Doped ZnO Nanocrystals Research Articles

Improved photocatalytic degradation of malachite green, crystal violet and antibacterial activity of mesoporous Zn1-xCoxO nanoflakes

ABSTRACT The photocatalytic ability of the Zinc Oxide nanoparticles can be significantly enhanced through transition metal doping. In this work, Zn1-xCoxO nanoparticles were synthesised by a chemical coprecipitation method at room temperature. XRD studies indicated non-monotonic decrease in the Scherrer’s crystalline size from 19.79 nm for the pristine ZnO NPs to 14.81 nm for 2.5% Co2+ doped ZnO nanocrystals. The morphology of the nanoparticles was revealed by FESEM to resemble nanoflakes, and the existence of Co, Zn and O elements was verified by EDS findings. Tauc plots demonstrated a shrinkage of the optical band gap from 3.15 eV to 2.98 eV. The mesoporous nature and noteworthy increase in specific surface area (from 22.198 m2/gm to 54.159 m2/gm) were revealed by BET analysis of the N2 adsorption of the nanoflakes. Further, the degradation experiments of cationic dyes Malachite Green (MG) and Crystal Violet (CV) were carried out in the presence of synthesised catalysts under a high-pressure mercury vapour (450W-UV) lamp. Among the doped ZnO nanocatalysts, 2.5% Co2+ doped ZnO nanoflakes showed the most efficient photocatalytic degradation against MG (97.87%) and CV (93.27%) in 20 minutes. Furthermore, antibacterial activity was investigated against pathogenic bacteria Staphylococcus aureus, Bacillus subtilis and Escherichia coli which revealed improved bactericidal effects of 2.5% Co2+ doped ZnO nanoflakes. Hence, the current finding explored 2.5% Co2+ doped ZnO nanoflakes can serve as dual role efficient and potential candidate for eradication of organic as well as microbial contaminants from wastewater.

Optical behavior, TL defect center analysis and photocatalytic characterization of Ag2O doped ZnO nano crystals

The objective of the study is to prepare Ag+-dopedZnO nano crystals and to assess their structure, optical behavior, TL analysis, photocatalytic activity. Using sol–gel technique the Ag+-doped ZnO nano crystals prepared. Using the FT-IR, XRD, BET surface area, SEM, EDX characterization techniques, we have examined the structure of the developed samples. The TL analysis of samples was done under a dose of 25 kGy gamma irradiation. Trap depth parameters of the samples evaluated. Which suggested the ZnO nano crystals doped with Ag+ ions are purely TL active. The optical studies such optical absorption, photoluminescence of samples reveal their optical behavior. The optical band gap of the samples also evaluated. Which is increasing with increase in concentration of Ag+ ions in ZnO nano crystals. Using the MV (10 ppm) and CR (10 ppm) dyes, under visible light the photocatalytic activity of the samples reported. At a reaction period of 90 min, the nano powdercontaining 1.0 mol% Ag+-dopedZnO nano crystals and a band gap energy of 2.75 eV demonstrated the best photocatalyticefficiency (∼91 %) in both colors. The evaluationwas done on the impact of radical scavenger research and catalyst dosage on degrading efficiency. The rate of reaction in the presence of 1.0 mol% Ag+-dopedZnO nano crystals was four times higher than the ZnO catalyst's rate constants, and the degradation reaction follows pseudo-first-orderkinetics. The degrading process shows pseudo-first-order kinetics, and the reaction rate was four times higher than the ZnO catalyst in the presence of 1.0 mol% Ag+-dopedZnO nano crystals. It took three cycle runs for the degrading efficiency to drop by 1 %. Thus, 1.0 mol% Ag+-dopedZnO nano crystals was employed as a promising photocatalyst with strong reusability potential for the degradation of textile colors, which will be very beneficial.

