Doped ZnO Research Articles

Gas Sensor Applications of Tungsten Doped ZnO Thin Films in Low Hydrogen Gas Concentration

Son zamanlarda, dizel, benzin ve propan gazı gibi fosil kaynaklardan gelen enerjinin yerini alabilecek alternatif bir enerji kaynağı olarak hidrojen gazının uygulanması için çalışmalar hızla önem kazanmaya başlamıştır. Bu nedenle, hidrojen gazı, vaat edilen alternatif bir enerji kaynağı olmanın ötesine geçtiği başlıca enerji kaynaklarından biri olarak kabul edilmektedir. Ancak, hidrojen gazı patlayıcıdır ve büyük bir yangına neden olabilir. Bu gazın rengi, kokusu ve tadı olmadığından, güvenlik için minimum miktarda hidrojen gazı algılayabilen çok hassas bir gaz sensörü üretimi yapılmalıdır. Dahası, hidrojen gazı son derece hafiftir ve atmosfere kolayca yayılır. Gaz konsantrasyonu % 4'ün üzerinde olduğunda bir patlama meydana gelebilir; bu nedenle ppm ölçekli hidrojen gazı algılayabilen sensörler geliştirilmelidir. Bu çalışmada Tungten (W) katkılı ZnO ince filmler kimyasal banyolama tekniği ile %1 ve % 2 katkılı olarak büyütüldü ve Hidrojen gaz (H2) algılama özellikleri incelendi. Üretilen numunenin farklı sıcaklıklarda (30°C-160°C) ve 5 ppm-100 ppm gaz konsantrasyonu aralığında elektriksel karakterizasyonu yapıldı. Çalışma sıcaklığı 100 C olarak bulundu. Sensörler, 5 ppm H2 gaz konsantrasyonuna karşı kabul edilebilir düzeyde duyarlılık sergiledi. %1 W-katkılı ZnO ince film 100 °C çalışma sıcaklığında diğer ince filmlere kıyasla daha yüksek algılama performansı gösterdi. %1 W-katkılı ZnO ince film 5 ppm H2 gazına karşı % 28,56 duyarlılık sergilerken, %2 W-katkılı ZnO ince film % 7 duyarlılık sergilediği hesaplandı. Ölçüm sonuçları, numunelerin gaz algılama özelliklerinin katkılamaya bağlı olarak değiştiğini gösterdi.

Optical Properties of Sn Doped ZnO Thin Film Prepared Using Sol-Gel Method for Optical Limiting Application

In this work, pure Zinc oxide and tin doped Zinc oxide thin films nanoparticles with various volume concentrations of 2, 4, 6, and 8V/V% were prepared by using the sol-gel method. The optical properties were investigated by using UV-Visible spectroscope, and the value exhibits the direct allowed transition. The average of transmittance was around ~(17-23) %in visible region. The optical energy band gap was calculated with wavelength (300-900) nm for pure ZnO and Sn doped ZnO thin films which decreases with increasing concentration from 3.4 eV to 3.1 eV respectively and red shift. The real dielectric(εr) and the imaginary dielectric εiare the same behavior of the refractive index(n) the extinction coefficient (k) respectively. The optical limiting properties were studied by using an SDL laser with a wavelength of 235 nm. ZnO and doping thin films an found efficient as optic limiting and depend on the concentration of the all samples.

Efficient photocatalytic degradation of crystal violet dye and electrochemical performance of modified MWCNTs/Cd-ZnO nanoparticles with quantum chemical calculations

