Hexagonal ZnO Phase Research Articles

Ternary ZnCdSO composite photocatalyst for efficient dye degradation under visible light retaining Z-scheme of migration pathways for the photogenerated charge carriers

Simple co-precipitation method followed by calcination was adopted to synthesize ZnCdSO ternary semiconductor photocatalyst. In the composite material highly crystalline and hexagonal wurtzite phase of ZnO along with hexagonal CdS and cubic CdO phases were separately identified. Oxygen vacancies (VO) in ZnO network and CdO phase resulted in narrowing the optical band gap, facilitating increased visible light absorption. CdS–CdO phase mediated surface defects increased significantly over the core/bulk defects in pristine ZnO, which simultaneously acted as trap-sites for charge carriers and active adsorption-sites for dyes for photocatalytic degradation. In photocatalytic Z-scheme charge transfer mechanism CdS with a higher negative potential conduction band acted as efficient reduction site and created superoxide radical anions (O2•−), whereas ZnO and CdO both having lower valence band potential acted as efficient oxidation sites and created hydroxyl radicals (•OH). Spatial separation of photo-generated charge carriers enhanced their lifetime and facilitated creating superoxide and hydroxyl radicals which, by virtue of their powerful and respective reducing and oxidizing properties, efficiently degraded the dye molecules. The hydroxyl radicals (•OH) and holes (h+) dominantly contributed in sequence, in the dye degradation process, while electrons (e−) and superoxide radicals (O2•−) had gradually lesser inputs to the phenomenon, as evidently identified via radical scavenging experiment with optical absorption study. Thus, photoactive ternary ZnO–CdS–CdO composite, produced via formation of CdO phase upon low temperature (300 °C) calcination, offered a significantly photostable superior photocatalyst for efficient dye degradation under visible light via a double Z-scheme charge transfer mechanism.

Solar active ZnO–Eu2O3 for energy and environmental applications

ZnO–Eu2O3 nanocomposite was fabricated by a simple hydrothermal route. This material forms a potential class of photocatalysts in which the increased absorption behaviour in ZnO–Eu2O3 is expected to couple with the existing characteristics of Eu2O3 and ZnO materials. ZnO–Eu2O3 was characterized using surface analytical (SEM, EDS, HR-TEM, AFM, XRD) and spectroscopic techniques (XPS, DRS,PL). From the XRD patterns, formation of well-crystallized cubic Eu2O3 and hexagonal wurtzite phase of ZnO were inferred. Presence of nanoflake like structure with hexagonal ZnO and cubical Eu2O3 is shown by SEM pictures. ZnO–Eu2O3 possesses higher UV and visible absorption than Eu2O3 and ZnO. ZnO–Eu2O3 produces larger methanol oxidation current indicating its anodic catalytic efficiency in direct methanol fuel cells (DMFCs). This reveals higher electrocatalytic activity of ZnO–Eu2O3 than ZnO. It is observed that at −1.6 V, cathodic current density (ipc) of ZnO–Eu2O3 (−103.17 mA cm−2) for Hydrogen evolution reaction (HER) is more than five times of ZnO (−18.19 mA cm−2) and the hydrogen evolved with ZnO–Eu2O3is 15.6 mL, which is higher than that of ZnO (6.8 mL). This indicates the superior catalytic property of ZnO–Eu2O3 in water splitting. This catalyst exhibited higher catalytic activity of 99.2% in the photodegradation of Rhodamine B (Rh-B) with natural sunlight in 75 min under neutral pH, whereas Eu2O3 and ZnO produced 60 and 82% degradations in the same time. Degradation quantum efficiency by ZnO–Eu2O3 is larger than ZnO and Eu2O3. ZnO–Eu2O3 was stable and reusable. The multifunctionality of this catalyst makes it suitable for energy and environmental applications.