Structural, optical and magnetic properties of Zn0.98Co0.02O dilute magnetic semiconductor nanoparticles

Doped ZnO nanoparticles (NPs) are one of the most promising dilute magnetic semiconductors (DMSs). Zn0.98Co0.02O NPs were synthesized using an aqueous co-precipitation technique. XRD confirmed phase purity and the crystallite size was determined to be 20 nm using the Debye-Scherrer method. For the doped NPs, distinct surface morphology was seen in FE-SEM images. The excitonic absorption peak appeared at 371 nm in pristine ZnO NPs which blue-shifted to 363 nm on 2 % Co-doping. The doped ZnO nanocrystals exhibit considerable ferromagnetic functionality at 7 K and 300 K. The asymmetry in the hysteresis curves confirm the presence of exchange bias and spin canting in the system. The results obtained suggests that the doped NPs could be potentially useful for enhanced optoelectronics and spintronics applications.

Magnetic anisotropy in the ferromagnetic Mn and N doped ZnO Nanocrystals

(Mn, N)- codoped ZnO thin films were prepared by two step process. Study of Magnetic anisotropy in Mn and N codoped ZnO nanoparticles was studied. X-ray diffraction (XRD) &X-ray photoelectron spectroscopy (XPS) reveals that Manganese (Mn) and Nitrogen (N) ions in ZnO lattice exists as Mn2+ and N3-. Hysteresis measurement shows that at room temperature the samples show ferromagnetic nature and also indicate that the samples are having magnetic anisotropic behavior. This ferromagnetic nature in the samples is because of the hybridization between 2p of N & 3d of Mn state. The compound formed, corresponds to Mn-N-Mn cluster, which is found to be energetically strong enough for ferromagnetism at room temp.

Dependence relationship between doping modes and surface plasmon resonance properties of doped ZnO nanocrystals

The use of doped semiconductors to achieve plasmonic frequencies in the middle infrared (mid-IR) has been recently proposed as a promising way to obtain high-quality and tunable plasmonic materials. Plasmonic materials have great static or dynamic tunability. In some cases, choosing a dopant over another is an important trade-off to optimize the plasmonic performance. In this work, trivalent metal ion (M)-doped ZnO nanoparticles (NPs, M=Al, In) were successfully synthesized with highly consistent synthesis conditions. Interestingly, surface plasmon resonance (SPR) can only be observed in Al-doped ZnO (AZO) NPs, but not in In-doped ZnO (IZO) NPs. We used X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and a first principle calculation to clarify the proper dopant sites (replaceable or interstitial doping) of Al3+ and In3+ in the host nanocrystals of ZnO. In addition, these results assisted by density of states (DOS) analysis indicate that different doping modes influenced the electronic structures in different degrees, and may lead to the opposite SPR phenomenon. Our work deepens the understanding of the relationship between doping sites in the host nanocrystals and SPR properties and give important guidance to the develop the doped semiconductors with plasmonic performance.

Crystallite size controlled doping of transition metals (Co, Cu) in ZnO nanocrystals to investigate microstructural, optical, magnetic, and photocatalytic properties

In this report, Co and Cu doped ZnO nanocrystals with controlled crystallite size were synthesized. The formation of the hexagonal wurtzite phase is confirmed by Rietveld refinement of X-ray diffraction (XRD) patterns. Crystallite size ranges (17 nm–20 nm) obtained from XRD analysis are complemented with transmission electron microscopy (TEM) observation. The presence of E2high mode recognized the unchanged phase and good crystalline nature of the nanocrystals. Photoluminescence (PL) spectra of Co and Cu doped ZnO nanocrystals are related to zinc and oxygen based defect states. The quenching of PL intensity is due to the incorporation of Co and Cu in the ZnO nanocrystals. ESR results complement the decrease in ferromagnetic order as obtained by VSM measurement in both Co and Cu doped ZnO nanocrystals. The superior photocatalytic activity of Cu doped ZnO nanocrystals could be due to the higher electron trapping capability of Cu2+ ions.