Several literature studies have defined the doping technique of ZnO with a Cd (transition metal) for photocatalysis applications. Despite this, the properties and the mechanism behind it have never been completely explained. The present work outlines the effect of Cadmium ion doping on structural, optical, electrochemical impedance and photocatalytic properties of ZnO and Cd-ZnO nanoparticles prepared via simple precipitation method. The prepared Cd-ZnO was characterized using advanced techniques such as powder X-ray diffraction, field-emission scanning electron microscope fitted with energy dispersive X-ray diffraction, UV-visible absorbance spectroscopy and electrochemical impedance were performed on modification of glassy carbon electrode (GCE) with multi-walled carbon nanotubes decorated with ZnO and Cd-ZnO. Moreover, Cd-ZnO NPs displayed superior electrochemical performance than undoped ZnO NPs. Experiments revealed that, as compared to ZnO and higher dopant concentration, the photodegradation of crystal violet was enhanced by the use of 0.5 mol% Cd-ZnO catalyst. The implantation of Cd is assisted by faster carrier separation and transfer efficiency. Tauc's plots illustrate that the incorporation of Cd ion has decreased the optical band-gap of ZnO samples. After Cd doping, the valence and conduction band of ZnO were modified, contributing to the development of O2‐, resulting in a significant improvement in photodegradation. Mulliken atomic charges and molecular properties computed at B3LYP/6-31G level of theory. The HOMO-LUMO band gap of alleged photocatalysts was calculated using density function theory (DFT). Further to support our hypothesis on degradation of the dye using Cd-ZnO was confirmed by the IR and Mass spectral studies. Computational prediction of toxicity of degraded organic molecules was estimated by Toxicity Estimation Software Tool (TEST) by United States Environmental Protection Agency. FTIR and Mass spectroscopy confirm the 100% degradation of CV-dye under UV chamber (6 W) after 30 min at basic medium. The enhanced electrochemical and photocatalytic efficiency of Cd doped ZnO nanoparticles suggest that they have dual activity in energy and water treatment applications.

Effect of glutathione capping on the antibacterial activity of tin doped ZnO nanoparticles

In the present study, we first synthesized pristine ZnO and Sn doped ZnO nanoparticles (NPs) then capped the synthesized NPs by Glutathione at different capping concentrations. The structural, morphological, and optical properties of synthesized NPs was examined by x-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), UV–visible, and energy resolved photoluminescence spectroscopy. The XRD analysis confirmed the formation of wurtzite crystal structure of ZnO nano-crystallites while TEM micrographs showed the formation of dendrite nanostructure at higher capping concentration of glutathione in Sn doped ZnO NPS. Strong absorption band around 2525 cm−1 in the IR spectrum indicates the presence of S–H stretching vibration of glutathione which has been disappeared in glutathione capped ZnO NPs. Blue shift in absorption edge was observed in glutathione capped ZnO NPs as compared to uncapped NPs. Moreover, the synthesized NPs showed excellent response against both Gram positive bacteria (Streptococcus sp. & Staphylococcus sp.) and Gram negative (E. coli & Klebsiella sp.) bacteria. Maximum zone of inhibition has been observed with glutathione capped Sn doped ZnO nanostructures. Hence, it can be concluded from the present investigation that the optimization of capping concentration will be very beneficial to exploit Sn doped ZnO semiconductor nanostructures as a coating material for different chemotherapeutic drugs.

Influence of Gd doping on morphological, toxicity and magnetic properties of ZnO nanorods

Properties of chemically synthesized Zn1−xGdxO (x = 0.01, 0.03 and 0.05) nanorods were explored through microstructural, morphological, toxicity and magnetic characterizations. Hexagonal wurtzite structure of all the samples without any impurity phases was confirmed by X-ray diffraction characterization. The micrographs of transmission electron microscope and field emission scanning electron microscope revealed the rod shape morphology of the samples. A systematic evolution of room temperature ferromagnetism of ZnO nanorods with increasing Gd doping content was observed. Gd doped ZnO nanorods showed narrow-spectrum antibacterial activity against the most opportunistic Gram-positive pathogen bacterium, S. aureus as compared to commercial ZnO. From cytotoxicity assessment with Caenorhabditis elegans and Danio rerio, the samples were non-toxic or showed little toxicity at the highest concentration tested (100 µg/mL) except for 1% Gd doped ZnO. Our study demonstrates the possible usefulness of the materials in spintronic, magnetic media device as well as bio-medical applications.

