Influence of PEO molecular weight on properties of ZnO/PEO composites

The removal of inorganic, organic and biological pollutants from drinking water and wastewater is one of the key steps in environmental protection. In recent 10 years a heterogeneous photocatalysis, as an efficient method for the degradation and mineralization of pollutants from water, has been widely studied and developed. For heterogeneous photocatalysis mostly used materials to initiate the photoreaction are oxide semiconductors such as TiO2 and ZnO. However, these oxide semiconductors, having band gap energies around 3.3 eV, can absorb UV light only. Since sunlight is a source of clean and cheap energy, where UV light makes no more than 3–5% of the total sunlight, it is highly desirable to modify the oxide semiconductor materials to be capable for visible light photocatalysts. Numerous approaches have been applied to modify the optical absorption properties and to improve the visible light photocatalytic activity including: (1) the incorporation of transition metal ions into the crystal structure, (2) sensitization of the particles’ surface, (3) hydrogenation, (4) the incorporation of crystalline defects in metal oxide semiconductors in the form of vacancies and interstitials, etc. Microwave processing is recognized as an attractive synthesis technique to introduce lattice defects. In this study ZnO spheroidal nanoparticles, synthesized by microwave processing, were used for preparation of composites with polyethylene oxide (PEO). The phase purity and crystal structure of the composites were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The composites' particles morphology and size distributions were studied by FE–SEM and laser diffraction particle size analyzer, respectively. The optical properties were studied using UV–Vis diffuse reflectance and photoluminescence spectroscopy. It is found that ZnO and ZnO/PEO composites absorb about 50% of visible light, also red-shift of band gap energy (0.12-0.15 eV) compared to bulk ZnO was determined. The effect of PEO molecular weights, 200.000, 600.000 and 900.000 g/mol, on photocatalytic activity of ZnO/PEO composites were examined via degradation of methylene blue (MB) under direct sunlight irradiation. A large efficiency of MB degradation was found after 6 h of irradiation. The enhanced photocatalytic activity of ZnO/PEO composites is attributed to the: (1) lattice defects introduced in ZnO crystal structure by rapid microwave processing, and (2) presence of PEO as a source of oxygen interstitials. In order to confirm and further clarify the experimental results ab initio calculations based on density functional theory (DFT) were performed

Photocatalytic activity of ZnO-PEO composites

The removal of organic pollutants from wastewater is very important for environmental protection. During the years different methods have been developed and applied on wastewater treatment. Between those methods a heterogeneous photocatalysis has received extensive attention since it allows a complete mineralization of pollutants. ZnO-based materials has established role in heterogeneous photocatalysis. However, major drawback of ZnO is a band energy gap of 3.37 eV (368 nm) which restricts the material to absorb only UV light. Since sunlight is a source of clean and cheap energy, where UV light makes no more than 3–5% while visible light is about 45% of the total sunlight, it is highly desirable to synthesize ZnO-based materials capable for visible light photocatalysis. To modify the optical absorption properties and improving the visible light photocatalytic activity of ZnO materials several approaches have been applied: (1) metal ion doping, (2) nonmetal doping, (3) defect induced doping, (4) surface sensitization of ZnO particles to extend the spectral response into the visible region, (5) band gap modification by creation of oxygen vacancies and oxygen sub-stoichiometry, etc. In this study, ZnO powder with nanospherical morphology was synthesized by microwave processing. In the continuation, the synthesized powder was used for preparation of composites with polyethylene oxide (PEO). PEO powders with three different molecular mass (200.000, 600.000 and 900.000) were used for composites preparation. The phase purity and crystal structure of the composites were investigated by X-ray diffraction and Raman spectroscopy. The particles morphology and size distributions were studied by FE–SEM and laser diffraction particle size analyzer, respectively. The optical properties were studied using UV–Vis diffuse reflectance and photoluminescence spectroscopy. The photocatalytic activity of ZnO-PEO composites was examined via decomposition of methylene blue (MB) under direct sunlight irradiation. A large efficiency of MB degradation was found after 6 h of irradiation. An enhanced optical and photocatalytical properties of ZnO-PEO composites were attributed to: (1) lattice defects introduced in crystal structure of ZnO by fast microwave processing, and (2) surface sensitization by polyethylene oxide (PEO)

Zinc oxide-based materials with enhanced sunlight-driven photo- and photo-electro-catalytic activity

