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Structural and optical properties of ZnO nanoclusters supported on mesoporous silica
Abstract
This paper reports on structural and optical properties of ZnO nanoclusters embedded in mesoporous silica (MPS) an inorganic host matrix. Small angle X-Ray Diffraction (SAXRD), Wide angle X-Ray Diffraction (XRD), N2-Sorption (BET), Scanning Electron Microscopy/Energy Dispersive spectroscopy (SEM/EDS), Fourier Transform Infrared Spectroscopy (FT- IR), Ultraviolet - visible spectroscopy (UV-Vis.) and Photoluminescence (PL) spectroscopy were used to characterize the samples. The UV-Vis absorption and PL spectra show a strong blue shift due to the quantum confinement effect of ZnO semiconductor nanoparticles. On excitation with 318 nm light source, ZnO/MPS nanocomposite shows emission peaks at 344, 372, 455, 485 and 512 nm. The emission peaks at 372 and 485 nm are the contribution of mesoporous silica. The PL band at 344 nm is attributed to the band gap luminescence of ZnO which is smaller than the previous reports. We suppose that the emission at 455 nm is not the blue shifted green emission but it is due to the formation of Zn-O-Si bonds at the interface. Singly ionized oxygen vacancies in ZnO nanocrystals are responsible for the green emission at 512 nm.
... The peak intensity decreased was attributed to the pore filling effects of clay at the maximum loading of 2wt% which could reduce the scattering contrast between the pores and improved the interfacial bonding within the nanocomposite formed. [21]. The existence of ST-co-GMA could be proven with the appearance of absorption peak at 1725cm -1 which corresponded to the carbonyl group vibration in all the samples [1]. ...
... This peak occurred when the carbon atom from GMA was attached to the carbon atom from styrene as well as forming aliphatic carbons in the main chain and side chain of the copolymer [23]. Broader peak was observed at 1wt% and 2wt% of ST-co-GMA-clay (1.30E) nanocomposites which indicated stronger bonding between styrene and glycidyl methacrylate compared to other nanocomposites as shown in Figure 1(c-d) [21]. The C-O stretching vibration was observed due to the carbonyl group in the nanocomposites which showed the peak at 1150cm -1 [25]. ...
Styrene-co-glycidyl methacrylate-fumed silica-clay (ST-co-GMA-fsi-clay) nanocomposites have been prepared via free radical polymerization in the presence of benzoyl peroxide. The nanocomposites are characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), adsorption isotherm, tensile test, thermogravimetric analysis (TGA) and moisture absorption. FT-IR shows the Si-O-C peak that represented ST-co-GMA-fsi bonding while Si-O-Si peak shows the bonding of fsi-clay. The surface morphology shows the well dispersion of clay (1.30E) into ST-co-GMA-fsi nanocomposite. 2wt% of ST-co-GMA-fsi-clay (1.30E) nanocomposite has higher specific surface area and average pore volume with less pore size. Incorporation of 2wt% of clay (1.30E) improves the tensile strength and modulus of the nanocomposites as well as higher thermal stability and activation energy. 2wt% of ST-co-GMA-fsi-clay (1.30E) nanocomposite shows the lowest moisture absorption value.
... As a result, numerous methods for fabrication of nanocomposites were developed. By variation of zinc precursors, post-synthetical modifying treatments, and sequences of zinc oxide introducing into the pores, the ZnO particles different by size, shape and localization in the pores were obtained [17][18][19][20]. Here, we present the studies of zinc oxide/SBA-16 nanocomposites fabricated by the new sublimation method. ...
The nanocomposite - nanoporous silica (SBA-16) containing ZnO quantum dots - was fabricated by the sublimation method. This novel route of synthesizing ZnO nanoparticles implies physical sorption of zinc acetylacetonate precursor into the matrix pores from the gaseous phase, hydrolysis of the precursor, and thermal decomposition of the product with formation of ZnO nanoclusters directly inside the pores. The nanocomposite was characterized by XRD, nitrogen adsorption-desorption isotherms, and photoluminescence spectroscopy methods. We observed the blue shift of the ultraviolet luminescence of nanocomposite as compared to the luminescence of ZnO nanopowder as well as the decrease of the linewidth of the ultraviolet luminescence line, which points to the encapsulation of ZnO nanoparticles in the silica pores and narrowing of their size distribution.
