SiO2 esaslı nanomalzemelerin hazırlanması ve karakterizasyonu
Preparation and characterization of SiO2 based nanomaterials
Yazar: BENGÜ ÖZUĞUR
Danışman: PROF. DR. FATMA TEPEHAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Fizik Mühendisliği Ana Bilim Dalı
Derece: Doktora, 2012
Konu: Fizik ve Fizik Mühendisliği = Physics and Physics Engineering
Dizin: Nanopartiküller = Nanoparticles ; Silisyum dioksit = Silicon dioxide ; İnce filmler = Thin films

Özet:
Nanoyapılı silisyumdioksit ince filmler sol-jel kaplama yöntemi kullanılarak elde edilmiştir. Parçacık boyutu dağılımı dar bir bölgede olan nanoparçacıklar içeren nano-SiO2, SiO2-nano-SiO2 (hibrit-silisyumdioksit) ve Al2O3-nano-SiO2 (aluminyumoksit-nanosilisyumdioksit) kompozit filmler sentezlenmiştir. Nano-SiO2 filmler, tetraethylortosilicate (TEOS), deiyonize su, etanol ve amonyak çözeltisinden Stöber yöntemine benzeyen bir karışım kullanılarak elde edilmiştir. Film içindeki parçacıkların büyümesini kontrol etmek için, nanoparçacıklı SiO2 solü ile SiO2 solünün ve Al2O3 solünün karışımından hibrit silisyumdioksit ve alüminyumoksit-nanosilisyumdioksit kompozit filmler hazırlanmıştır. Sonuç olarak, SiO2 ve Al2O3 jelleri nano-SiO2 parçacıklarını çevreleyerek büyümelerini engellemektedir. Parçacık boyutunun çözelti içindeki NH3/TEOS oranına ve 450-1100°C aralığındaki ısıl işlem sıcaklığına göre değişimi incelenmiştir. Karakterizasyon için, X-ışını kırınım cihazı (XRD), atomik kuvvet mikroskobu (AFM), taramalı elektron mikroskobu (SEM), geçirmeli elektron mikroskobu (TEM), NKD, ultraviyole-görünür bölge (uv-vis), Fourier dönüşümlü kızıl ötesi (FTIR) spektrofotometreler kullanılmıştır. XRD çalışmaları nano-SiO2 filmlerin 1100 ? C hariç diğer tüm ısıl işlem sıcaklıklarında amorf yapıda olduğunu göstermektedir. Alfa-cristobalite kristal yapısı 1100 ? C ısıl işlem sıcaklığında oluşmaktadır. Kırılma indisi ve söndürme katsayısı gibi optik parametrelerin NH3/TEOS oranına ve ısıl işlem sıcaklığına göre değişimleri NKD Analiz Cihazı ile elde edilmiştir. Hibrit-SiO2 ve Al2O3-nano-SiO2 kompozit filmlerinin kırılma indisi ve söndürme katsayısı değerleri ısıl işlem sıcaklığı ile paketleme yoğunluğunun artmasına yol açarak (parçacıkların sıkı bir yapı oluşturması) büyümektedir. Hibrit filmin aktivasyon enerjisi 22.3 kJ/mol iken aktivasyon entalpisi 14.7 kJ/mol olarak bulunmuştur. Hesaplanan aktivasyon enerjisi değeri literatürde silisyumdioksit filmler için verilen aktivasyon enerjisi değerine göre daha düşüktür. Toplam yüzey enerjisini arttırmak amacıyla nanoparçacıklı ince filmler daha yüksek yüzey alanına sahiptir. Böylece, nanoyapılı silisyumdioksit filmlerin parçacıklarının büyümesini sağlamak için daha az enerji gerekmektedir. Nanoyapılı filmlerin soğurma sınırı karışım içindeki NH3/TEOS oranı arttığında uzun dalgaboylarına kaymaktadır. Diğer taraftan, ısıl işlem sıcaklığı 450 ? C'den 900 ? C'ye arttığında, silisyumdioksit nanoparçacıklarının boyutları arttığından hibrit filmlerin soğurma sınırında kırmızıya kayma gözlenmektedir ki bu da silisyumdioksit nanoparçacıklarının kuantum kuşatma etkisini doğrulamaktadır. Bu çalışma, farklı parçacık boyutlu hibrit-silisyumdoksit ve alüminyumoksit kompozit filmler hazırlanarak optik özelliklerin değiştirilebileceğini göstermektedir.