Photocatalytic activity of magnetic core-shell CoFe2O4@ZnO nanoparticles for purification of methylene blue

Magnetic core–shell CoFe2O4@ZnO nanoparticles have been successfully synthesized by using coprecipitation method to provide easy separated nanomaterials and high photocatalytic activity performance. Core–shell nanoparticles with various CoFe2O4-to-ZnO molar ratio (1:2, 1:3, 1:4, 1:5) have been investigated over x-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible spectroscopy, and vibrating sample magnetometer. XRD spectra confirm the cubic spinel ferrite phase structure of CoFe2O4 and the hexagonal wurtzite phase of ZnO. The crystallite size is found within the range of 14.9–20.6 nm. TEM measurement confirms the good crystallinity of the samples. The magnetic hysteresis shows that CoFe2O4@ZnO has high saturation magnetization of about 30 emu g−1 and coercivity of about 300 Oe. Photocatalytic investigation was carried out using methylene blue (MB) under UV irradiation. Our result yields the enhancement of MB degradation as ZnO content increases. The maximum photodegradation achieved by the core–shell nanoparticles is 57.2%, 60.5%, 65.5%, and 78.3% for molar ratio of 1:2, 1:3, 1:4, and 1:5, respectively. The enhancement of MB degradation can be attributed to the formation of internal structure between CoFe2O4 and ZnO in the form of heterojunction structure. The magnetic properties of core–shell lead to the easy separation between the magnetic core–shell nanoparticles and the final degraded solution by permanent magnet.

Effects of growth conditions on properties of CBD synthesized ZnO nanorods grown on ultrasonic spray pyrolysis deposited ZnO seed layers

ZnO nanorods were synthesized on a seed layer coated glass substrate using chemical bath deposition (CBD). Prior to growth, a seed layer had been prepared via ultrasonic spray pyrolysis method. The aim was to explore the influence of varying the chemical bath deposition conditions namely: growth time, bath temperature and concentration levels of the precursor on the orientation, structural, optical and vibrational properties of the subsequently grown nanorods. The presence of ZnO nanorods resembling the hexagonal-wurtzite structure having preference of orientation along the c-axis and varying crystallinity under different growth parameters was confirmed by X-ray diffraction (XRD). Scanning Electron Microscopy (SEM) acquired images of uniformly arranged and vertically oriented ZnO nanorods grown at a relatively higher bath temperature of 90 °C and shorter growth period of 2 h. UV/Vis/NIR spectrophotometer measurements revealed an optical transmittance of between 50 – 70 % for the nanorods. Raman spectroscopy results confirmed the presence of Raman active E2(low) and E2(high) modes corresponding to 98 cm−1 and 478 cm−1 belonging to the hexagonal ZnO phase. This work shows that the orientation, structural, optical and vibrational properties of the grown nanorod structures are controlled via alteration of the growth parameters.

Novel Low Temperature Route to Produce CdS/ZnO Composite Nanofibers as Effective Photocatalysts

In this work, CdS/ZnO composite nanofibers (NFs) were prepared by the electrospinning of a sol–gel comprised of poly(caprolactone), zinc acetate dihydrate, cadmium acetate dihydrate, and ammonium sulfide. The electrospun NF mats were calcined under vacuum in an argon (Ar) atmosphere at 200 °C for 1 h. Standard physiochemical analysis techniques demonstrated the formation of the crystalline hexagonal phase of CdS and ZnO. Composite NFs showed good photocatalytic degradation of methylene blue (MB) dye under visible light irradiation compared to their counterparts. CdS nanoparticles, ZnO nanofibers, and composite NFs photodegraded 35.5%, 47.3%, and 90% of the MB dye, respectively, within 100 min. The reaction kinetics of MB photodegradation using the composite NFs followed the pseudo-first-order relation. Owing to their facile preparation and good photodegradation ability, the proposed method can be used to prepare various photocatalysts for wastewater treatment.