Silica-based ZnO-MCM-41 mesoporous nanocomposites with high Zn content (5≤Si/Zn≤50) have been synthesized by a one-pot surfactant-assisted procedure from aqueous solution using a cationic surfactant (CTMABr = cetyltrimethylammonium bromide) as structure-directing agent, and starting from molecular atrane complexes as inorganic hydrolytic precursors. This preparative technique allows optimization of the dispersion of the ZnO nanodomains in the silica walls. The mesoporous nature of the final materials is confirmed by x-ray diffraction (XRD), transmission electron microscopy (TEM) and N(2) adsorption-desorption isotherms. The ZnO-MCM-41 materials show unimodal pore size distributions without blocking of the pore system even for high Zn content. A careful optical spectroscopic study (using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and UV-visible spectroscopy) of these materials shows that, irrespective of the Si/Zn ratio, the Zn atoms are organized in well-dispersed, uniform low-defect ZnO nanodomains (radius about 1 nm) and are partially embedded within the silica walls.
The two-solvent method was employed to prepare ZnO encapsulated in mesoporous silica (ZnO/SBA-15). The prepared ZnO/SBA-15 samples have been studied by X-ray diffraction, transmission electron microscope, X-ray photoelectron spectroscopy, nitrogen adsorption-desorption isotherm, and photoluminescence spectroscopy. The ZnO/SBA-15 nanocomposite has the ordered hexagonal mesostructure of SBA-15. ZnO clusters of a high loading are distributed in the channels of SBA-15. Photoluminescence spectra show the UV emission band around 368 nm, the violet emission around 420 nm, and the blue emission around 457 nm. The UV emission is attributed to band-edge emission of ZnO. The violet emission results from the oxygen vacancies on the ZnO-SiO(2) interface traps. The blue emission is from the oxygen vacancies or interstitial zinc ions of ZnO. The UV emission and blue emission show a blue-shift phenomenon due to quantum-confinement-induced energy gap enhancement of ZnO clusters. The ZnO clusters encapsulated in SBA-15 can be used as light-emitting diodes and ultraviolet nanolasers.
ZnO nanocrystals are embedded in an amorphous matrix of SiO2 using the sol-gel method and their optical properties were studied. A strong Blue-violet luminescence was observed. The photoluminescence’s maximum, located at 400 nm approximately, depends on the rate of the gelation process and also shows a slight dependence with the wavelength of excitation. The mean size of ZnO particles is about 27 nm leading with a weak quantum confinement. XRD patterns did not reveal the presence of ZnO particles but TEM investigation demonstrates agglomeration of ZnO crystallites in the composite with different sizes and shapes.
This review discusses two classes of organic template-derived amorphous silicas distinguished by the nature of template-matrix interactions and the extend to which subsequent processing dictates the final pore morphology. First we discuss surfactant-templated silicas where the template-matrix interaction is via non-covalent bonding mechanisms and the pore structure is established in the solution stage. We then discuss silicas templated by organic ligands covalently bonded to the siloxane network where subsequent processing strongly influences the final pore structure. 176 refs., 19 figs., 2 tabs.
ZnO crystallites have been anchored within the pore channels of silica MCM-41 (named ZnO-MCM-41) through a chelation method of Zn2+ by ethyldiamino group (EN) on the inner surface. The structural and elemental composition analyses of the material were characterized by the SAXRD, XRD, BET, XPS and EDS under HRTEM techniques. The ZnO crystallites were about 1.7 nm estimated from the UV–Vis spectra. The bright UV (377 nm), violet (420 nm) and blue–green (480 nm) photoluminescence (PL) bands have been detected. The unfamiliar violet PL band is related to the oxygen vacancies on the ZnO–SiO2 interface traps.