Ball Milling synthesis and characterization of highly crystalline TiO[formula omitted]-ZnO hybrids for photovoltaic applications

In this work, Ball Milling is investigated as a viable synthesis method for highly crystalline TiO2-ZnO composites. The composites were verified using various standard techniques. XRD measurements confirmed the presence of hexagonal wurtzite ZnO and tetragonal TiO2 nano particles. Both XRD and transmission electron microscopy show a mean crystallite size between 12.7 and 15.0 nm. The blend compatibility of the two oxides was investigated by varying the molar ratio of ZnO from 0 to 30%. It is apparent that the morphology compatibility of ZnO and TiO2 plays a significant role in the performance of the final device. The composite specific surface area is seen to increase with ZnO doping. UV–Vis measurements show that its band gap decreases from 3.281 to 3.221 eV. UV–Vis further demonstrated a red-shift of TZHO absorption band which enhances the ability of hybrids to absorb in the visible wavelength range. Scanning electron microscopy suggests that TiO2 and ZnO are morphologically well-matched, and can be used as the electron transport layer in a blended perovskite solar cell. A maximum efficiency of 8% was measured on the PSC with 30% ZnO with Isc=18.4 mA, Voc=0.69 V, and FF=0.65. This efficiency is comparable for PSCs with the hybrid oxide synthesized using other methods, showing that Ball Milling is also a viable method.

A Structural and ac Conductivity Study on Li Doped ZnO

In the present work, the incorporation of Li in a wurtzite ZnO host synthesized through a chemical route has been studied experimentally. Starting with Yoshio’s model, [K. Yoshio, A. Onodera, H. Satoh, N. Sakagami and H. Yamashita, Ferroelectrics, 264, 133 (2001)] structural refinement of the powder diffraction data suggests that Li has a mixed occupancy at both the substitutional 2b (≈1.2%) and interstitial 2a (≈12.6%) Wykoff sites. In comparison with a pure ZnO sample, the net result appears to be an overall expansion of the unit cell by ∼0.07% along both a and c -axes. Temperature dependant ac conductivity measurements carried out using a two electrode assembly and the influence of superimposed dc bias suggest that the conduction may be governed by both the presence of interstitial Li and polar defect complexes also. The grains seem to respond more to an applied dc bias (an order of magnitude change in resistivity) than grain boundaries.

Fabrication of Cr2O3: ZnO Nanostructure Thin Film Prepared by PLD Technique as NH3 Gas Sensor

Chromium oxide (Cr2O3) doped ZnO nanoparticles were prepared by pulsed laser deposition (PLD) technique at different concentration ratios (0, 3, 5, 7 and 9 wt %) of ZnO on glass substrate. The effects of ZnO dopant on the average crystallite size of the synthesized nanoparticles was examined By X-ray diffraction. The morphological features were detected using atomic force microscopy (AFM). The optical band gap value was observed to range between 2.78 to 2.50 eV by UV-Vis absorption spectroscopy, with longer wavelength shifted in comparison with that of the bulk Cr2O3 (~3eV). Gas sensitivity, response, and recovery times of the sensor in the presence of NH3 gas were studied and discussed. In the present work, we found that the sensitivity was increased upon increasing the concentration ratio from 3 to 5%wt of ZnO, whereas it was decreased again over that value. Also, we found that the sensitivity was increased when increasing operating temperature, while the response time was decreased. The optimum concentrations ratio for NH3 gas sensitivity at 5%wt ZnO revealed sensitivity of 66.67% and response time of 14s at operating temperature of 300oC and 700mJ PLD energy.

Green synthesis strategy for producing doped and undoped ZnO nanoparticles: their photocatalytic studies for industrial dye degradation.

A facile and green method for synthesizing Zinc Oxide nanoparticles (ZnO NPs) was successfully carried out using unutilized sweet lime; i.e., Citrus Limetta rind pulp (U-CLRP) extract. The structural, morphological and optical studies were elucidated to confirm the crystallinity, size, and shape of the synthesized NPs. Copper doping on ZnO NPs (CZnO NPs) was carried out to enhance the optical properties. The as-prepared and doped nanoparticle's potential for efficient degradation of a commercial dye, Methylene Blue (MB), was tested under Ultraviolet (UV) and visible light radiation. ZnO NPs showed promising results for dye degradation while an improved result was witnessed for CZnO NPs. ZnO NPs showed 74% of degradation of MB dye under UV irradiation and 57% degradation under visible light radiation. CZnO NPs presented 85 and 90% degradation in the UV and visible radiation, respectively. This green reduction method utilizing biological waste sources shows a promising path for photocatalysts to be developed economically as well as efficiently in the future. These CZnO/ ZnO NPs proved their potential for embedding them as efficient catalysts for wastewater treatment plants of textile industries.