Current trend in photocatalysis is to develop efficient semiconductors which can be activated by absorbing sunlight. Which wavelength of sunlight will be absorbed depends on the semiconductor band gap; semiconductors with a wide band gap (> 3 eV) can absorb only UV light (5% of sunlight), while those with a narrow band gap (< 3 eV) can be activated by visible light (45% of sunlight). Zinc oxide (ZnO) is promising semiconductor with band gap of 3.37 eV. Various approaches have been applied to modify its optical properties, for example: incorporation of different metal and nonmetal ions or defects into the crystal structure, particles’ surface sensitization or hydrogenation. In this study, we examined the influence of different defects present in ZnO particles on their photo- and photo-electro-catalytic properties. Processing of ZnO particles were carried out in order to introduce: (1) lattice defects, through microwave procedure, (2) surface defects, through mechanical activation, and (3) surface defects, trough composite with polyethylene oxide. Synthesized particles were characterized by XRD, FESEM, laser diffraction particle size analyzer, Raman, UV-Vis diffuse reflectance and photoluminescence spectroscopy. The results of achieved photo- and photo-electro-catalytic tests indicate that both, structural and surface, defects enhanced sunlight-driven activity of ZnO particles

Sinteza i optičke karakteristike nanostrukturnih prahova ZnO i ZnO/PEO

In this paper, microwave processing of nanostructured ZnO powder as well as preparation of nanostructured ZnO/PEO composite were described. As a fast processing method which introduces a large amount of energy in the reaction system, the role of microwave processing was to modify ZnO crystal structure, while the role of PEO was to additionally sensitivize surface of ZnO particles; both of the approaches were used in the aim to improve optical properties of zinc oxide in comparison with bulk one. The synthesized powders were characterized by X-ray powder diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and photoluminescence (PL). It was found that point defects (oxygen vacancies and zinc interstitials) were created in the crystal structure of zinc oxide. However, PEO has two-fold role, it passivate surface of the ZnO particles, but also introduce oxygen interstitials on the surface. The influence of the point defects on optical properties of ZnO was studied; it was found that oxygen vacancies, zinc interstitials and oxygen interstitials improved percent of thevisible light absorption, also shift band gap energy toward visible range of the spectrum.U ovom radu opisan je postupak mikrotalasnog procesiranja nanostrukturnog praha ZnO kao i metoda pripreme nano strukturnog kompozita ZnO/PEO (PEO - polietilen oksid). Uloga mikrotalasnog procesiranja, kao brze metode koja u sistem uvodi veliku količinu energije, bila je da modifikuje kristalnu strukturu ZnO dok je uloga PEO bila da dodatno senzitivizuje površinu čestica ZnO, a sve u cilju modifikovanja (poboljšanja) standardnih optičkih osobina cink oksidnog materijala. Sintetisani prahovi su analizirani metodama rendgenske difrakcije (XRD), Ramanove spektroskopije, skanirajuće elektronske mikroskopije (FESEM), UV-Vis difuzione refleksione spektroskopije (UV-Vis DRS) i fotoluminiscencije (FL). Uočeno je da su se u kristalnoj strukturi ZnO formirali tačkasti defekti, tačnije kiseonične vakancije i intersticije cinka, dok je prisustvo PEO sa jedne strane dovelo do pasivizacije površine čestica, dok je sa druge strane došlo do formiranja kiseoničnih intersticija. Ispitan je uticaj tačkastih defekata na optičke karakteristike prahova ZnO; kiseonične vakancije, kao i intersticije cinka i kiseonika utiču kako na procenat apsorpcije vidljive svetlosti (%) tako i na pomeraj energetskog procepa (eV) ka vidljivom delu spektr

Influence of point defects concentration on optical and photocatalytic properties of ZnO ceramics

Zinc oxide is one of the most studied materials due to its wide bandgap (3.37 eV) and large exciton binding energy (60 meV) which enables application in electronics, optoelectronics and spintronics. In the forms of single crystal and thin-film ZnO are used as UV and blue light emitter, while sintered ZnO-based ceramics are important as varistors, thermistors or semiconductors. It has been found that point defects in the crystal structure of a ZnO strongly influenced its electrical and optical properties. Neutral oxygen vacancies are considered to be a major component of the defect structure of ZnO. Thus, correlation of the oxygen vacancies concentration with band gap energy of ZnO product is important to its application in optoelectronic devices. In this study we investigated the influence of point defects concentration in ZnO crystal structure on its optical and photocatalytic properties. We analyzed ZnO powders prepared by different techniques: (a) microwave processing of precipitate and (b) hydrothermal processing, which yield different ordered crystal structure. To increase a concentration of the point defects in the crystal structure, the powders were sintered in air atmosphere by heating rate of 10 °/min up to 1100 °C, with dwell time of 1 h. The crystal structure, average crystallite size and phase purity of the ZnO ceramics were determined by X-ray diffraction and Raman spectroscopy. The optical properties, in particular, absorption capacity and bang gap energy, were studied using UV–Vis diffuse reflectance spectroscopy. To reveal the role of microstructures and point defects in ZnO crystal lattice, which are receptive for luminescence and photocatalytic activity of this functional oxide, photoluminescence (PL), photoluminescence excitation (PLE) and EPR spectra were analyzed. The influence of point defects concentration in the ZnO crystal structure on photocatalytic properties was examined via decolorization of methylene blue under direct sunlight irradiation. Correlation between amount of the point defects, absorption capacity and photocatalytic efficiency were established. In order to clarify the experimental results ab initio calculations based on density functional theory (DFT) were performed