ZnO quantum dots (QDs)-SiO2 nanocomposite films were prepared using the target-attached radio-frequency sputtering. The transmission electron microscopy revealed the uniform dispersion of ZnO QDs with diameters about 2-7 nm in amorphous SiO2 matrix. The photoluminescence showed that small ZnO QDs are able to emit white light with luminescence spectra similar to those of the present GaN-based light emitting diode (LED). The calculated chromaticity coordinates of emitting light evidenced the feasibility of ZnO QDs-SiO2 nanocomposite films as the fluorescence material in optoelectronic devices.
The synthesis of ZnO–SiO2 nanocomposites through impregnation of a commercial mesoporous silica with zinc nitrate aqueous or ethanolic solutions is reported. During thermal treatment the samples evolve toward the formation of nanocrystalline ZnO particles (zincite phase) dispersed onto amorphous silica stable up to the temperature of 700 °C. The nanoparticle size distribution is affected by the process parameters used in the impregnation. A narrower and more homogeneous distribution is obtained using ethanolic instead of aqueous solutions. At higher temperatures the samples evolve towards the formation of a zinc silicate phase.
Mesoporous silica films fabricated on a conductive substrate are employed as templates for the electrochemical deposition of nanosized zinc oxide (ZnO). The ZnO-deposited mesoporous silica (ZnO-MPS) films are characterized and proved to retain the three-dimensional (3-D) cubic structure even after the electrodeposition process. Photoluminescence (PL) spectrum of ZnO-MPS film shows three main emission peaks at 451.5, 415 and 369 nm at room temperature. The emission peak at 369 nm presents apparent blue-shift than that of bulk ZnO at 384 nm, and the corresponding band-gap energy of nanosized ZnO significantly increases compared with that of bulk ZnO. These results demonstrate that the MPS films act as important templates for the electrochemical preparation of nanostructured ZnO. They also suggest the potential utility of ZnO-MPS films as optical nanodevices.
A new aqueous chemical growth method for in situ generation of ZnO clusters inside MCM-41 without modification scheme has been developed. Different from classical methods, the host MCM-41 does not need to be surface modified. Furthermore, ZnO clusters were prepared without calcinations, which could pack in the channels of mesoporous materials to produce quantum size effects. The PL emission peak was blue shifted to 420nm.
ZnO nanoparticles have been successfully loaded in the all-silica molecular sieve MCM-41. Combined powder X-ray diffraction and infrared spectral measurements show that ZnO particles have been formed and dispersed in the nanoporous channels host. The marked blue shifts in the electronic absorption band and visible fluorescence are ascribed to the quantum size effects. The visible photoluminescence is attributed to the defects that are related to the anionic oxygen vacancies.
Zinc oxide species hosted in the siliceous matrix MCM-48 were prepared by an organometallic route using ZnEt2 as the ZnO precursor. Gas phase as well as liquid phase infiltration of the precursor was studied in detail by ICP-AES, FT-RAIRS, 1H- and 13C-MAS-NMR, PXRD, TEM/EDX and UV–VIS. Highly loaded ZnO@MCM-48 materials with a Zn-content of up to 29.6 wt% were synthesized. A comparison of the different preparation techniques was carried out in order to find a convenient way of preparing ZnO@MCM-48 species for catalytic applications.
The size distribution and dispersion of nanomaterials, in addition to their dimensions, are crucial for their performance. Ordered mesoporous materials (OMMs), due to their periodic and size-controllable pore channels (2–10 nm) and high surface areas, have been regarded as “a natural micro-reactor” to construct novel ordered and well dispersed nanocomposites with controlled size and size distribution. In this review, we overview the recent developments in nanomaterials preparation and properties from ordered mesoporous materials over the last five years, focusing on materials preparation methodology and special properties. Finally, the present and future research interests of nanocomposites from OMMs will also be discussed.