Influence of surface modification by organic molecules on optical properties of Eu3+-doped ZnO nanocrystals

We have fabricated Eu3+-doped ZnO (ZnO:Eu) nanocrystals (NCs) surface-modified with organic molecules such as dodecylamine (DDA) and tri-n-octylphosphine oxide (TOPO) and have studied their optical properties. In the ZnO:Eu NCs without surface modification, strong broad photoluminescence (PL) band due to surface defects is observed, so the Eu3+ PL peaks are not observed because they are covered by the strong surface-defect PL band. In the DDA-capped ZnO:Eu NCs, the Eu3+ PL and the exciton PL are observed because of suppression of the surface-defect PL caused by the inactivation of the surface defects. Contrary to expectations, the surface modification with TOPO suppressed not only the surface-defect PL but also the exciton PL. As a result, the ZnO:Eu NCs with red PL due to the Eu3+ ions have been successfully prepared. We discuss the influence of surface modification by DDA and TOPO on optical properties.

Coexistence of ferromagnetic-antiferromagnetic ground state, exchange bias effect and bandgap narrowing in Cr-doped ZnO nanocrystals derived by simple chemical method.

A detailed investigation of the structural, optical and magnetic properties of Cr-doped ZnO nanostructures obtained via a simple chemical method has been carried out. The structural study using X-ray powder diffraction indicates the hexagonal wurtzite structure for undoped ZnO and ZnO doped with 1% Cr, whereas the appearance of a secondary cubic phase (ZnCr2O4) is witnessed with the incorporation of Cr content ≥3% in the ZnO compounds. Furthermore, the secondary phase is observed to increase systematically with the increase of the Cr concentration. Field emission scanning electron microscopy and high-resolution transmission electron microscopy studies indicate cuboid, hexagonal and rod-type structural morphology in all the nanocrystals. The presence of the cubic structure along with the hexagonal structure is further confirmed from the selected area electron diffraction pattern. Raman spectroscopy has been used to study the crystalline quality, defects and disorder present in the host lattice. UV-visible spectra were obtained to study the effect of Cr doping on the optical absorption and hence to determine the bandgap, and show a decrease in bandgap with increasing Cr concentration. PL spectra show near-band-edge emission along with visible emission, which decreases with a higher concentration of Cr-doped nanocrystals. X-ray photoelectron spectroscopy analysis indicates the incorporation of Cr3+ and Cr2+ ions in the ZnO lattice. Detailed magnetic studies reveal the co-existing ferromagnetic (FM) and antiferromagnetic (AFM) ground states, which result in the observation of an exchange bias (EB) effect in all the doped compounds. The observation of an EB effect arises from the coupling between the FM and AFM components in the Cr-containing ZnO nanocrystals, and provides a way to design new principles and materials platform that are useful for futuristic spintronic devices.

Densification of Doped Zinc Oxide Nanocrystal Films via Chemical Bath Infiltration

AbstractColloidal nanocrystals (NCs) hold great promise for the fabrication of thin film devices due to the ability to precisely control their properties and deposit coatings via solution‐based protocols. The role of interfaces, surface defects, and (electrical) connectivity between NCs is a major bottleneck to achieving the desired performance. Here, a novel method to infiltrate and densify nanocrystalline coatings deposited from doped ZnO Ncs is presented. Reduced porosity, enhanced connectivity, and a reduction in surface defects are observed, culminating in vastly improved electrical conductivity. Doped ZnO NCs are processed into an ink and used for thin film deposition. Bulky native ligands are then removed from the film to promote better electrical contact between neighboring NCs and render their surfaces hydrophilic. Afterward, a modified chemical bath deposition (CBD) is adopted to slowly infill the pores between the NCs with pure ZnO via a controlled heterogeneous nucleation process. The optimized CBD avoids excessive surface growth and premature sealing of the film surface, confirmed by combined spectroscopic and morphological characterizations. The resultant hybrid films demonstrate enhanced electrical properties, with conductivity increasing by two orders of magnitude after pore infiltration. These results pave the way to hybrid functional coatings with enriched interparticle communication.