Effects of substrate temperature and sodium source temperature on Na doping of ZnO by MOCVD

In this paper, Na-doped ZnO thin films were grown by MOCVD using cyclopentadienylsodium (CpNa) as the sodium source. The effects of substrate temperature and sodium source temperature on the properties of Na-doped ZnO thin films were studied. SIMS measurement verified that higher substrate temperature would result in higher Na doping content in ZnO:Na thin films. SEM and Hall measurements revealed that both surface morphology and electrical properties of ZnO:Na thin films showed nonlinear variation with substrate temperature and sodium source temperature. When the substrate temperature or the sodium source temperature was increased, the surface morphology of ZnO:Na thin films turned to sheet structures tilted in all directions first and then back to perpendicular hexagonal structure dominant morphology, and the hole concentration increased first and then decreased. This nonlinear variation trend was ascribed to be related to the variation of doping content and impurity position in ZnO:Na thin films, especially the Na impurity position.

Electrochemical reduction of CO2 using oxide based Cu and Zn bimetallic catalyst

Electrochemical reduction of carbon dioxide (ERCO2) driven by non-conventional sources of energy is a favorable approach to generate closed loop CO2 cycle. It will reduce CO2 emissions by lowering CO2 levels in the atmosphere and converting it into high-energy-density carbon-based fuels like alcohols and hydrocarbons. Monometallic copper based catalysts are largely reported for ERCO2 to a variety of industrially important carbon products. But, challenges such as high over potential, uncontrollable product selectivity, and catalyst deactivation inhibit their progress. Oxide derived bimetallic CuZnx electrocatalysts have been suggested as alternatives for achieving high selectivity for different valuable products. In this study, the co-precipitation approach has been used to prepare CuO with various quantities of ZnO dopants. HRTEM, FESEM, XRD, and XPS were used to conduct detailed morphological and elemental studies on the electrode. The electrochemical performance was measured using H-type electrochemical cell in aqueous 0.1 M KHCO3 electrolyte. The evaluation of different CuO-ZnOx (where x=5, 10, 15 and 20 wt.%) catalysts for ethanol productivity and Faradaic efficiency are of particular importance. Amongst studied electrocatalysts, the highest Faradaic efficiency of 22.27 % was obtained for ethanol using CuO-ZnO10 at -0.80 V (vs RHE) with the production rate of about 121 µmol h−1 L−1. The optimized electrode (CuO-ZnO10) shows long term stability for at least 12 h. Post characterization of the catalyst was also conducted to obtain an insight into the active sites, which indicated that CO2 reduction took place on reduced oxide sites (i.e. metallic sites) rather than metal oxides.

Dielectric Properties of ZnO-Based Nanocomposites and Their Potential Applications

Energy storage devices constitute one of the research areas in recent years. Capacitors are commonly used for the storage of electrical energy. The current research is focusing on not only the improvement in energy density but also the materials which are environment friendly. Polymer composites are known to be technically essential materials owing to their wide range of applications. Enormous research has been devoted to zinc oxide- (ZnO-) based polymer nanocomposites, due to their extraordinary dielectric properties. This review article presents a detailed study of the dielectric properties of ZnO-based nanocomposites. The dielectric constant study includes the effect of transition metals and rare earth metals as a dopant in ZnO. This review gives an insight into the mechanism responsible for the variation of dielectric constant in ZnO nanocomposites due to various factors like size of nanoparticles, thickness of the thin film, operating frequency, doping concentration, and atomic number. The observations have been summarized to convey the mechanism and structural changes involved in the ZnO nanocomposites to the researchers. The deployment of biodegradable nanocomposite materials is expected to open an innovative way for their outstanding electronic applications as storage materials.