Two novel synthesis routes for the preparation of mesoporous MCM-41 materials are introduced. Both methods use tetra-n-alkoxysilanes such as tetraethoxysilane (TEOS) or tetra-n-propoxysilane (TPS) as a silica source which are added to an aqueous solution of a cationic surfactant in the presence of ammonia as catalyst. In this study, n-alkyltrimethylammonium bromides and n-alkylpyridinium chlorides were employed as templates. The addition of an alcohol (e.g. ethanol or isopropanol) leads to a homogeneous system which allows the formation of spherical MCM-41 particles. The main advantages of these methods are short reaction times, excellent reproducibility and easy preparation of large batches.
In this work, mesoporous silica (MCM-41) was modified by triethanolamine (TEA) by a wet impregnation method. The structure and morphology of modified MCM-41 particles were characterized by X-ray diffraction, N2 adsorption/desorption, Fourier-transform infrared spectra, scanning electron microscopy, particle size analyzer and thermogravimetric analysis. The results show that the interior channels of MCM-41 are modified by TEA without destroying its mesoporous structure. Further, pure MCM-41and TEA-modified particles were adopted as dispersed phase in silicone oil for electrorheological (ER) investigation. The TEA-MCM-41 (modified with 7wt.% TEA)-based ER fluid with a volume fraction of 5% exhibits stronger ER activity than pure MCM-41 fluid. The possible mechanism related to dielectric properties is also provided.
Improved methods for the preparation of colloidal ZnO solutions of different particle size are described, and the relation between absorption threshold and particle size is reported. CHâOH radicals, radiolytically generated, transfer electrons to ZnO particles. The electrons are long-lived and cause a substantial blue shift of the absorption spectrum of ZnO in a wavelength range of 60 nm below the threshold. The wavelength of maximum bleaching is shifted to shorter wavelengths with decreasing particle size (size quantization effect). Maximum bleaching occurs with a negative absorption coefficient of 1.1 x 10âµ Mâ»Â¹ cmâ»Â¹. Electrons are also stored upon UV illumination of colloidal ZnO. The stored electrons react rather slowly with oxygen, the rate constant becoming lower with increasing particle size, and more rapidly with peroxy radicals.
This report presents the results of steady-state and time-resolved luminescence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes and at different temperatures. In all cases two emission bands are observed. One is an exciton emission band and the second an intense and broad visible emission band, shifted by approximately 1.5 eV with respect to the absorption onset. As the size of the particles increases, the intensity of the visible emission decreases while that of the exciton emission increases. As the temperature decreases, the relative intensity of the exciton emission increases. In accordance with the results presented in a previous paper, we assume that the visible emission is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the bulk ( ) of the ZnO particle. The temperature dependence and size dependence of the ratio of the visible to exciton luminescence and the kinetics are explained by a model in which the photogenerated hole is transferred from the valence band to a level in the bulk of the particle in a two-step process. The first step of this process is an efficient surface trapping, probably at an O2- site.
Wide band-gap semiconductorzinc oxide nanoclusters have been prepared in the channels of MCM-41 materials by functionalizing the MCM-41 with ethylenediamine groups, absorbing zinc cations, and calcinating at high temperatures. The products have been characterized by XRD, TEM, EDS, nitrogen adsorption and desorption, and UV−vis and PL spectroscopies. ZnO clusters were mostly confined and dispersed in the pores of mesoporous hosts. No large ZnO particles on the external surfaces have been detected. A massive blue-shift in UV−vis absorption spectra has been observed and large band increase can be expected. The nature of the PL spectrum has been attributed to the defects related to oxygen vacancies. In addition, the assembly of cobalt, nickel, and copper oxides inside MCM-41 materials has also been tried by this scheme, but at the moment, only the cobalt oxide can be prepared with good results. Unfortunately, noble metals have usually grown into large particles on the outside surface of MCM-41 by this scheme, e.g., a lot of silver particles with sizes much larger than the pore diameter of MCM-41 host have been obtained. However, the explanation is not yet clear.