Structural and optical investigations in undoped and rare-earth doped zinc oxide nanoparticles for photon upconversion based applications

The present work reports comprehensive investigations of the physical properties of undoped and rare-earth (RE3+) doped ZnO based upconversion nanoparticles. X-ray diffraction analysis of these nanoparticles reveals the hexagonal wurtzite structure. The induced-strain value of the RE3+ doped ZnO nanoparticles increased by almost one order of magnitude, and their mean particle size reduced from 36 nm to 17 nm. The scanning electron microscopic images showed that the particles retained the spherical shape in the undoped and doped conditions. The elemental maps confirmed the uniform distribution of the RE3+ dopants. The recorded absorbance spectrum for the doped nanoparticles showed significant absorption in ultraviolet and near infra-red regions owing to the band-band transitions and the deep defect level transitions that are induced due to intrinsic and extrinsic defect states. A detailed analysis of the defect states of these samples is described based on the Urbach energy studies. An intense peak observed at 980 nm suggests the possibility of photon upconversion in the doped nanoparticles. A simplified energy diagram (as depicted below) has demonstrated the multi-photon absorption and possible upconversion mechanisms. The optical absorption spectrum of RE3+ doped ZnO nanocrystals encourages the promising application of these materials in diverse fields, particularly in biomedical and photovoltaic applications.

Photoluminescent neodymium-doped ZnO nanocrystals prepared by laser ablation in solution for NIR-II fluorescence bioimaging

The work reports on the use of laser ablation and post-ablation irradiation techniques for the preparation Nd3+ doped ZnO nanoparticles (NPs). The focus has been made on photoluminescence of Nd-doped ZnO NPs in the second near infrared (NIR-II) spectral window (1000–1700 nm) of the biological transparency. Morphology, phase composition and optical properties of the synthesized NPs were studied by absorption and photoluminescence spectroscopy, X-Ray diffraction (XRD) and transmission (TEM) electron microscopy. Near-infrared luminescence of Nd3+ doped ZnO nanocrystals in the region of 1000–1400 nm was detected both upon excitation from the ground state (800 nm) and upon UV excitation. The latter proves the incorporation of the Nd3+ into ZnO lattice as photoluminescence occurs through the transfer of excitation energy from the ZnO matrix to the Nd3+ ion. The possibility of control over the luminescence properties by a variation of solvent composition and by additional laser irradiation was demonstrated.

Significant Enhancement in Electrical Conductivity of Al doped ZnO Nanocrystals

Significant Enhancement in Electrical Conductivity of Al doped ZnO Nanocrystals

Humidity sensing enhancement and structural evolution of tungsten doped ZnO nanosensors fabricated through co-precipitation synthesis

In this work, pure and tungsten (W) doped ZnO nano-crystals have been synthesized through the co-precipitation technique. The prepared samples are employed to study the effect of tungsten doping on the structural, morphological, and humidity sensing properties of ZnO nanomaterials. The structural and morphological studies have been carried out using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM), respectively. The XRD of the synthesized nanomaterials reveals a hexagonal wurtzite structure and a reduction of crystallite size from 44.46 nm to 37.38 nm with increase in doping concentration. The SEM images reveal a cuboidal columnar growth. Finally, the samples have been exposed to a humidity range from 10% to 90% RH and observed that the sensitivity of the sensor is enhanced as the concentration of tungsten is increased. The sample doped with 1.75 mol% tungsten demonstrates the highest sensitivity of 144% among all the samples with good stability.

The sensitivity of pristine and Pt doped ZnO nanoclusters to NH3 gas: A transition state theory study

The sensitivity of pristine and Pt doped ZnO nanocrystals to ammonia gas are calculated using density functional theory combined with transition state theory. NH3 is adsorbed on the surface of pristine ZnO without reaction. However, Pt catalyzed ZnO reacts by oxidizing NH3 to nitrogen oxide and water. A small increase in the energy gap of pristine ZnO nanocrystal at the order of 0.05 eV transforms the pristine ZnO to a weak p-type ammonia sensor. On the other hand, the oxygen is removed by NH3 reaction from the surface of Pt/ZnO with a reduction of the energy gap (n-type). The sensitivity of the Pt/ZnO sensor increases with temperature due to an increase in the number of reactions of the NH3 molecules until nearly 330 °C. However, when the temperature is increased further, the sensor sensitivity decreases due to the reaction of NH3 with oxygen in the ambient air instead of picking oxygen from Pt/ZnO particles. As we reach the autoignition temperature of ammonia at 650 °C, most NH3 gas reacts with ambient air before it reaches the Pt/ZnO surface of the sensor. The present results are in agreement with available experimental results.