ZnO-CuxO heterostructure photocatalyst for efficient dye degradation

A simple aqueous solution process was used for the synthesis of Cu-loaded ZnO nanorod structures in which highly crystalline and hexagonal wurtzite ZnO, cubic CuO and Cu2O phases coexist. Absorption/emission characteristics identify band gap narrowing, which facilitates visible light absorption, as a consequence of hybridization of O 2p bands with Cu 3 d bands of the ZnO-CuxO heterostructure system. With use of the specific band energy position of the individual components (i.e., ZnO, Cu2O, and CuO), photogenerated electrons and holes could be made to migrate separately and thereby the carrier recombination could be delayed, making them amply available for photocatalytic reactions. The available electrons plausibly produce superoxide radical anion (O2·−), while holes generate hydroxyl radicals (·OH), which are reactive agents for dye degradation. Furthermore, the formation of a depletion region via Cu-induced charge separation within the ZnO-CuxO heterostructure and thermal-energy-mediated abstraction of the electric field barriers and subsequent migration of the positively charged defects from the subsurface region toward the interface lead to efficient photocatalytic performance.

Polythiophene doped ZnO nanostructures synthesized by modified sol-gel and oxidative polymerization for efficient photodegradation of methylene blue and gemifloxacin antibiotic

The present work aims to enhance the photocatalytic performance of mesoporous ZnO by doping with polythiophene (PTh) for the treatment of toxic pollutants. Polythiophene doped ZnO (PTh/ZnO) photocatalysts with various weight percentages of PTh (1–10%) were synthesized via modified sol-gel and oxidative polymerization techniques. The XRD patterns revealed a well-crystalline hexagonal wurtzite phase of ZnO, whereas the XPS, FTIR and Raman spectra confirmed the successful coupling between PTh and ZnO nanostructures. TEM results illustrate the uniform distribution of PTh onto the mesoporous ZnO. UV–vis spectroscopy revealed a small lowering in bandgap energies upon PTh incorporation. The prepared PTh/ZnO nanostructures have been successfully exploited for the photocatalytic degradation of methylene blue dye (MB) and gemifloxacin mesylate (GFM) antibiotic. All doped samples exhibited superior performances in comparison to either pure PTh or undoped ZnO. The 5% PTh/ZnO showed the highest activity with 95 % degradation efficiency of MB dye in 180 min of irradiation with rate constant value 0.0156 min−1 which is almost 2.6 higher than pure ZnO. In addition to this, 5% PTh/ZnO proven to be highly active towards the degradation of gemifloxacin mesylate antibiotic after 180 min of irradiation time, yielding ∼80 % degradation efficiency. Repeated cyclic use of the current photocatalyst indicated excellent reusability and operational stability. The reaction mechanism involved during the photocatalytic degradation is also proposed.

Surface, optical and photocatalytic properties of Rb doped ZnO nanoparticles

The present work describes the synthesis of pure and Rb doped ZnO (ZnO:Rb) nanoparticles by a simple combustion technique. The surface, structural, optical, morphology and photocatalytic activities of the ZnO:Rb nanoparticles were studied with respect to pure ZnO. X-ray diffraction studies confirmed the presence of a hexagonal wurtzite phase of ZnO and a secondary phase Rb2ZnO2 was also observed with increasing content of Rb into the ZnO. Field emission scanning electron microscopy confirmed an irregular shaped morphology for the ZnO:Rb samples. The X-ray photoelectron spectroscopy results confirmed both ZnO and Rb2ZnO2, as a new phase. Time of flight secondary ion mass spectroscopy revealed the homogeneous distribution of the Rb in the ZnO and the enrichment of Rb2ZnO2 at the outside and surface of the particles. In the photocatalytic degradation of RhB dye under natural sunlight irradiation, 97.0%, 20.2%, 16.6%, 15.7% and 12.8% degradation was evidenced for ZnO, Rb 5%:ZnO, Rb 7%:ZnO, Rb 10%:ZnO and Rb 15%:ZnO samples, respectively. The result demonstrated that doping ZnO with Rb affects the recombination process of the electron-hole pairs produced by the photon absorption and consequently influence the photocatalytic activity of ZnO.

