 44.35 and 34.42%, respectively. Therefore, with the low cost of natural Z and moderate Zn intake, these treatments can be used to enhance the performance of canola, especially in regions frequently subjected to water stress.
An innovative nano-base polymer that scavenges radicals and reactive oxygen species exhibits potential antibacterial properties, which are crucial in the biomedical field, particularly in reducing nosocomial infections. However, the safety of this nano-based polymer, which has direct contact with the human system, has not been fully understood. The present study investigated the cytocompatibility and hemocompatibility responses of linear low-density polyethylene polymer (LLDPE) embedded with difference ratios of heterogeneous TiO2/ZnO nanocomposites. Exposure of the blood and fibroblast cells to LLDPE/100Z and LLDPE/25T75Z/10% nanocomposite films for 48 and 72 h decreased their viability by less than 40%, compared with LLDPE, LLDPE/100T and LLDPE/25T75Z/5% nanocomposite films. It also presented possible cellular damage and cytotoxicity, which was supported by the findings from the significant release of extracellular lactate dehydrogenase profiles and cell survival assay Further observation using an electron microscope revealed that LLDPE films with heterogeneous 25T75Z/5% promoted cell adhesion. Moreover, no hemolysis was detected in all ratios of heterogeneous TiO2/ZnO nanocomposite in LLDPE film as it was less than 0.2%, suggesting that these materials were hemocompatible. This study on LLDPE film with heterogeneous TiO2/ZnO nanocomposites demonstrated favorable biocompatible properties that were significant for advanced biomedical polymer application in a hospital setting.
In this paper, we address the synthesis of nano-coalesced microstructured zinc oxide thin films via a simple thermal evaporation process. The role of synthesis temperature on the structural, morphological, and optical properties of the prepared zinc oxide samples was deeply investigated. The obtained photoluminescence and X-ray photoelectron spectroscopy outcomes will be used to discuss the surface structure defects of the prepared samples. The results indicated that the prepared samples are polycrystalline in nature, and the sample prepared at 700 °C revealed a tremendously c-axis oriented zinc oxide. The temperature-driven morphological evolution of the zinc oxide nano-coalesced microstructures was perceived, resulting in transformation of quasi-mountain chain-like to pyramidal textured zinc oxide with increasing the synthesis temperature. The results also impart that the sample prepared at 500 °C shows a higher percentage of the zinc interstitial and oxygen vacancies. Furthermore, the intensity of the photoluminescence emission in the ultraviolet region was enhanced as the heating temperature increased from 500 °C to 700 °C. Lastly, the growth mechanism of the zinc oxide nano-coalesced microstructures is discussed according to the reaction conditions.
Multi-layer structure of functional materials often involves the integration of different crystalline phases. The film growth orientation thus frequently exhibits a transformation, owing to multiple possibilities caused by incompatible in-plane structural symmetry. Nevertheless, the detailed mechanism of the transformation has not yet been fully explored. Here we thoroughly probe the heteroepitaxially grown hexagonal zinc oxide (ZnO) films on cubic (001)-magnesium oxide (MgO) substrates using advanced scanning transition electron microscopy, X-ray diffraction and first principles calculations, revealing two distinct interface models of (001) ZnO/(001) MgO and (100) ZnO/(001) MgO. We have found that the structure alternatives are controlled thermodynamically by the nucleation, while kinetically by the enhanced Zn adsorption and O diffusion upon the phase transformation. This work not only provides a guideline for the interface fabrication with distinct crystalline phases but also shows how polar and non-polar hexagonal ZnO films might be manipulated on the same cubic substrate.
A series of glass samples with chemical formula {[(TeO2)0.7(B2O3)0.3]0.7(ZnO)0.3}1-x(Dy2O3)xwhere x=0.01, 0.02, 0.03, 0.04 and 0.05M fraction were synthesized through conventional melt-quenching method. The most common way to fabricate a glass material is by fusion of two or more component oxides followed by their quenching. This technique is known as melt-quenching technique. Kaur et al. (2016) [1] highlighted that the melt-quenching method able to enhance the mechanical properties like hardness and flexural strength of the material. The nature of the glass systems is proven to be amorphous based on the XRD pattern. The FTIR spectra of the glass systems confirm the existence of five bands which are assigned for the BO4, BO3,TeO4and TeO3vibrational groups. The density of the glass systems is increased with the addition of Dy2O3while the molar volume is found to be inversely proportional to the density of the proposed glass. The optical properties of the glasses are determined through the absorption spectra obtained from the UV-VIS spectrophotometer. From the absorption spectra, the indirect and direct optical band gaps and the Urbach energy are found to be inversely proportional to each other. As the molar fraction of the Dy2O3increased, the optical band gaps are observed to increase as opposed to the Urbach energy. For this glass system, the values of refractive index, electronic polarizability, oxide ion polarizability and the optical basicity are found to decrease as the addition of the dysprosium oxide is increased. From the emission spectra, two intense blue and yellow emission bands are observed, which correspond to the4F9/2→6H15/2and4F9/2→6H13/2transitions of Dy3+ions respectively. The CIE chromaticity coordinates of the zinc borotellurite glass systems are found to be located in the white light region.