Scanning and Transmission Electron Microscopy Investigation of SrGd2O4: Yb,Tm Up-conversion Luminescent Material

In recent decades, inorganic luminescent materials have gathered significant attention due to their great potential for various applications [1-4]. The rare-earth (RE)-based UC luminescent materials are particularly interesting for their exceptional optical, electronic, and magnetic properties. These materials have distinct intra-4f electronic transitions and existence of plenty long-living electronic excited states at different energies, all of which makes electron promotion to high-energy states possible [5, 6]. RE-based UC luminescent materials are composed of a host material (matrix), a sensitizer (absorbs the IC radiation), and an activator (provides emission in the visible and UV part of the spectrum) [7]. So far, the best results have been gained by co-doping the matrix using Yb3+ as sensitizer and Er3+, Ho3+, Tm3+, etc. as activators [8-10]. As for the hosts, rare-earth oxides (ARE2O4; A = Ca, Sr, Ba and RE = trivalent rare-earth ions) have great perspective for producing highly efficient luminescent materials. To the best of our knowledge, SrGd2O4 has been poorly investigated so far, although it has an enormous potential for variety of applications since it is environmentally friendly, has high thermal stability and good chemical durability [11]. In this work, we will present new UC luminescent material composed of SrGd2O4 (host) doped with Yb3+ (sensitizer) and Tm3+ (activator). Control of particle morphology has attracted a great deal of attention from researchers, so efforts for finding appropriate synthesis method are still very current issue. The morphology of the obtained particles is mostly influenced by the synthesis methods used for preparing the material. Luminescent properties mainly interested for us, are in very close connection with the morphology. Here, samples were synthesized using glycine-assisted combustion method, with constant concentration of Tm3+ (1 at%) and different concentration of Yb3+ (2, 4, 6 at%). All samples were heated in the furnace at 500 °C for 1.5h and then thermally treated for 2.5 h at 1000 °C. X-ray diffraction (XRD) was used to see phase crystallinity and purity, and revealed that all peaks are assigned to the pure orthorhombic lattice of SrGd2O4 with space group Pnma (JCPDS Card No.:01-072-6387). Luminescent properties were investigated after recording UC luminescence spectra at room temperature under 980 nm excitation for all samples. The spectra revealed strong blue emission bands which originates from Tm3+ ions 1D2 → 3F4 and 1G4 → 3H6 and weak red emission 1G4 → 3F4 transitions. Morphology and structure were thoroughly studied by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), whilst energy dispersive spectroscopy (EDS) was used to provide additional information about constituting elements and their distribution. FE-SEM analysis revealed irregular spherical-like morphology with all samples, and particle size of around 100 nm. TEM examination showed nanostructures organized as a group of agglomerated nanoparticles. EDS verified uniform distribution of all composing elements through every sample [12]

Point defect-enhanced optical and photoelectrochemical water splitting activity of nanostructured Zn1-xFeyO(1-x+1.5y)

Even has been under study since 1935, zinc oxide (ZnO) based materials still attract a huge scientific attention. Owing to a wide band gap energy (3.37 eV at room temperature) and a large exciton binding energy (60 meV) ZnO has a variety of application, e.g. in electronics, optoelectronics, spintronics and photocatalysis. Besides, it has been shown that zinc oxide-based materials have a great potential as photoelectrocatalysts in the processes of water splitting, yielding an increased both photocurrent density and photoconversion efficiency. However, with a band gap energy of 3.37 eV, ZnO is restricted to absorb UV light only. This restriction can be overcome by modifying optical properties of zinc oxide particles. During the years different approaches have been applied to modify the visible light photocatalytic activity of ZnO materials, for example: (1) metal and nonmetal ion doping, (2) hydrogenation, (3) the incorporation of crystalline defects in the form of vacancies and interstitials, (4) the modification of particles morphology and surface topology, etc. In this study we employed 3d metal ion substitution to improve visible light-driven photoactivity of zinc oxide particles. We investigated the influence of Fe concentration in Zn1-xFeyO(1-x+1.5y) nanoparticles on crystal structure, textural, optical and photoelectrocatalytic properties. Zn1-xFeyO(1-x+1.5y) nanoparticles with nominally 5, 10, 15 and 20 at.% of Fe ions were synthesized by microwave processing of a precipitate. The crystal structure and phase purity of the samples were investigated by X-ray diffraction, Raman and ATR-FTIR spectroscopy. Mössbauer spectroscopy was carried out to clarify the valence state of the iron ions in the ZnO crystal structure. Effects of the iron ions concentration on particles morphology and texture properties were observed with field emission scanning electron microscopy (FE–SEM), transmission electron microscopy (TEM) with elemental mapping, and nitrogen adsorption–desorption isotherm, respectively. The optical properties were studied using UV–Vis diffuse reflectance and photoluminescence (PL) spectroscopy. Photoelectrochemical activity of the Zn1-xFeyO(1-x+1.5y) samples as anode material was evaluated by linear sweep voltammetry in Na2SO4 electrolyte; the oxygen evolution kinetics were determined and compared. In addition, a series of first principles calculations were performed to address the influence of the iron concentration on the electronic structure of Zn1-xFeyO(1-x+1.5y) samples

