Characterisation of Nanostructured ZnO for Photocatalysis Applications - Report Example

Name : xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Tutor :xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Title : Institution : xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Date :xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx @ 2013 Characterization of Nano-structured ZnO for Photo catalysis Applications Introduction Characterization of nanostructured ZnO involves the description of the qualities of the compound that are important in photocatalysis applications. ZnO has several qualities for these applications. Characterization of ZnO also involves different processes and methods such as the traditional microscopic techniques. Some of the traditional microscopic methods of characterization include the combination of spectroscopy and microscopy with ion etching methods. According to Djurisic and Leung, (2006, pp. 945) Zinc Oxide is a material that has immense potential for many practical applications. Some of these applications include piezoelectric transducers, phosphors, spin functional equipment, transparent conductive oxides, UV-light emitters, optical wave guides and surface acoustic wave devices. ZnO is a very good material for phototonic applications within the blue spectral range and the UV range (Djurisic and Leung 2006, pp. 945). This is because of its wide band gap of about 3.37 eV at room temperature. It also has high exciton binding energy makes it possible for effective exciton emissions even at room temperature. Doping ZnO with transition metals makes it suitable for spintronic applications. ZnO is also good for sensing applications because of its sensitivity to different species of gases such as acetylene, carbon monoxide and ethanol. In addition its nancentrosymmetric structure gives it the piezoelectric property which makes it applicable to actuator applications and electromechanical sensors (Bai, Liu & Daren 2012, pp. 127). The biocompatibility of ZnO also makes it suitable for biomedical applications. The compound is also friendly to the environment and is chemically stable (Djurisic and Leung 2006, pp. 945). Scientists have used photoluminescence spectroscopy to study the optical properties of different forms of ZnO. Many of luminescence spectra of ZnO nanostructures were measured at room temperature and variable temperatures as well (Bai et al pp. 130). Photoluminescence in UV and visible spectral regions Researchers have produced results about low temperature photoluminescence nanostructures including nanowall systems, nanowires, nanowalls, faceted nanorods, nanosheets, nanoflowers, nanoblades and nanoparticles. ZnO PL spectra at low temperature of about 4-10 K shows many peaks which reflect bound excitons. A commom bound exciton line in the nanostructures of ZnO is the I4 line existing at approximately 3.3628 eV. All ZnO nanostructures, films and single crystals are similar in that all of them have their bound exciton peaks disappearing at a range of temperatures between 50 and 150K. At room temperature what can be seen are the free exciton emissions alone. PL spectra at room temperature show many peaks within the visible spectral region which are associated with defect emission. Studies have revealed the existence of emission lines at 405, 420, 446, 466, 485, 510, 544, 583 and 640 nm. ZnO nanostructures have the green emission as the most common defect emission. This is true for other ZnO forms. The diameter of the nanowire determines the intensity of the blue-green defect emission. However, there were also increased and decreased defect emission intensities going with a lower diameter of wires. The defect that brings about the green emission is unknown. However, there is proof that it exists at the surface (Liu, Kang, Chen, Shafiq, Zapien, Bello, Zhang, & Lee 2009, pp. 229). Experiments have shown that when ZnO nanostructures are coated with a surfactant then the green emission is concealed. Polarized experiments on luminescence from ZnO nanorods that are aligned also showed that the green emission starts on the nanorod surface. Nanorod structures have also been found to have yellow defect emissions. The emission is related to the oxygen interstitial although an impurity of Li may be responsible. There was a difference between the deep levels that caused the yellow and green emissions (Liu et al pp. 237). Contrary to the defect that causes the green emission, the one that causes the yellow emission is not found at the surface. To add to the green and yellow emission, there was orange-red emission that also comes out. The intense emission that can be seen in ZnO nanosheets was believed to temporarily result from the dislocation at the surface. The orange red emissions resulted from the oxygen interstitials. There are many methods that are being used in combination with photoluminescence to get the source of the defect emissions in ZnO. Electron paramagnetic resonance (EPR) spectroscopy has been used frequently to identify paramagnetic defects.EPR is useful in the study of ZnO defects. A combination of photoluminescence and positron annihilation spectroscopy is also useful in the study of defects in ZnO (Liu et al pp. 230). Stimulated emission in ZnO nanostructures