The optical absorption and photoluminescence (PL) of ZnO loaded in mesoporous silica have been investigated. These ZnO/SiO2 composites are synthesized by impregnation of mesoporous silica with an aqueous solution of zinc acetate, followed by firing at 550 °C. A significant blue shift of the absorption edge compared to that of bulk ZnO in the absorption spectra is observed, and the band gap luminescence of ZnO is predominant in the emission spectrum, while the defect induced green luminescence, typical of ZnO, does not appear. However, the above sample, after additional annealing at 350 °C in air for three hours, exhibits an absorption edge (365 nm) near to that of the bulk ZnO; the colour of the sample also turns to yellowish from colourless, and green luminescence also appears. Combining the analysis of x-ray photoelectron spectroscopy (XPS), we propose that Zn-O-Si cross linking bonds formed in the interface between ZnO and the pore walls of silica have a great influence on the optical properties of ZnO/SiO2 before the additional annealing at 350 °C and the corresponding optical properties reflect both those of SiO2 and ZnO. After annealing, the Zn-O-Si cross linking bonds mentioned above are destroyed, and the optical properties of ZnO are exhibited alone.
ZnO nanoparticles were encapsulated in the porous activated carbon matrix by incipient-wetness impregnation. The use of the small host matrix allowed the size confinement of ZnO by utilizing the porous nature of the host matrix. Partial fixation of ZnO in the porous matrix determines the size and the dispersion of the particles. Experiments at different calcination temperatures were carried out to investigate structural and optical properties of ZnO nanoparticles in the porous activated carbon matrix using X-ray diffraction, scanning electron microscopy, Raman spectroscopy and photoluminescence. The optimal calcination temperature was found to be ∼450 °C in order to confine ZnO nanoparticles in the porous ACP matrix. Near-band-edge UV emission and green emission were both associated with the deep-level defect state. A decrease in full width at half maximum of E2 mode in Raman spectrum confirmed an increase in crystallite size due to higher calcination temperature, causing an increase in phonon lifetime.
This paper reports on the synthesis and characterization of a nanocomposite incorporating zinc oxide (ZnO) in mesoporous silica (MPS), a host medium for stabilizing the nanoparticles. The composite is prepared using a chemical route involving in-situ synthesis of ZnO in pores of host material in an acidic medium. The samples are characterized by X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), high resolution Transmission Electron Microscopy (TEM), UV–Vis absorption and Photoluminescent (PL) spectroscopy. The UV–Vis spectra and the PL spectra of the composite show blue shift in the position of the absorption band, indicating quantum confinement effect. On excitation at 320 nm, the samples exhibit a relatively narrow UV fluorescence emission band around 350 nm and an exciton recombination luminescence peak around 405 nm. The generally observed defect-related green emission is not observed, possibly being too low in intensity when compared to UV/blue emission.
Via electrochemical deposition, nanosized zinc oxide (nano-ZnO) is prepared within a template of mesoporous silica (MPS) film fabricated on a conductive substrate. Enhanced dark current from the nano-ZnO in MPS film is obtained because nano-ZnO is located in the nanosized pores of MPS. Most of the nano-ZnO surface is prevented from contacting ambient oxygen, and the combination of oxygen with free electrons of this n-type semiconductor is avoided; thus the free electrons increase the conductivity. Photodetection to ultraviolet (UV) light is examined at the exciting wavelength of 365 nm. The photocurrent with fast growing and decay times is observed due to the photo-generated holes being trapped at the interface between ZnO and pore walls of MPS film, while producing the photocurrent by photo-generated electrons. Photoluminescence (PL) spectrum of nano-ZnO in MPS film at room temperature shows increased amount of oxygen vacancies in these nano-ZnO, which might contribute to the conductivity.