INVESTIGATING THE EFFECTS OF Zr DOPING ON THE TITANIUM DIOXIDE NANOFIBRES

In this work, titanium dioxide (TiO2) nanofibers doped with 0.5–5 mol% zirconium ions (Zr4+) were synthesized by combining the sol-gel process and electrospinning method, and calcined at 500 °C. The morphological, structural and optical properties of pure and Zr-doped TiO2 nanofibers were investigated. According to the XRD and FTIR analyses, the addition of Zr as a dopant suppressed the transformation of anatase to rutile phase. Scanning electron microscopy showed that all fibers were smooth, fragile and randomly oriented after the calcination process. HRTEM analysis revealed that Zr4+ ions were incorporated at the substitutional sites in the anatase TiO2 crystalline lattice. The photocatalytic efficiency for degradation of methylene blue (MB) was examined for both pure and Zr-doped TiO2 samples. Nanofibers doped with 1% of Zr4+ ions have shown the highest photocatalytic activity of 98%, wich can be explained by considering lower PL intensity in the PL spectrum of this sample, indicating suppressed electron-hole recombination

Surface functionality as a key parameter for the conductivity of microwave synthesized CQDs thin films

The carbon quantum dots (CQDs) as zero-dimensional carbon nanomaterials with extraordinary physicochemical properties have a broad range of applications. Some of the most intriguing is the CQD thin films for electronic devices and membrane nanofiltration. Here we present the simple and affordable microwave-assisted synthesis method for the production of the nitrogen-doped CQDs (N-CQD) and iron/nitrogen co-doped CQDs (FeN-CQD). The comprehensive study of the morphology and electrical properties of N-CQD and FeN-CQD was discussed after manufacturing obtained nanomaterials in the form of a thin film on different substrates using different deposition procedures. The morphological features of generated thin films altered dramatically when the substrate and nanomaterial properties, as well as the number of deposited layers of CQDs and the chosen deposition process, were taken into consideration

Examination of the doping effects of samarium (Sm3+) and zirconium (Zr4+) on the photocatalytic activity of TiO2 nanofibers

Pure and samarium (Sm3+) - or zirconium (Zr4+)-doped TiO2 nanofibers were synthesized by electrospinning method followed by calcination at 500 °C for 1 h. As-spun fibers were smooth, straight and continuous, whilst EDS analysis confirmed the fiber composition and incorporation of dopants in the fibers. Doping with Sm3+ and Zr4+ greatly inhibited the phase transformation of anatase to rutile, by surrounding of Sm3+ ions through formation of Ti-O-Sm bonds and by replacement of Ti4+ ions with larger Zr4+ ions. This was confirmed by HRTEM and SAED analysis. The size of nanofibers was determined to be 133 nm, 175 nm and 155 nm for pure, (0.5%)Sm3+:TiO2 and (1%)Zr4+:TiO2, respectively. After calcination, TiO2 crystal lattice with interplanar spacing of 0.353 nm of (101) crystal plane was not significantly disturbed by Sm doping whilst crystal lattice spacing of 0.357 nm of (101) planes of anatase phase in case TiO2:1.0%Zr4+, significantly differs from the value of pure TiO2 (0.352 nm), thus implying that Zr was doped into substitutional sites of the TiO2 lattice. The indirect band gaps were calculated to be in the range 3.07–3.24 eV. TiO2:0.5%Sm3+ and TiO2:1.0%Zr4+ exhibited higher specific surface area, of 47.1 and 59.4 m2/g, respectively, than pure TiO2 fibers (19.7 m2/g). Effects of Sm3+ and Zr4+ dopant content on the photodegradation efficiency of methylene blue (MB) were studied. TiO2:0.5%Sm3+ and TiO2:1.0%Zr4+ nanofibers have shown the highest photocatalytic activity of 97% and 98% with constant rates 0.01768 min−1 and 0.01939 min−1, respectively, within180 min irradiation under visible light