Investigation of Photodeposition of Gold on Titanium Dioxide Nanoparticles - Term Paper Example

Student Name/Number: Institution: Date: Investigation of Photodeposition of Gold on Titanium Dioxide Nanoparticles Analytical Techniques i. X-ray Photoelectron Spectroscopy (XPS) XPS surface analysis is performed by irradiating a sample using monoenergetic soft x-rays of sufficient energy and analyzing the electrons detected [26]. The X-rays can be from monochromatic sources of radiation, Mg K, or Al K. XPS is used to provide the following information: The type of elements present on the surface of a sample (within 1-12 nm) and their quantities. Empirical formula of materials. Contamination that may exist in a material and elements that contaminate it. Electronic or chemical state of elements present on the surface of a sample, including local bonding of atoms. The BE and density of electronic states. Uniformity of elemental composition on the sample surface, and as a function of ion beam etching. The thickness of thin layers of the materials within 1-12 nm of the sample surface. When a beam of X-ray hits the sample, the electrons in the sample undergo excitation. The photons produced after excitation to excitation interact with the atoms on the sample surface, resulting in emission of electrons due to the photoelectric effect. Some of the emitted electrons scatter through the sample as they move towards the sample surface, while others are promptly emitted without experiencing energy loss and escape into the vacuum chamber. As photoelectrons move through the specimen, they undergo inelastic collisions, recombination, and trapping in different material excitation states. These effects make surface signals stronger than the signals from the deeper layers of a specimen [26]. Hence, XPS signal is exponentially surface sensitive because only the electrons that are ejected from the surface are detected. The average distance moved by an electron through a solid before energy loss is called inelastic mean free path. Once in the vacuum, the electrons are collected by an electron analyzer which measures the kinetic energy of the electrons. Figure 1 shows a schematic representation of XPS. Figure 1: A schematic diagram of XPS (Source: [30]) An electron analyzer produces a spectrum of electron intensity versus binding energy with energy peaks that correspond to a particular element present in a sample. A part from identifying elements, peak intensities can also be used to determine the quantity of an element within the specimen. The peak area is proportional the quantity of atoms in an element. The chemical composition of a specimen is obtained by determining the respective contribution by every peak area [30]. The actual binding energy of an electron is not only dependent on the level of occurrence of photoemission and chemical bonding in an element, but also on the local physical and chemical environment, and formal oxidation state of an atom. Changes in these factors produces chemical shifts in the peak positions in the XPS spectrum. The chemical state of the specimen being analyzed can be determined using chemical shifts. The initial and final states effects of the elements in a material determine the peak position and the full-width-half-maximum (FWHM) of the peaks. These effects are attributed to relaxation of electrons and the difference in electronic structure of final states [16]. In our project, we will use XPS to provide the information on surface composition and chemical states which may be responsible for features exhibited by the Au/TiO2 composites. XPS will also be used to determine electron intensity and the binding energies of the Au NPs generated. ii. Neutral Impact Collision Ion Scattering Spectroscopy (NICISS) NICISS typically involves directing a pulsed beam of inert gas ions, preferably helium ions (H+) at low energy ( 3 keV) at a sample [29]. As the ions travel to the sample surface, they become neutralized and continue travelling through the sample until they collide with an atom, then are backscattered. The backscatter ions are detected by microchannel plates to produce time-of-flight (TOF) spectrum. The energy of the backscattered projectiles is determined by their TOF from the target atom to the detector. During the backscattering process, the projectiles lose energy in amount proportional to the mass of the element on target. As the ions penetrate through a sample, there is continuous energy loss consisting of electronic excitations and small angle scattering [29]. In analyzing data from NICISS spectrum, there are two ways to consider the inelastic energy. First, inelastic energy loss has to be considered for determining the energy of the backscattered projectile. Second, the inelastic energy loss due to small angle scattering forms part of the continuous energy loss of the projectiles that pass through the material and depends on the energy loss straggling and stopping power of the section. The TOF spectrum is an energy loss spectrum that can be used to provide information on the composition, the crystalline structure, and the concentration depth profiles of elements (in non-crystalline samples) on the surface or near-surface regions of a material when projectile ions interact with a target atom in matter. The interaction of projectiles with the target atom involves processes such as charge transfer, inelastic energy transfer, and elastic transfer of kinetic energy. The continuous inelastic energy loss (also called the stopping power) depends on the element in the sample from which the projectiles are backscattered, and is used to establish the depth from which backscattering of the projectile occurred [28]. NICISS will be used in the project in the investigation of the Au/TiO2 hybrid and the concentration depth profiles of Au NPs to provide information that is essential to understand the crystal structure and orientation of the hybrid surface. iii. UV photoelectron spectroscopy (UPS) This technique typically involves measurement of kinetic energy of photoelectrons which are emitted when molecules absorb UV photons, so that the molecular orbital energies can be determined. The UV light with energy in the order of 0-100 eV is used to excite the photoelectrons [25]. Analysis of the kinetic energy as well as the angular distribution of the excited photoelectrons can be used to provide information on the band structure (electronic structure) of the material being investigated [24]. A UPS can also be used to determine the electronic work function of a material, a property that is applied in electronic devices. UPS is similar to XPS in terms of the principle of operation, with the only difference being that in UPS, the photoelectric effect is induced using ionization radiation at energies in the range of 0-100 eV, while in XPS photon energy > 1000 eV are usually used [25]. In a lab setting, UV photons are generated by use of a gas discharge lamp, usually filled with gases such as helium, neon or argon [25]. Figure 2 shown below is a diagrammatical illustration of how UPS operates. The ionization energy required to remove electrons from the orbitals of a molecule is given by the difference between the final state and the initial state of the ionized molecule. Figure 2: The principle of operation of a UPS We can apply UPS in our project to determine the valence band structure of the Au/TiO2 composite. This will be accomplished by irradiation the Au/TiO2 samples with UV photons from helium gas with energies of 21.2 eV (He I) and 40.8 eV (He II) depending on the operating conditions in order to obtain the binding energy. iv. Scanning Electron Microscopy (SEM) Unlike the other methods, SEM has the capability to image and analyze bulk material. SEM operates on the principle illustrated in figure 3. The technique uses a beam of electrons to produce an image of a material under investigation, and magnifies the image by electromagnetic fields. A typical SEM machine consists of a component called the gun which generates the electrons, a tunnel through which the electrons are accelerated, a series of lenses to help in shaping the beam of electrons, a sample holding chamber, and pumps that maintain a vacuum environment for imaging. Electrons from a field emission cathode, a thermionic, or a Schottky cathode in the gun are accelerated through a cathode and an anode with a voltage difference in the range of 0.1 keV – 50 keV. Low-voltage SEM ranges from 0.1-5 keV, while high voltage SEM ranges from above 5 – 50 keV [27]. The accelerated beam of electrons is focused over the surface of the material on the sample chamber in order to create an image. The beam of electrons is raster scanned on the sample surface, and the position of the beam is combined with the signals detected to form an image. As the electrons in the beam interact with the sample, secondary electrons and other rdiations are produced from the surface of the sample. The secondary electrons are collected by a detector which converts them into signals which are scanned using a television system to create an image on a cathode ray tube and their number depends on the surface topography. The image produced can be used to obtain information on the sample surface composition, distribution of varous elements in the sample, surface topography, and mineral orientation. Figure 3: Working principle Of SEM (SE – secondary electrons; BSE – backscatterd electrons; EBIC -electron beam-induced current; SC – specimen current; CRT – cathode ray tube; X – x-rays). Source: [27]. Thus, we can use SEM imaging technique in our project to confirm the elemetal composition and distribution of elements, as well as the surface topography of the Au/TiO2 nanomaterial. References Read More