Studies have shown there is a possibility of ZnO producing amplified spontaneous emissions when optically pumped lasing is applied to it. There is amplified spontaneous lasing for self organized ZnO fibres. Researchers have also showed that lasing happens in various structures such as nanowires, nanoribbns and tetracombs. The coherent coherent result in the nanostructures of ZnO or nanostructured films may be achieved through two fundamental ways. In the first method multiple reflections bring about coherent feedback from the final sides of the nanostructure. The emission of UV increases at the edges of the nanowire while green emissions come from all over the nanowires. In method two the coherent feedback comes from several dispersed events (Liu et al pp. 230). Random lasers have the coherent feedback coming from recurring dispersing events. The wavelength of the emissions is bigger than the scatter size. The lasing is dependent on the area of excitation and this has been displayed in ZnO polycrystalline films, nanowires and nanorods (Liu et al pp. 236). Polycrystalline thin films have random lasing in ZnO. Others with random lasing include ZnO powder films, nanoneedles, nanorods and nanowires. The threshold of lasing is affected by crystallinity and the structure of the film. Strain also determines the characteristics of lasing. A major sign of random lasers is that it is possible to observe stimulated emission from every any direction and the measured spectra has its mode structure displaying angular dependence. Researchers have also reported about lasing with ZnO nanowires whereby each of the wires from a Fabry Perot reasonator surrounded by reflecting faces. After that, many other studies have reported of stimulated emissions from many different ZnO nanostructures. Studies have reported about lasing in tetrapod nanostructures, nanoribbons, nanocombs, whiskers, nanowires, nanocoral reefs, nanofibers and microtubes. Some measurements were experimented on nanostructure ensembles but stimulated emissions emanating from each nanostructure were seen and well as those nanostructure with low density (Sajjad 2011, pp. 56). This was evidence that there was no possibility of getting feedback from multiple random scattering. In certain cases for example where there were specific nanoribbons and nanowires cavity identification is very clear and studies have also shown lasing with nanostructures have very intricate morphologies. Again, stimulated emission was not just obtained for those nanostructures whose fabrication had been done at hot temperatures through deposition of vapor. It was also done for those nanostructures made at reduced temperatures using aqueous solutions (Liu et al pp. 231). Studies show that when there is poor crystallinity very high thresholds of lasing can be achieved in ZnO films. It is also possible to get stimulated emission in nanostructures that have varying times of decay and different defect emissions. However, if the quality of crystals is poor and there are high cavity loses then it cannot be attained. Nanostructure dimensions, conditions of experimentation and the quality of the cavity will therefore determine the threshold of lasing. Again, the threshold of lasing acquired for different wavelengths of excitation must not be subjected to direct comparison because they absorb ZnO differently at different wavelengths (Sajjad 2011, pp. 56). Many groups of scientists have done time resolved studies of stimulated emission. These studies were done on many ZnO nanostructures including nanowires, nanoribbons, nanoneedles, thin films, nanorods, tetrapods and highly faceted rods (Bergmann 2011, pp. 81). Normally, the stimulated time of decay emission goes faster than in spontaneous emission. This puts it below the limit of detection for certain time resolved photoluminescence films. The EE and the EHP regimes have emissions that exhibit different behaviors at different times. The rise time for the EHP emission is shorter and this may be caused by the hot carriers being thermalized. The emission peak of the EHP shows some changes as time passes. The changes are caused by direct lasing spectra measurements and also the measurements of the transient profiles of the lasing changes as a function of wavelength. The EHP emission only decays in a few picoseconds. Contrary to EHP emission, the stimulated emission within the EE regime may display a longer time of delay before the emission begins. This is because a lot of time was required to get high excitons concentration in the state of excitement (Sajjad 2011, pp. 56). Non linear optical properties of ZnO Zinc Oxide has non-zero second order susceptibility. This results from its non centrosymmetric crystal structure. Djurisic & Leung (2006, pp. 955) studied different forms of Zinc Oxide for the non linear properties they display. These Researchers have also measured the non linear optical reaction of C excitons. They used the 4 wave technique of mixing on ZnO samples with single crystals. ZnO crystals, nano wires, nano ribbons and thin films were also measured for the second harmonic generation (SHG) were also measured. In thin films of ZnO, SHG was found to be reliant on the thickness of the films, conditions of deposition, shape of the