Calcination effects on europium doped zinc oxide as a luminescent material synthesized via sol-gel and precipitation methods

This work is aimed at the determination of the preferential calcination conditions for maximum thermoluminescence (TL) and optically stimulated luminescence (OSL) emissions of europium doped zinc oxide (ZnO:Eu) pellets after exposure to β-particles when they are synthesized using the precipitation and sol-gel methods, in micro/nanoscales. The verification of the photoluminescence (PL) signals were investigated. The calcination temperature range of 800–1200 °C and the calcination times between 2 h and 24 h were applied in the production of ZnO:Eu pellets. The phase composition and morphology of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The transmittance measurements were carried out by Fourier transformed infrared (FTIR) spectroscopy to understand the vibrational properties of ZnO:Eu nanocrystals. In the XRD and SEM patterns, the formation of hexagonal wurtzite phase of ZnO was observed. It was observed that the samples exhibited a weak green emission when the excitation wavelength is 350 nm. In addition, the ZnO:Eu pellets exhibited two sharp peaks at 579 and 615 nm which related to the Eu transitions. The PL intensities dropped with the increasing calcination temperatures. The TL peaks settled at 130 and 300 °C with 5 oCs−1 heating rate, were found similar to each other when the pellets calcined using different temperatures and times. β-dose linearity extended from 1 Gy to 0.2 kGy for all of the studied samples.

Optical properties and defects study in Zn1-x (CrGa) x/2O

Luminescence properties due to defects in sol–gel prepared parent and Ga3+/Cr3+ co-doped ZnO are investigated. The difference of charge and ionic size creates defects in between the energy bands i.e. conduction band (CB) and valence band (VB). A hexagonal ZnO phase (Wurtzite) with a space group of P63mc is revealed in these samples using X-ray diffraction. A detailed study of native defects or trap states like oxygen vacancies, zinc interstitials, etc. present in undoped and Ga/Cr co-doped samples were investigated using photoluminescence study. Significant reduction in the visible spectrum i.e. deep level emission is revealed with the introduction of higher charges.

Temperature-dependent heterojunction device characteristics of n-ZnO nanorods/p-Si assembly

Heterojunction diode based on n-ZnO nanorods/p-Silicon (Si) assembly was fabricated, examined and reported here. Horizontal quartz tube thermal evaporation technique was used for the growth of ZnO nanorods on Si substrate. The nanorods were characterized by several techniques to examine the structural, morphological, scattering and electrical properties. Wurtzite hexagonal phase of the grown aligned nanorods was observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The appearance of a sharp Raman peak at 438 cm–1 was observed and it is related to the E2(high) mode of the wurtzite hexagonal phase of ZnO. The electrical properties of the fabricated heterojunction assembly were examined at different temperatures (298∼398 K) in both reverse and forward biased conditions, and a good stability was observed over the entire temperature range. A reduction in the turn-on and breakdown voltage was observed with increasing temperature. By increasing the temperature, the effective potential barrier height was increased, while quality factor was decreased. The observed activation energy was found to be ∼93.4 meV, higher than the exciton binding energy of ZnO.

Gas Sensing Properties of ZnO-SnO2 Nanocomposite

In present study, ZnO-SnO2 nanocomposite was synthesized by co-precipitation method and its sensing properties with respect to carbon monoxide gas were investigated. X-ray diffraction pattern shows the exhaustive evolution of hexagonal wurtzite phase of ZnO and rutile phase of SnO2. Morphological study was done by FE-SEM and optical characterization was done by UV-visible spectrophotometer. To study the sensing properties, material was layered on conducting substrate and resistance was recorded in the presence of air and CO gas at different operating temperature. Sensing responses of pure ZnO and ZnO-SnO2 composite was also compared. ZnO-SnO2 showed much enhanced response along with better response and recovery time compared to pure ZnO.