Growth of (0001) facet-dominated, free-standing, piezoelectric zinc oxide (ZnO) nanostructures is challenged by the divergence of the surface energy due to intrinsic polarization. By controlling growth kinetics, we show the success of growing nanobelt-based novel structures whose surfaces are dominated by the polarized ±(0001) facets. Owing to the positive and negative ionic charges on the zinc-and oxygen-terminated ±(0001) surfaces, respectively, a spontaneous polarization is induced across the nanobelt thickness. As a result, right-handed helical nanostructures and nanorings are formed by rolling up single-crystal nanobelts; this phenomenon is attributed to a consequence of minimizing the total energy contributed by spontaneous polarization and elasticity. The polar-surface-dominated ZnO nanobelts are likely to be an ideal system for understanding piezoelectricity and polarization-induced ferroelectricity at nanoscale; and they could have applications as one-dimensional nanoscale sensors, transducers, and resonators.
A simple solvothermal impregnation method was used to prepare ZnO nanoparticles supported on MCM-41 and SBA-15. X-ray powder
diffraction, N2 adsorption–desorption, Electron Probe Micro Analysis (EPMA), and UV–vis spectroscopy were used to characterize the prepared
materials. The influence of the ZnO loading of different supports on the structural characteristics and the photocatalytic
activity toward degradation of methylene blue in water under ultraviolet irradiation were investigated. Wide angle X-ray diffraction
and UV–vis Diffuse Reflectance confirmed the existence of ZnO phase. A much smaller influence of impregnation with ethanolic
zinc salt solution on the porosity was observed for SBA-15 compared with MCM-41. Finally, the adsorption and photocatalytic
activity of the ZnO/mesoporous materials depend on porous characteristics of the support materials.
Different photoluminescence (PL) techniques have been used to study the emission of light from MCM-41 and MCM-48. It was found that the intensity of PL is enhanced after rapid thermal annealing (RTA). This enhancement is explained by the generation of the oxygen-related defects according to the surface properties. Through the analysis of PL with RTA and photoluminescence excitation (PLE), we suggest that the NBOHC and the twofold coordinated silicon centers are responsible for the red and blue–green PL of MCM meso-porous materials, respectively. The PL intensity in MCM meso-porous materials degrades with time during photoexcitation. The dominant mechanism of PL degradation involves the formation of the chemisorbed oxygen-related complexes on the surface, which are adsorbed onto the surface and act as an efficient quencher of PL.
Extremely small ZnO particles (< 2.5 nm) are formed in the precipitation of Zn2+ by OH− in alcoholic solution. The absorption threshold and fluorescence band shift towards longer wavelengths upon growth of the colloidal particles. These shifts are typical for the transition from molecular to semiconductor material. The colloidal ZnO could be separated in the form of a powder with a specific surface of 108 m2/g. A Cu2+-doped colloid was also prepared. Colloidal ZnO has a strong fluorescence, λmax = 540 nm, which decays with τ = 10 ns, and a weak one, λmun = 370 nm, τ < 1 ns. Intense illumination in the absence of O2 results in the disappearance of the 540 nm fluorescence and a blue-shift of the absorption spectrum. Both changes are immediately reversed upon admission of air. The results are explained on the basis of the Hauffe mechanism for the dissolution of ZnO, in which intermediate Zn+ plays a role. Prolonged illumination of a propanolic sol leads to precipitation of zinc metal. In the presence of 1 × 10−1 M methylviologen. MV+ is formed with a quantum yield of 60%.
In the present work a novel variant of synthesis of ZnO nanoparticles in the mesoporous silica matrix was suggested. The method is based on the introduction of a hydrophobic Zn compound, Zn acetylacetonate, into the hydrophobic part of silica–surfactant composite, followed by thermal modification. The ZnO content in the as-synthesized composites attains 14.6 wt.%, which exceeds most previously reported values for ZnO/mesoporous silica nanocomposites. The samples were characterized by SAXS, chemical analysis, TEM, XRD, EDX, UV–Vis and photoluminescent (PL) spectroscopy. The UV–Vis spectra of the composites show a significant blue shift of an absorption band demonstrating an increase of ZnO band gap energy due to the quantum confinement effect. The PL spectra under the excitation with 337 nm light show a narrow UV emission band of ZnO at 370 nm, while emission in the visible range of spectra almost disappeared. The intensity ratio for the as-synthesized nanocomposites is up to 65 (at 77 K).