grain, crystallite orientation, and the structure of crystals and thickness of the film (Sajjad 2011, pp. 94). The important parts of the SHG signal originate from the interfaces and boundaries of grains. Thin films displayed that they have the enhancement of non linear susceptibility in comparison to the bulk values. There was a decline in second order susceptibility as the film thickness increased. This was caused by altering the orientation of the polar axis with increasing film thickness. Djurisic & Leung (2006, pp. 955) also found out that in some experimental set ups ZnO oxide films displayed third harmonic which is comparable to a conventional SHG signal. Sputtered ZnO films showed the non linear properties of the second and third order although there was no preferential direction for growth. However the bulk values were higher than the second order non linearities. Studies have however focused more on SHG in ZnO than THG (Dubbaka, 2008, pp. 78). Researchers studied ZnO nanowires and nanoribbons for carrier dynamics and SHG and THG signals in nanowires but failed to do anything on the nonlinear optical properties of ZnO nanostructures. ZnO was also reported to display three photon and three photon spectroscopy and measurements of non linear refraction and absoption. Single crystalline ZnO had two and three photon induced luminescence while ZnO microtubes had the two and three photon induced luminescence as well. Nevertheless, studies on multi photon spectroscopy and the characterization of non linear Nanostructured ZnO optical properties are rare because the theory and measurements are very complex than the characterization of linear optical properties (Du, Chen, Yao & Wang 2013, pp. 128). Doped ZnO Many studies have also been done on the optical properties of ZnO. In order to study how doping affects the optical properties of ZnO researchers examine UV Spectra of low temperature to find if there is any proof of bound exciton peaks that do not appear in samples that have no doping. They also inspect the emissions to see the changes appearing in the defect emission spectra. Doping may be done in four ways. The first way is to use donor impurities to get high n-type conductivity, the second one is doping with acceptor impurities to get p-type conductivity. The third way is doping by use of rare earth elements to get the needed optical properties and finally doping by use of transition metals to get the needed magnetic properties. Doping to get n-type conductivity is easy but the achievement of p-type conductivity is made difficult because of native defects. Generally doping using different donors results in the production of a broader UV emission peak. However, the shift of the peak is determined by the dopant (Morgen, Henriksen, Kyrping, Sorensen, Yong, Holstm, Aelsen & Hansen, 2009, pp. 107). Researchers discovered that Sn doping results in the biggest red shift of UV emission. It also causes a green emission when ZnO nanowires are doped. A different experiment on the optical properties of ZnO Sn-doped nanowires did not show any shift in UV emission as well as the occurrence of new shifts. It is obvious that the doped and undoped ZnO show different optical properties depending on the conditions of fabrication it is not easy to know the pattern of change in the properties when doping in done (Du et al pp. 130). It is anticipated that new luminescence peaks will appear but is there is a major increase in carrier density then it is expected that there should be a red shift of the near band edge emission. When ZnO is Sn-doped, its optical properties are also determined by the type of gas used. This shows that fabrication conditions are very important. Doping with Pb made the emission peak to turn red and the position of the peak was determined by the content of the Pb. Doping with Sc also caused the PL Spectra to have a red shifting with increased concentration. This result had huge similarities with the results of Pb doping (Djurisic & Leung 2006, pp. 948). Bibliography Bai, H., Liu, Z. & Daren D.S. 2012. Hierarchical ZnO Nanostructured Membrane for Multifunctional Environmental Applications. Colloids and Surfaces A. Physicochemical and Engineering Aspects, 410, 11-17. Bergmann, C.P. 2011. Nanostructured Materials for Engineering Applications. Springer. Djurisic, A. B., & Leung, Y.H. 2006. Optical Properties of ZnO nanostructures. Small journal Vol. 2, No. 8-9 pp. 944-961. Du, Y., Chen, R.Z., Yao, J.F. & Wang H.T. 2013. Facile Fabrication of ZnO by thermal treatment of Zeolitic imidazolate Framework-8 and its Photocatalytic Activity. Journal of Alloys and Compounds. Dubbaka, S. 2008. Branched Zinc Oxide Nanostructures: Synthesis and Photocatalysis. ProQuest. Liu, Y., Kang, Z.H., Chen, Z.H., Shafiq, I., Zapien, J.A., Bello, I., Zhang, W.J.& Lee, S.T. 2009. Synthesis, Characterization and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations. American Chemical Society. Morgen, P., Henriksen, B.O, Kyrping, D., Sorensen, T., Yong, Y.X., Holstm, J., Aelsen, J.R. & Hansen, P.E. 2009. Nanostructured Materials for Advanced Technological Applications. Springer Science + Business Media B.V. Sajjad, S. 2011. Synthesis, Characterization and Applications of Nanomaterials in the Field. GrinVerlag. Read More