Plant litter decomposition is not only the major source of soil carbon and macronutrients, but also an important process for the biogeochemical cycling of trace elements such as iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu). The concentrations of plant litter trace elements can influence litter decomposition and element cycling across the plant and soil systems. Yet, a global perspective of the patterns and driving factors of trace elements in plant litter is missing. To bridge this knowledge gap, we quantitatively assessed the concentrations of four common trace elements, namely Fe, Mn, Zn, and Cu, of freshly fallen plant litter with 1411 observations extracted from 175 publications across the globe. Results showed that (1) the median of the average concentrations of litter Fe, Mn, Zn, and Cu were 0.200, 0.555, 0.032, and 0.006 g/kg, respectively, across litter types; (2) litter concentrations of Fe, Zn, and Cu were generally stable regardless of variations in multiple biotic and abiotic factors (e.g., plant taxonomy, climate, and soil properties); and (3) litter Mn concentration was more sensitive to environmental conditions and influenced by multiple factors, but mycorrhizal association and soil pH and nitrogen concentration were the most important ones. Overall, our study provides a clear global picture of plant litter Fe, Mn, Zn, and Cu concentrations and their driving factors, which is important for improving our understanding on their biogeochemical cycling along with litter decomposition processes.
Xenobiotic Organic Compounds (XOCs) have been widely considered to be pollutant compounds due to their harmful impacts on aquatic life. However, there have been few rigorous studies of cutting-edge technology used to eradicate XOCs and their presence in bathroom greywater. The present review provides a comprehensive examination of current methodologies used for removing XOCs by photocatalysis of green nanoparticles. It was appeared that zinc oxide nanoparticles (ZnO NPs) have high efficiency (99%) in photocatalysis process. Green synthesis provides proven processes that do not require dangerous chemicals or expensive equipment, making photocatalysis a potential solution for the status quo. XOCs residue was decomposed, and pollutants were eliminated with varied degrees of efficiency using green synthesis ZnO nanoparticles. It is hypothesized that the utilization of photocatalysis can create a greywater treatment system capable of degrading the toxic XOCs in greywater while increasing the pace of production. Hence, this review will be beneficial in improving greywater quality and photocatalysis using green nanoparticles can be an immediate platform in solving the issue regarding the existence of XOCs in greywater in Malaysia. Researchers in the future may benefit from focusing on optimizing photocatalytic degradation using green-synthesis ZnO. It might also help with the creativity and productivity of the next generation of authoritative concerns, notably water conservation.
Nowadays, the current synthesis techniques used in industrial production of nanoparticles have been generally regarded as nonenvironmentally friendly. Consequently, the biosynthesis approach has been proposed as an alternative to reduce the usage of hazardous chemical compounds and harsh reaction conditions in the production of nanoparticles. In this work, pure, iron (Fe)-doped and silver (Ag)-doped zinc oxide (ZnO) nanoparticles were successfully synthesized through the green route using Clitoria ternatea Linn. The optical, chemical, and physical properties of the biosynthesized ZnO nanoparticles were then analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), UV-Vis diffuse reflectance spectroscopy (DRS), zeta potential measurement, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and surface analysis. The biosynthesized ZnO nanoparticles were crystallized with a hexagonal wurtzite structure and possessed smaller particle sizes than those of commercially or chemically produced samples. The existence of biomolecules to act as reducing and stabilizing agents from C. ternatea Linn aqueous extract was confirmed using FTIR analysis. The biosynthesized ZnO nanoparticles mainly comprised of negatively charged groups and responsible for moderately stable dispersion of the nanoparticles. All these properties were favorable for the sonocatalytic degradation of Congo red. Sonocatalytic activity of ZnO nanoparticles was studied through the degradation of 10 mg/L Congo red using ultrasonic irradiation at 45 kHz and 80 W. The results showed that the sonocatalytic degradation efficiency of Congo red in the presence of biosynthesized ZnO nanoparticles prepared at 50 °C for 1 h could achieve 88.76% after 1 h. The sonocatalytic degradation efficiency of Congo red in the presence of Ag-doped ZnO was accelerated to 94.42% after 10 min which might be related to the smallest band gap energy (3.02 eV) and the highest specific surface area (10.31 m2/g) as well as pore volume (0.0781 cm3/g). Lastly, the biosynthesized ZnO nanoparticles especially Ag-doped ZnO offered significant antibacterial potential against Escherichia coli which indicated its ability to inhibit the normal growth and replication of bacterial cells. These results affirmed that the biosynthesized ZnO nanoparticles could be used as an alternative to the current chemical compounds and showed a superior sonocatalytic activity toward degradation of Congo red.
The present study investigated the sustainable approach for wastewater treatment using waste algal blooms. The current study investigated the removal of toxic metals namely chromium (Cr), nickel (Ni), and zinc (Zn) from aqueous solutions in batch and column studies using biochar produced by the marine algae Ulva reticulata. SEM/EDX, FTIR, and XRD were used to examine the adsorbents' properties and stability. The removal efficiency of toxic metals in batch operations was investigated by varying the parameters, which included pH, biochar dose, initial metal ion concentration, and contact time. Similarly, in the column study, the removal efficiency of heavy metal ions was investigated by varying bed height, flow rate, and initial metal ion concentration. Response Surface Methodology (Central Composite Design (CCD)) was used to confirm the linearity between the observed and estimated values of the adsorption quantity. The packed bed column demonstrated successful removal rates of 90.38% for Cr, 91.23% for Ni, and 89.92% for Zn heavy metals from aqueous solutions, under a controlled environment. The breakthrough analysis also shows that the Thomas and Adams-Bohart models best fit the regression values, allowing prior breakthroughs in the packed bed column to be predicted. Desorption studies were conducted to understand sorption and elution during different regeneration cycles. Adding 0.3 N sulfuric acid over 40 min resulted in the highest desorption rate of the column and adsorbent used for all three metal ions.