Abstract:
Sol?gel spin coating technique was used to produce nanostructured silicondioxide thin films. The nano-SiO2, SiO2-nano-SiO2 (hybrid-silica), and Al2O3-nano-SiO2 (aluminiumoxide-nano-silica) nanocomposite films containing narrow size distributed nanoparticles were synthesized.The nano-SiO2 film was prepared with a Stöber-like process using a mixture of tetraethylortosilicate (TEOS), deionized water, ethanol and ammonia solution. The starter SiO2 solution was prepared using tetraethoxysilane (TEOS) solved in ethanol and mixed with deionized (DI) water for the hydrolysis reaction. Then hydrochloric acid (HCl) used as a catalyst, was added to the regular SiO2 solution at room temperature, until it became homogeneous and transparent. The Al2O3 sol was prepared using aluminium butoxide, distilled water and acetic acid.To control the growth of the particles inside the film, the hybrid silicondioxide and nanocomposite aluminiumoxide-nano-silicondioxide films were prepared by the mixture of the SiO2 nanoparticled sol with the regular SiO2 sol and the Al2O3 sol. Both the SiO2 and Al2O3 sols were mixed with nano-SiO2 sol at room temperature for 30 min. in volume ratio of 1:10. Then, the sols were spin coated on the corning (2947) and quartz glasses substrates at 1000 rpm for 30 s. The final coatings were heat treated at 450, 550, 650, 900 C for 1 h. and at 1100 C for 48 h. by a furnace.For the characterization, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), profilometer, NKD, ultraviolet?visible (uv?vis), fluorescence and Fourier Transform Infrared (FTIR) spectrophotometers were used.The structure of the silica films was characterized by an X-Ray Diffractometer using monochromatized Cu-K ? radiation. The XRD spectra of the films were recorded by scanning 2 ? in the range 20?80 ? , with a grazing angle of 1 ? .Atomic force microscope (AFM) and scanning electron microscope (SEM) were used to study the detailed morphological surface analysis and nanostructure of the thin films. The particle size of all silica powders were investigated by the transmission electron microscope (TEM).The transmittance data of the films were carried out by the spectrophotometer (NKD) in the s and p polarization modes in between 300 nm to 1000 nm wavelength. Refractive indices and extinction coefficients were evaluated by the Pro-Optix software incorporated with this device. The thickness of the films was calculated using the profilometer.The absorbance spectra of nanostructured silica, hybrid silica and aluminumoxide-siliaca nanocomposite films were measured by uv-visible spectrophotometer in between 200 nm and 1000 nm wavelength. Fluorescence properties of the nanostructured silica films was studied by spectrometer, excited with the wavelength of 242 nm at room temperature. The Fourier transform infrared (FT-IR) spectra of the films in the transmission mode were recorded in a wave number range of 650?4000 cm?1.The XRD studies showed that nano-SiO2 thin films were amorphous at all annealing temperatures except 1100 ? C. The alfa-cristobalite crystal structure formed at the annealing temperature of 1100 ? C. A slow XRD scan was used to calculate the average crystallite size of the film, using Scherrer?s formula. The change in the crystallite size was investigated according to the NH3/TEOS ratio in solutions. The weakening and broadening of the XRD peaks were attributed to the decrease of the crystallite size due to the reduced NH3/TEOS ratio in the compositions.AFM and SEM measurements showed that SiO2 nanoparticles were distributed almost uniformly in the SiO2 thin film matrix at the annealing temperature of 450 ? C. In comparison with the SiO2 film, the nano-SiO2 thin film manifested a granular structure. According to the graph of maximum diameter of particles versus the number of particle distributions of the different SiO2 films, Gaussian-like particle distributions centered about the mean particle size of the films shifted to a smaller particle diameter when the NH3 /TEOS ratio in compositions is decreased. The change in the