Conduction mechanisms and dielectric constant features of Fe doped ZnO nanocrystals

Fe doped ZnO nanocrystals (NCs) were synthesized from low cost sol gel method and the effect of iron content on their structural, transport and dielectric properties was studied. X-ray diffraction (XRD) measurements exhibit good crystalline quality of all the prepared NCs and a reduction of the crystallite size with Fe doping. Raman spectra confirmed the good crystallinity and revealed the formation of ferric phases for highly doped samples. Investigations of the impedance characteristics indicate that the samples are perfectly modulated from equivalent circuits using a constant phase element (CPE). Analysis of complex impedance and electrical modulus revealed a relaxation process dependent on temperature and of non-Debye type. Single activation energy was estimated for the electronic conduction, based on the Arrhenius plots. The conduction mechanism was governed from small polaron hopping in all the ZnO:Fe materials. The frequency dependence of the electric conductivity followed a simple Jonscher power behavior σac(ω)=σdc+Aωs (0.5≤ s ≤ 1), where the exponent s increased with temperature.The dielectric measurements, in both medium and high frequency regions, exhibited high dielectric constant values and low dielectric losses for all the samples, which could favor the use of Fe doped ZnO a as promising candidate for energy storage. The electrical modulus provides double relaxation times with low values. All the properties indicate that the ZnO:Fe material is suitable for device applications.

A comprehensive study on the impact of Gd substitution on structural, optical and magnetic properties of ZnO nanocrystals

Abstract We report here the impact of Gd substitution on structural, optical, photoluminescence and magnetic properties of Zn1−xGdxO (x = 0.02–0.10) nanocrystals. Rietveld refinement of PXRD patterns, SAED and FTIR analysis revealed that Zn1−xGdxO nanoparticles crystallize in single phase wurtzite type hexagonal symmetry. HRTEM & SEM analysis confirmed good crystallinity of Zn1−xGdxO nanocrystals with a wide size distribution in 10–40 nm range. Concentration of Gd3+ ions at the Zn site causes a systematic reduction in the average crystallite size of Zn1−xGdxO nanocrystals. UV-Vis-NIR and photoluminescence spectroscopic studies showed decrement in band gap energy and suppression of UV emission (391–416 nm) with Gd concentration in Zn1−xGdxO. Analysis of the visible band emission spectra demonstrated that 325 nm photoexcited Zn1−xGdxO nanocrystals emit light in the orange-red region due to formation of complex defects in the ZnO lattice. M-H curves of 2% and 4% Gd doped ZnO nanocrystals recorded at 300 K showed thin ferromagnetic hysteresis loop (almost equal Mr =~1.8 memu/g and Hc=~210 & ~136 Oe for 2% and 4% Gd doped ZnO respectively) at lower magnetic fields but in the high field range, the magnetization responded linearly with the applied magnetic fields and, thus establishes coexistence of paramagnetic and weak ferromagnetic ordering in 2% and 4% Gd substituted ZnO nanocrystals. The higher 6%, 8% and 10% Gd substituted ZnO nanocrystals are found to be paramagnetic at 300 K. At 5 K, all the samples showed S-shaped M-H curves with negligible coercivity & retentivity and the magnetization monotonically incremented (5–25 emu/g) with increasing Gd concentration in Zn1-xGdxO. Superparamagnetic clusters and antiferromagnetic states coexist in Zn1−xGdxO nanocrystals at 5 K. Magnetic and optical properties of Zn1−xGdxO nanocrystals are suitable for magneto-optoelectronic applications.

Input of IBA for the study of plasmonic properties of doped ZnO nanocrystals

In this work, we used ion beam analyses (IBA) to characterize doped ZnO crystalline nanoparticles elaborated by different methods: (i) low energy cluster beam deposition (LECBD) for ZnO:Ga, (ii) metal organic framework (MOF) for ZnO:Li and (iii) sol-gel for ZnO/SiOx core-shell system.The analyses were performed either in RBS mode using 4He++ ions of 6 MeV energy (characterization of Ga and Si) or in NRA mode using 1H+ ions of 2.5 MeV energy (characterization of Li). The experimental data allowed determining the mean concentration of dopants (Ga, Li) or shell atoms (Si) and their areal masses. These elemental information, coupled with structural (TEM, X-ray) and optical (IR spectroscopy, photoluminescence) ones, were of prime importance permitting to better understand the optical properties of doped ZnO nanocrystals.