Evidence of Piezoelectric Potential and Screening Effect in Single Highly Doped ZnO:Ga and ZnO:Al Nanowires by Advanced Scanning Probe Microscopy

Evidence of Piezoelectric Potential and Screening Effect in Single Highly Doped ZnO:Ga and ZnO:Al Nanowires by Advanced Scanning Probe Microscopy

Morphological and optical property study of Li doped ZnO produced by microwave-assisted solvothermal synthesis

ZnO materials have been at the centre of many studies for decades. Doping of ZnO by lithium atoms is a prospective approach for compensation of n-type conductivity in the unintentionally doped ZnO aimed at obtaining p-type semiconductor. In this study, we have synthesized ZnO rod-like powders doped with lithium ions (0–0.65 atom%) by the new microwave-assisted solvothermal method in order to obtain greater photoluminescence intensity of ZnO emissions in the UV region. The obtained powders contain nanoparticles from 20 nm up to 250 nm depending on Li content. X-ray diffractometry and Raman spectroscopy were employed to characterise the structure of ZnO powders, scanning electron microscopy was used to determine the morphology, and most importantly the photoluminescence technique was used to investigate the optical properties of the obtained materials. As a result, the photoluminescence intensity has been increased up to an order of magnitude with the Li content in the ZnO crystals. The increase of the excitation laser power has led to an increased carrier lifetime and photoluminescence response intensity that is observed for all the samples.

Recent Advances in ZnO-Based Carbon Monoxide Sensors: Role of Doping.

Monitoring and detecting carbon monoxide (CO) are critical because this gas is toxic and harmful to the ecosystem. In this respect, designing high-performance gas sensors for CO detection is necessary. Zinc oxide-based materials are promising for use as CO sensors, owing to their good sensing response, electrical performance, cost-effectiveness, long-term stability, low power consumption, ease of manufacturing, chemical stability, and non-toxicity. Nevertheless, further progress in gas sensing requires improving the selectivity and sensitivity, and lowering the operating temperature. Recently, different strategies have been implemented to improve the sensitivity and selectivity of ZnO to CO, highlighting the doping of ZnO. Many studies concluded that doped ZnO demonstrates better sensing properties than those of undoped ZnO in detecting CO. Therefore, in this review, we analyze and discuss, in detail, the recent advances in doped ZnO for CO sensing applications. First, experimental studies on ZnO doped with transition metals, boron group elements, and alkaline earth metals as CO sensors are comprehensively reviewed. We then focused on analyzing theoretical and combined experimental–theoretical studies. Finally, we present the conclusions and some perspectives for future investigations in the context of advancements in CO sensing using doped ZnO, which include room-temperature gas sensing.

Highly Effective Self-Propagating Synthesis of Lamellar ZnO-Decorated MnO2 Nanocrystals with Improved Supercapacitive Performance.

A series of MOx (M = Co, Ni, Zn, Ce)-modified lamellar MnO2 electrode materials were controllably synthesized with a superfast self-propagating technology and their electrochemical practicability was evaluated using a three-electrode system. The results demonstrated that the specific capacitance varied with the heteroatom type as well as the doping level. The low ZnO doping level was more beneficial for improving electrical conductivity and structural stability, and Mn10Zn hybrid nanocrystals exhibited a high specific capacitance of 175.3 F·g−1 and capacitance retention of 96.9% after 2000 cycles at constant current of 0.2 A·g−1. Moreover, XRD, SEM, and XPS characterizations confirmed that a small part of the heteroatoms entered the framework to cause lattice distortion of MnO2, while the rest dispersed uniformly on the surface of the carrier to form an interfacial collaborative effect. All of them induced enhanced electrical conductivity and electrochemical properties. Thus, the current work provides an ultrafast route for development of high-performance pseudocapacitive energy storage nanomaterials.