particle size was investigated according to the NH3/TEOS ratio in solutions and heat treatment temperature range from 450 to 1100°C for all the silica films.Similar results were observed from the TEM measurements of the powder samples were prepared in the same conditions.The decrease of the calculated crystallite sizes of the thin films at the annealing temperature of 1100 ? C are in agreement with the AFM, SEM and TEM measurements.The optical studies revealed that the transmittance of the films increased and stabilized by doping nanoparticles in the sol. The higher transmittance observed in the ratio of 1:64 can be attributed to the structural homogeneity. Optical parameters such as refractive indices and extinction coefficients were obtained by the NKD spectrometer with respect to NH3/TEOS ratio and the annealing temperature of the films. The refractive index and the extinction coefficient of the hybrid-SiO2 and Al2O3-nano-SiO2 composite films increased with the temperature leads to an increase of packing density of the film (tighter packing of particles).The absorption edge of the nanostructured films shifted to the longer wavelengths with the increasing ratio of NH3/TEOS in compositions. The quantum confinement effect of nanoparticles was confirmed by the cut-off wavelength shift with both UV-vis and fluorescence measurements. On the other hand, a red shift in the absorption threshold of the films indicated that the size of silica nanoparticles was redounded by an increase at the annealing temperatures from 450 to 900 ? C, and these also confirm the quantum confinement effect of the silicondioxide nanoparticles. As seen from the fluorescence measurements, the position of maximum emission peak is shifted to the higher wavelength (red shifted) as the NH3/TEOS ratio increase. The other maxima of the fluorescence spectra are attributed to the vibrational effect of the energy levels of the solid state, and also observed that the intensity increased with the decrease of NH3/TEOS ratios in composition. The shifts in the fluorescence emission maxima values are compatible with the shifts in the cut-off wavelength values of the nanostructured-SiO2 films from the uv-vis measurements.The Si-O-Si stretching motion, which was expected to be about 1080 cm?1, shifted to lower wavenumber values with the decrease of the size of the silica nanoparticles. The transverse optical (TO) component of the asymmetric stretching vibration of Si-O-Si bond shifted to the lower wavenumber values (from 1049 to 1020 cm?1) when particle size of the films decreased. Chemically, the decrease in amount of the catalyzer material (in this case NH3 solution) effects the particle distribution in the solution, and it leads to the coagulation in the clustered medium. The possible reason is that owing to the effect of finite size of nanoparticles, the bonds of surface atoms are breaking. Therefore, the inlocalized electrons on the surface of particles are rearranged and the lattice constrictions like change in symmetry occur. Meanwhile, the lower wavenumbers which correspond to lower frequencies may caused by the cross-linking of the Si-O-Si chains while the synthesized particles are smaller due to the decreasing ratio of NH3/TEOS in compositions. All that leads to the shifting of the transmission bands of FT-IR spectrum to the lower wavenumbers.The activation energy and enthalpy of the hybrid silicondioxide film were evaluated as 22.3 and 14.7 kJ/mol. Calculated activation energy is much lower in comparison to the activation energy of silica films reported in the literature. In order to increase the total surface energy, nanoparticled thin films have a higher surface area. Thus, less energy is required to induce the particle growth of nanostructured SiO2 films.This study indicates that it is possible to alter optical properties through the preparation of hybrid-silica and and aluminiumoxide-nano-silica composite thin films with different particle sizes. As a result, the SiO2 and Al2O3 gels encapsulate the nano-SiO2 particles to prevent their growth.