Chapter 2: Experimental Techniques
2.2. Characterization techniques
2.2.2. Optical characterization
2.2.2. Optical characterization
nanostructure samples. Fig. 2.7 is the photograph of micro-Raman set up used to observe the Raman spectrum for the present study. This instrument is used to study the identification of various phases of TiO2 nanostructures, oxygen vacancy defects and stress present in our samples. All the measurements are carried out at room temperature. Some of the samples are characterized using 488 nm and others using 514 nm Ar+ laser with source laser power fixed at 22 mW.
Fig. 2.8. Schematic diagram, illustrating the displacements of each atom for Raman active lattice vibrations of rutile TiO2. Red ball: Ti atom, Blue ball: O atom. Adapted from Ref. [5].
Rutile TiO2 is tetragonal structure having space group P42/mnm and has 6 atoms in the primitive cells. Thus there should be 15 optical and 3 acoustic modes. From the group analysis, the optical modes at Γ point belong to the following irreducible presentations:5, 6
Γopt = 1A1g + 1A2g + 1A2u + 2B1u + 1B1g + 1B2g + 1Eg + 3Eu,
where symbol “g” represents Raman active, “u” infrared (IR) active and “E” degenerate modes. The vibrational modes B1g, Eg, A1g and B2g are Raman active and A2g and B1u modes, however, are Raman and infrared silent modes.6 The Raman active modes consists of motions of anions with respect to central cations either perpendicular to c axis (modes A1g, B1g and B2g) or along the c axis (mode Eg). The illustration of the displacements of each atom for the Raman active lattice vibration in rutile TiO2 is shown in Fig. 2.8.5 Anatase is tetragonal structure with space group I41/amd and has 15 optical modes at the Γ point of
Brillouin zone are expected on the basis of factor group analysis with the following irreducible representation of the normal vibrational modes:7
Γopt = 1A1g + 1A2u + 2B1g + 1B2u + 3Eg + 3Eu,
Six Raman modes, i.e., the A1g, 2B1g, and 3Eg are Raman active, whereas the three modes A2u and 2Eu are IR active. The mode B2u is silent. TiO2(B) is monoclinic structure having space group C2/m and its primitive cell contains four formula unit, i.e., 12 atoms with following irreducible representation:8
Γopt = 12Ag + 6Bg + 6Au + 12Bu,
Among these modes, the selection rules give 18 Ag and Bg Raman active modes and 15 IR active Au and Bu modes whereas one Au and two Bu correspond to cell translations.8
B. Diffuse reflectance spectroscopy (DRS)
DRS is a technique generally used to study the reflectance or absorbance properties and to measure the band gap energy, i.e., the energy gap between the valence band maximum (VBM) and conduction band minimum (CBM) of semiconductor materials. The energy gap (()*) is an important feature of semiconductors which determines their applications in optoelectronics, photocatalysts and photovoltaics. The optical reflectance or absorbance is a result of interaction of light with the material. The basic principle is that UV, visible or near infrared (NIR) light is used to excite electrons from VB to empty CB. A sharp increase in absorption (or reflection) at energy close to the band gap that manifests the absorption edge (or reflectance threshold) in the UV-visible-NIR absorbance (or reflectance) spectra. This technique is mostly applicable to powder samples and thin films. For powder samples, usually UV-visible absorption spectroscopy is carried out by dispersing the sample in a liquid medium like water, ethanol or methanol. If the particle size of the sample is not small enough or not well dispersed in the solvent media, it precipitates and the absorption spectrum is more difficult to interpret. In order to avoid these complications, it is desirable to use DRS, which enables to obtain absorbance as well as the band gap energy (()*) of un-supported materials with more accuracy.9 Reflectance of samples can be measured using either an integrating sphere or a specular reflectance accessory. Integrating spheres are used for samples with a significant diffuse reflectance component such as powders and other ‘rough’ materials.
Samples are placed at the back of the sphere and the light is reflected back off the sample and
collected by the sphere. Measurements typically provide the total reflectance but if required the diffuse reflectance (specular excluded) portion can be measured independently. The latter measurement is achieved by allowing the specular component to exit the sphere through the open specular port. The theory which makes possible to use diffuse reflectance spectra was proposed by Kubelka and Munk.10 The absorbance can be calculated using Kubelka-Munk formula as
+(,-) = (. ./)! = 0
1 (2.4)
+(,-) is called the Kubelka-Munk (K-M) function, where ,- = .234567
.2839:3;: , < = K-M absorption coefficient and = K-M scattering coefficient.