Green synthesis of p-NiO/n-ZnO nanocomposites: Excellent adsorbent for removal of congo red and efficient catalyst for reduction of 4-nitrophenol present in wastewater

Globally water pollution caused by mainly the discharge of industrial effluents, human activities and natural processes is the major issue and concern. Adsorption and heterogeneous catalysis among various developed technologies for the remediation of wastewater treatment have attracted much attention due to the involvement of versatile and cheap materials such as mixed metal oxides. NiO/ZnO nanocomposites with different molar ratios (Ni:Zn 1:4, 2:3, 1:1, 3:2, 4:1 abbreviated as N1Z4, N2Z3, N1Z1, N3Z2, N4Z1, respectively) were synthesized via sol-gel method using zinc 2,4-pentanedionate and nickel acetate as precursors. Nanocomposites, NiO and ZnO were characterized by powder X-ray diffraction (PXRD), field emission–scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-Ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FT-IR) spectroscopy, Brunauer-Emmett-Teller surface analysis (BET) and ultraviolet diffuse reflectance spectroscopy (UV-DRS). XRD demonstrated the presence of cubic NiO and hexagonal ZnO phases. FE-SEM and TEM images demonstrated mixed morphology including spheres, hexagons and rods, and their particle size varies from 15 to 45 nm. Among all the composites, N3Z2 has probed as excellent adsorbent towards congo red (CR) (% removal efficiency: N3Z2, N2Z3, N1Z1, N4Z1, N1Z4, NiO, ZnO: 94.9, 88.8, 86.5, 67.9, 57.4, 52.3, 22.8, respectively, for 60 ppm CR in 1 h). N3Z2 can also be reused upto 5 cycles. On the other hand, N1Z1 has catalysed reduction of 4-nitrophenol (4-NP, 0.2 mM) to 4-aminophenol (4-AP) within 9 min with 100% conversion efficiency upto 3 cycles. The molar ratio of NiO and ZnO in the synthesized nanocomposites plays an important role in controlling their morphology, surface area and their selective application for wastewater treatment.

Structural, Optical, and Thermoelectric Properties of the ZnO:Al Films Synthesized by Atomic Layer Deposition

Aluminum-doped zinc oxide thin films have been grown by atomic layer deposition at a temperature of 200°C. Using X-ray diffraction, it has been established that the ZnO:Al thin films exhibits the reflections from the (100), (002), (110), and (201) ZnO hexagonal phase planes. The (101) and (102) planes have also been detected by electron diffraction. The ZnO:Al thin films grow smooth with a root-mean-square roughness of Rq = 0.33 nm and characteristic nanocrystallite sizes of ~70 and ~15 nm without additional aluminum or aluminum oxide phases. The transmission at a wavelength of 550 nm with regard to the substrate has been found to be 96%. The refractive indices and absorption coefficients of the ZnO:Al thin films in the wavelength range of 250–900 nm have been determined. The maximum refractive indices and absorption coefficients have been found to be 2.09 at a wavelength of 335 nm and 0.39 at a wavelength of 295 nm, respectively. The optical band gap is 3.56 eV. The resistivity, Seebeck coefficient, and power factor of the ZnO:Al thin films are ∼1.02 × 10–3 Ω cm, –60 μV/K, and 340 μW m–1 K–2 at room temperature, respectively. The maximum power factor attains 620 μW m–1 K–2 at a temperature of 200°C.

Visible light response and heterostructure of composite CdS@ZnS–ZnO to enhance its photocatalytic activity

In this work, CdS@ZnS–ZnO composites were prepared by the temperature-programmed hydrothermal method combined with the microwave-assisted hydrothermal method. The composite consisted mainly of cubic phase ZnS, hexagonal phase CdS and hexagonal phase ZnO, and presented a spherical structure with relatively uniform size and shape. There were heterostructures between CdS, ZnS and ZnO in the spherical structure. At the same time, the grain size, specific surface area, average pore diameter and pore volume of the composited material changed significantly, and its light absorption performance in the visible light region was obviously enhanced, and the photocatalytic activity was also greatly improved. Under ultraviolet light, CdS@ZnS–ZnO can degrade simulated pollutants by more than 90% within 20 min. In addition, the results of hydrogen evolution from photolysis of water showed that the ternary composite CdS@ZnS–ZnO had excellent hydrogen evolution capacity compared with pure CdS, ZnS and ZnO, whose hydrogen evolution amount can reach 3647 μmol g−1 in 8 h, was 298 times than that of P25. Moreover, it still maintained a high hydrogen evolution capacity after 4 cycles.