Impact of ultralow yttrium concentration on formation, morphology and optical properties of DC magnetron co-sputtered yttrium-doped ZnO films

In this article, we reveal the effect of ultralow concentration of Y dopant on the formation, structure peculiarities and optical properties of yttrium-doped ZnO (YZO) films. The YZO films were deposited on glass substrates at 250 °C and at room temperature (HT and RT, correspondingly) with the magnetron co-sputtering method in a direct current (DC) regime. The Y content in the ZnO matrix was varied by DC power applied to the Y target, and did not exceed 0.14 and 0.11 at.% for RT and HT regimes, correspondingly. For the first time it was revealed that the presence of ultralow Y dopant in the co-sputtering process at RT abruptly increases the growth rate of the YZO films by 20%, enlarges the size of crystallites and degree of texture, and cardinally changes the morphology. For the HT regime, the smooth increasing of the deposition rate and the morphological changing of the YZO films with an increase in the Y content were registered. The deposited YZO films demonstrated a high level of optical transparency in the visible and IR region. Ultralow Y doping of ZnO led to decreasing of the bandgap width. The ZnO and YZO films possessed the resistivity typical for insulators even at the highest Y content at both RT and HT deposition regimes.

Tuning of Optical Band Gap: Genesis of Thickness Regulated Al Doped ZnO Nano-Crystalline Thin Films Formulated by Sol–Gel Spin Coating Approach

Tuning of Optical Band Gap: Genesis of Thickness Regulated Al Doped ZnO Nano-Crystalline Thin Films Formulated by Sol–Gel Spin Coating Approach

Tailoring the Performance of ZnO for Oxygen Evolution by Effective Transition Metal Doping.

In the quest for active and inexpensive (photo)electrocatalysts, atomistic simulations of the oxygen evolution reaction (OER) are essential for understanding the catalytic process of water splitting at solid surfaces. In this paper, the enhancement of the OER by first-row transition-metal (TM) doping of the abundant semiconductor ZnO was studied using density functional theory (DFT) calculations on a substantial number of possible structures and bonding geometries. The calculated overpotential for undoped ZnO was 1.0 V. For TM dopants in the 3d series from Mn to Ni, the overpotentials decreased from 0.9 V for Mn and 0.6 V for Fe down to 0.4 V for Co, and rose again to 0.5 V for Ni and 0.8 V for Cu. The overpotentials were analyzed in terms of the binding to the surface of the species involved in the four reaction steps of the OER. The Gibbs free energies associated with the adsorption of these intermediate species increased in the series from Mn to Zn, but the difference between OH and OOH adsorption (the species involved in the first, respectively the third reaction step) was always in the range 3.0-3.3 eV, despite a considerable variation in possible bonding geometries. The bonding of the O intermediate species (involved in the second reaction step), which is optimal for Co, and to a somewhat lesser extend for Ni, then ultimately determined the overpotential. These results implied that both Co and Ni are promising dopants for increasing the activity of ZnO-based anodes for the OER.

Signature of strong localization and crossover conduction processes in doped ZnO thin films: synergetic effect of doping fraction and dense electronic excitations

Ga x Zn1−x O thin films with varying Ga fraction within the solubility limit were irradiated with high-energy heavy ions to induce electronic excitations. The films show good transmittance in the visible region and a reduction of about 20% in transmittance was observed for irradiated films at higher ion fluences. The Urbach energy was estimated and showed an augmenting response upon increase in doping fraction and ion irradiation, this divulges an enhancement of localized states in the bandgap or disorder in the films. The evolution of such localized states plays a vital role in charge transport and thus the temperature dependent electrical conductivity of irradiated thin films was studied to elucidate the dominant conduction mechanisms. The detailed analysis unfolds that in the high-temperature regime (180 K < T < 300 K), the charge conduction was dominated by thermally activated band conduction followed by the nearest neighbor hopping (NNH) mechanism. Whereas in the lower temperature regime (25 K < T < 170 K), the conduction mechanism was governed by Mott-VRH (variable range hopping) followed by Efros–Shklovskii (ES)-VRH. A sudden and steep rise in resistivity below 30 K was observed for GZO films with higher doping fraction at higher ion fluence and proclaims the presence of strong localization of carriers.