ii. Neutral Impact Collision Ion Scattering Spectroscopy (NICISS) NICISS typically involves directing a pulsed beam of inert gas ions, preferably helium ions (H+) at low energy ( 3 keV) at a sample [29]. As the ions travel to the sample surface, they become neutralized and continue travelling through the sample until they collide with an atom, then are backscattered. The backscatter ions are detected by microchannel plates to produce time-of-flight (TOF) spectrum. The energy of the backscattered projectiles is determined by their TOF from the target atom to the detector.

During the backscattering process, the projectiles lose energy in amount proportional to the mass of the element on target. As the ions penetrate through a sample, there is continuous energy loss consisting of electronic excitations and small angle scattering [29]. In analyzing data from NICISS spectrum, there are two ways to consider the inelastic energy. First, inelastic energy loss has to be considered for determining the energy of the backscattered projectile. Second, the inelastic energy loss due to small angle scattering forms part of the continuous energy loss of the projectiles that pass through the material and depends on the energy loss straggling and stopping power of the section.

The TOF spectrum is an energy loss spectrum that can be used to provide information on the composition, the crystalline structure, and the concentration depth profiles of elements (in non-crystalline samples) on the surface or near-surface regions of a material when projectile ions interact with a target atom in matter. The interaction of projectiles with the target atom involves processes such as charge transfer, inelastic energy transfer, and elastic transfer of kinetic energy. The continuous inelastic energy loss (also called the stopping power) depends on the element in the sample from which the projectiles are backscattered, and is used to establish the depth from which backscattering of the projectile occurred [28].

NICISS will be used in the project in the investigation of the Au/TiO2 hybrid and the concentration depth profiles of Au NPs to provide information that is essential to understand the crystal structure and orientation of the hybrid surface. iii. UV photoelectron spectroscopy (UPS) This technique typically involves measurement of kinetic energy of photoelectrons which are emitted when molecules absorb UV photons, so that the molecular orbital energies can be determined. The UV light with energy in the order of 0-100 eV is used to excite the photoelectrons [25].

Analysis of the kinetic energy as well as the angular distribution of the excited photoelectrons can be used to provide information on the band structure (electronic structure) of the material being investigated [24]. A UPS can also be used to determine the electronic work function of a material, a property that is applied in electronic devices. UPS is similar to XPS in terms of the principle of operation, with the only difference being that in UPS, the photoelectric effect is induced using ionization radiation at energies in the range of 0-100 eV, while in XPS photon energy > 1000 eV are usually used [25].

In a lab setting, UV photons are generated by use of a gas discharge lamp, usually filled with gases such as helium, neon or argon [25]. Figure 2 shown below is a diagrammatical illustration of how UPS operates. The ionization energy required to remove electrons from the orbitals of a molecule is given by the difference between the final state and the initial state of the ionized molecule. Figure 2: The principle of operation of a UPS We can apply UPS in our project to determine the valence band structure of the Au/TiO2 composite.

This will be accomplished by irradiation the Au/TiO2 samples with UV photons from helium gas with energies of 21.2 eV (He I) and 40.8 eV (He II) depending on the operating conditions in order to obtain the binding energy. iv. Scanning Electron Microscopy (SEM) Unlike the other methods, SEM has the capability to image and analyze bulk material.

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