Effect of B2O3 Concentration and Sintering Temperature on Microstructure and Electrical Properties in the ZnO-Bi2O3-Based Varistors

The ZnO-Bi2O3-MnO2-Co2O3-based (ZBMCO) varistors were prepared via the sol–gel method. The effects of B2O3 additive on the phase composition, microstructure, sintering temperature and electrical properties of the ZBMCO ceramics were studied. A single hexagonal ZnO phase was detected in all ZBMCO-xB2O3 varistors sintered at high temperature (1100°C). Secondary phases Mn0.31Bi1.69O2.85, Zn3B2O6, Bi24B2O39 and Bi2O3 were detected in ZBMCO-1.0 wt.%B2O3 varistors at a lower sintering temperature (900°C, 1000°C). The average grain size increased remarkably in the range of 15.27–26.12 μm with an increase of the B2O3 content. The ZBMCO-1.0 wt.%B2O3 varistor showed the maximum relative density of 97.2% with high nonlinear coefficient (56.5) and low leakage current (0.07 μA/cm2) at 1100°C. The E1mA decreased noticeably from 865 V/mm to 64 V/mm with an increase of the sintering temperature. When the sintering temperature is lower at 1000°C, the varistor exhibits relatively good electrical properties (α = 48.7, IL = 0.68 μA/cm2). These results demonstrate that low melting point B2O3 plays multiple roles in grain growth and microstructure.

Nanocrystalline Cu-ZnO as an Green Catalyst for One Pot Synthesis of 4,4'-((phenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) Derivatives.

Synthesis of 4,4'-((phenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) derivatives was successively carried out using Cu doped ZnO nanomaterials. The nanocrystalline Cu-ZnO was obtained by decomposing as-synthesized copper-zinc oxalate intermediate at 520 °C. The prepared Cu-ZnO nanostructured catalyst was characterized with FTIR, X-ray diffraction, field emission scanning electron microscope and electron diffraction techniques. XRD analysis indicates the formation of highly crystalline hexagonal phase of ZnO along with the presence of monoclinic CuO. FESEM photographs shows the existence of plate like structures made up of small spherical shaped particles having size in the range of 30-50 nm. As-synthesized Cu-ZnO was used as heterogeneous catalyst for one pot synthesis of 4,4'-((phenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) derivatives using phenyl hydrazine, ethyl acetoacetate and aromatic aldehydes. The 3-methyl-1-phenyl-1H-pyrazol-5-ol was obtained as in-situ precursor to the series of bis-pyrazolone derivatives. The progress of reaction was monitored by thin layer chromatography. The obtained organic product was further characterized and confirmed by FT-IR, 1H-NMR, 13C-NMR and HRMS spectroscopic techniques. The Cu-ZnO catalyst confers upto 96% yield of pyrazolone derivatives in ethanol solvent at refluxing condition. The Cu-ZnO catalyst was used successfully up to 5 cycles without much loss of catalytic activity. Overall, the use of environmental friendly Cu-ZnO nano-structures as a heterogeneous catalyst shows higher yield and lower reaction time towards the synthesis of bispyrazolone derivatives by Tandem Knoevenagel/Michael reaction.