Precession controlled synthesis and ligands assisted modulation of optical properties and Raman scattering in Ag doped ZnO nano-egg

Abstract Highly luminescent Ag doped ZnO nanocrystals were synthesized in controlled manner by wet chemical route at room temperature and effects of Ag doping and three different ligands polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and biotin (BT) attached were investigated. The phase purity, crystallinity and crystallite sizes of as prepared nanoparticles (NPs) were characterized by X-ray diffraction (XRD) technique, which confirms high crystalline hexagonal wurtzite phase for all the samples and has no any toxicity issues. We reported here the egg shape of the Ag doped ZnO nanoparticles in biotin matrix having single domain crystallites as observed from high resolution transmission electron microscopy (HRTEM) images. The electron diffraction results obtained are consistent with the XRD results. The optical properties of the synthesized NPs were examined using UV–visible spectroscopy which shows exceptional band gap of 4.25 eV for the BT caped NPs and is believed to be tuned by Ag doping and quantum confinement in biotin matrix. Photoluminescence (PL) measurements demonstrated that the how Ag+ and different ligands attached can alter the excitonic and defect related emission in the Ag doped ZnO NPs. Room temperature Raman scattering were also reported from all the Ag doped ZnO NPs which shows new phonon scattering mode.

Solar driven photocatalytic properties of Sm3+ doped ZnO nanocrystals

ZnO nanocrystals (NCs) doped with different Sm3+ concentrations were prepared by sol gel method. XRD analysis showed that the ZnO:Sm3+ NCs crystallized in the hexagonal wurtzite structure with a grain size varying from 61.4 nm to 72.6 nm, with Sm3+ concentration. Transmission electron microscopy (TEM) images indicated that ZnO NCs adopted a bimodal size distribution. X-ray photoelectron spectroscopy (XPS) revealed that Sm ions existed in trivalent state and substituted at the Zn2+ sites in the ZnO lattice. Raman spectra highlighted the presence of the LO mode, confirming the successful substitution of Zn2+ by Sm3+. Excitation and emission spectra highlighted the typical 4f-4f transitions of Sm3+. A photoluminescence (PL) quenching accompanied by a decrease of PL lifetime was observed for Sm3+ concentrations above 1.5%. The processes of excitation and de-excitation of the Sm3+ ions in ZnO NCs were discussed based on dipolar interactions between the excited ions. The ZnO:Sm3+ (1.5%) photocatalyst induced complete and fast photodegradation of RhB under sunlight irradiation. The photocatalytic mechanism is discussed based on the analysis of PL lifetimes. The role of oxygen vacancies on the reduction of Sm3+ ions and its impact on the photocatalytic process is also discussed.

Correlation among lattice strain, defect formation and luminescence properties of transition metal doped ZnO nano-crystals prepared via low temperature technique

The present study focuses on the structural-optical correlation of dilute Fe doped ZnO nano-particles synthesized via low temperature chemical route. Polycrystalline Hexagonal wurtzite structure was confirmed from x-ray diffraction. Lattice parameters calculated using the Scherrer’s method are appended with substitutional doping induced deformations, leading to dopant-defect induced broadening, lattice strain and decreased crystallanity using modified W-H analysis. The direct correlation of the peak broadening with defect formation, contraction and expansion of band gap is carried out. The mechanism responsible for the formation of defects, lattice strain and shifting of band gap due to finite size effect of dopant is discussed. The presence of such defects is confirmed from the Photoluminescence spectroscopy. CIE coordinates indicate emission in blue region with variation from cool to warm under the UV excitation. Thus, making them promising candidates for the use as cost-effective phosphors in UV- blue region and in down conversion based luminescent devices.