Contrary to the defect that causes the green emission, the one that causes the yellow emission is not found at the surface. To add to the green and yellow emission, there was orange-red emission that also comes out. The intense emission that can be seen in ZnO nanosheets was believed to temporarily result from the dislocation at the surface. The orange red emissions resulted from the oxygen interstitials. There are many methods that are being used in combination with photoluminescence to get the source of the defect emissions in ZnO.

Electron paramagnetic resonance (EPR) spectroscopy has been used frequently to identify paramagnetic defects.EPR is useful in the study of ZnO defects. A combination of photoluminescence and positron annihilation spectroscopy is also useful in the study of defects in ZnO (Liu et al pp. 230). Stimulated emission in ZnO nanostructures Studies have shown there is a possibility of ZnO producing amplified spontaneous emissions when optically pumped lasing is applied to it. There is amplified spontaneous lasing for self organized ZnO fibres.

Researchers have also showed that lasing happens in various structures such as nanowires, nanoribbns and tetracombs. The coherent coherent result in the nanostructures of ZnO or nanostructured films may be achieved through two fundamental ways. In the first method multiple reflections bring about coherent feedback from the final sides of the nanostructure. The emission of UV increases at the edges of the nanowire while green emissions come from all over the nanowires. In method two the coherent feedback comes from several dispersed events (Liu et al pp. 230). Random lasers have the coherent feedback coming from recurring dispersing events.

The wavelength of the emissions is bigger than the scatter size. The lasing is dependent on the area of excitation and this has been displayed in ZnO polycrystalline films, nanowires and nanorods (Liu et al pp. 236). Polycrystalline thin films have random lasing in ZnO. Others with random lasing include ZnO powder films, nanoneedles, nanorods and nanowires. The threshold of lasing is affected by crystallinity and the structure of the film. Strain also determines the characteristics of lasing.

A major sign of random lasers is that it is possible to observe stimulated emission from every any direction and the measured spectra has its mode structure displaying angular dependence. Researchers have also reported about lasing with ZnO nanowires whereby each of the wires from a Fabry Perot reasonator surrounded by reflecting faces. After that, many other studies have reported of stimulated emissions from many different ZnO nanostructures. Studies have reported about lasing in tetrapod nanostructures, nanoribbons, nanocombs, whiskers, nanowires, nanocoral reefs, nanofibers and microtubes.

Some measurements were experimented on nanostructure ensembles but stimulated emissions emanating from each nanostructure were seen and well as those nanostructure with low density (Sajjad 2011, pp. 56). This was evidence that there was no possibility of getting feedback from multiple random scattering. In certain cases for example where there were specific nanoribbons and nanowires cavity identification is very clear and studies have also shown lasing with nanostructures have very intricate morphologies.

Again, stimulated emission was not just obtained for those nanostructures whose fabrication had been done at hot temperatures through deposition of vapor. It was also done for those nanostructures made at reduced temperatures using aqueous solutions (Liu et al pp. 231). Studies show that when there is poor crystallinity very high thresholds of lasing can be achieved in ZnO films. It is also possible to get stimulated emission in nanostructures that have varying times of decay and different defect emissions.

However, if the quality of crystals is poor and there are high cavity loses then it cannot be attained. Nanostructure dimensions, conditions of experimentation and the quality of the cavity will therefore determine the threshold of lasing.

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