A facile one pot flash combustion synthesis of ZnO nanoparticles and their characterizations for photocatalytic applications

Herein, we have facilely synthesized high purity ZnO nanoparticles by flash combustion technique in presence of different citric acid (CA) content. X-ray diffraction study rules out the existence of any extra phase except single hexagonal phase of ZnO. Further, crystallite size, dislocation density and strain values were estimated, and the crystallite size was found to be reduced from 20to 10 nm with rising the concentration of CA. Vibrational study also confirm the single phase in accordance with XRD results and revealed that the modes of vibration are slightly shifted owing to quantum size effect and relaxed nanostructure formation. To confirm the purity of synthesized ZnO the energy dispersive X-ray spectroscopy was carried out which confirm the high purity of synthesized products. Spherical nanoparticles (SNPs) morphology was confirmed through scanning electron microscopy and the size is reduced with raising the concentration of CA. TEM study was also carried out which clearly shows the formation of spherical nanoparticles (SNPs) and the mean size is around 10 nm for 1 g CA:ZnO and < 6 nm for 10 g CA:ZnO samples. The blue shift in absorption edge was noticed in DR spectra with raising the CA concentrations and the energy gap was estimated using Kubelka-Munk theory. The estimated direct energy gap of ZnO SNPs was noticed to be increased from 3.25 to 3.28 eV. An enhancement in dielectric constant from 14 to 17 and ac conductivity was observed with raising the CA content. Furthermore, the prepared SNPs of ZnO were subjected to photocatalytic study from application point of view. The photocatalytic performance of the prepared materials is found to be improved several times. All results displayed that the CA is playing the vital role in improving the optoelectronic and photocatalytic performance of ZnO and hence will be more applicable.

Fabrication and magnetic transformation from paramagnetic to ferrimagnetic of ZnFe2O4 hollow spheres

Abstract ZnFe2O4 hollow spheres (ZFHs) with sizes of 200–302 nm were synthesized by simple impregnating method using the as-prepared phenolic formaldehyde (PF) spheres as templates and subsequent annealing at 500–700 °C. The prepared ZFHs are assembled by a large number of small nanoparticles with sizes of 15–20 nm, and many mesopores exist among these nanoparticles. The samples annealed at 500–550 °C exhibit a single cubic spinel structure, while higher annealing temperature leads to the formation of hexagonal ZnO and rhombohedral α-Fe2O3 secondary phases. The size of the assembled nanoparticles increases with the increase in annealing temperature. Novel magnetic transformation from paramagnetic to ferrimagnetic is induced by the reduction of annealing temperature and the saturation magnetization significantly increases from 2.3 to 13.5 A·m2/kg. The effect of the formation of hollow sphere structure on the redistribution of Fe3+ and Zn2+ in the spinel structure was studied.

Inexpensive and quick photocatalytic activity of rare earth (Er, Yb) co-doped ZnO nanoparticles for degradation of methyl orange dye

The present work provides an eco friendly rout to synthesize erbium and ytterbium co-doped ZnO (EYZO) photocatalysts by combustion method followed by calcinations at 700 °C for 3 h and then characterized by different techniques (XRD, EDS, SEM, XPS, FTIR, UV–Vis DRS, PL, CV and BET surface area). XRD confirm the formation of hexagonal wurtzite phase of ZnO. Further, doping of Er and Yb in to ZnO lattice was evidenced by shift of XRD planes towards lower 2θ values. The addition of Er and Yb introduces significant changes in light absorption characteristics with important absorbance in visible region due to Er and Yb role as acceptor to increase charge separation. UV–Vis DRS shows red shift of ZnO band gap from 3.23 eV to 3.16 eV due to Er and Dy doping. All synthesized photocatalysts were tested in photocatalytic degradation of methyl orange (MO) under visible light irradiation. It was observed an increase of photocatalytic MO degradation under visible light irradiation with enhance of Er and Yb co-doping content for ZnO. In particular, EYZO with maximum Er and Yb content allowed obtaining the almost complete MO degradation only in 90 min under visible light irradiation. In addition, the impact of various operational factors on the photo-degradation of MO was studied and explained. Moreover, a feasible catalytic process for photo-degradation of MO over EYZO photocatalysts is suggested and participation of hydroxyl radicals as reactive species is explained by radical trapping analysis.