In the parabolic band structure, the band gap (()*), and absorption coefficient (α) of a direct band gap and indirect band gap semiconductor are related through the well known equation (2.5) and (2.6), respectively:11, 12
(=ℎν) = ? (ℎν − ()*) (2.5) (=ℎν)/ = ? (ℎν − ()*) (2.6)
Where = is the linear absorption coefficient of the material, ℎν is the photon energy and ?,
? are the proportionality constant. When the material scatters in a perfectly diffuse manner, the scattering coefficient is considered as constant with respect to wavelength. So, by comparing equation (2.4) with equations (2.5) and (2.6), one can obtain expressions for relation between the K-M function and band gap of the material as
(+(,-)ℎν) = ?B(ℎν − ()*), direct band gap (2.7) (+(,-)ℎν)/ = ?(ℎν − ()*), indirect band gap (2.8) Here ?B, ? are the proportionality constant.
Fig. 2.9. Photograph of the Diffuse Reflectance Spectrometer (Perkin Elmer, LAMBDA 750).
We used a Perkin Elmer (LAMBDA 750) spectrophotometer for the DRS measurements. Fig. 2.9 shows a photograph of the DRS spectrophotometer used in this study. It performs the measurement in the UV/visible/ NIR (i.e., 200 – 2500 nm wavelength range) region. The deuterium and tungsten halogen light sources are provided to perform the measurements in the UV/Visible/NIR region. An integrating sphere along with PMT (photomultiplier), InGaAs and PbS detectors is used to collect the diffuse reflectance light being reflected from the sample in all direction. Calibrated spectralon diffuse reflectance standard is used as reference. A sample holder kit with a quartz piece on the front is used to hold the powder sample. A schematic diagram of diffuse reflectance mechanism is shown in Fig. 2.10. We measured the diffuse reflectance in absorbance mode, i.e., using K-M function.
The band gap is calculated from the linear fit of linear portion of (+(,-)ℎν) vs ℎν plot with (+(,-)ℎν) = 0 for direct band gap and (+(,-)ℎν)/ vs ℎν plot with (+(,-)ℎν)/
= 0 for indirect band gap.
Fig. 2.10. Schematic diagram illustrating diffuse reflectance mechanism, excluding the specular reflectance.
C. Photoluminescence (PL) spectroscopy
Photoluminescence (PL) is the emission of light from a material under optical excitation.
Excitation energy is required to excite the electron from lower energy level (ground state/
equilibrium state) to higher energy level (excited state). Light of suitable energy is directed
onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. This causes electron-hole pair generations within the material and those electrons move into permissible excited states. The excess energy can be released by the sample through the emission of light or luminescence, when these electrons return to their equilibrium states and the whole process is called photoluminescence. The energy of the emitted light (photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state.
The quantity of the emitted light is related to the relative contribution of the radiative process between the various excited states (as defect states near the conduction act as a luminescence centers) and the equilibrium states. Features of the emission spectrum provide the information about the band gap energy, impurity level/ defect level detection, and electron- hole recombination mechanism. The intensity of the PL signal can be used to extract the qualitative information about the contribution of various defect states. The most common radiative transition in semiconductors is between the states in the conduction and valence bands, with the energy difference being known as the band gap. Radiative transitions in semiconductors also involve localized defect levels within the band gap of the semiconductor. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their relative concentration. The return of the excited electrons to equilibrium state to recombine with holes, also known as "recombination," can involve both radiative and non- radiative processes. The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process.
Analysis of photoluminescence helps to understand the underlying physics of the recombination mechanism. The fundamental limitation of PL analysis is its reliance on radiative events. Materials with poor radiative efficiency, such as low-quality indirect band gap semiconductors, are difficult to study via ordinary PL. Similarly, identification of impurity and defect states depends on their optical activity. More intense laser PL is generally preferred to study the defect/ impurity states such as oxygen vacancy as compared to ordinary Xenon-lamp based PL system.
We used 325 nm He-Cd laser and 405 nm diode laser (Coherent, Cube) excitation with the help of a spectrometer (focal length: 15 cm; blaze wavelength: 500 nm; groove
density: 150 g mm-1) equipped with a cooled charge-coupled device (Princeton Instruments, PIXIS 100B) detector. Extended NIR PL measurements were carried out using a liquid N2
cooled InGaAs detector (OMA-V-SE, Roper Scientific). Fig. 2.11 shows a photograph of the PL instrument which is used in the present study. The PL measurements were carried at room temperature for most of the samples. We have also performed low temperature (10 K- 280 K) PL measurement and room temperature PL under high vacuum (5 × 10-5 torr), and with oxygen gas into the vacuum chamber for some of the samples. We put the powder samples on a conductive black carbon tape for PL measurements. Each spectrum was corrected for the detector response as a function of wavelength after background subtraction. The steady state PL peaks are usually of Gaussian shape expressed by
= + ' E$F [−(( !)!)] (2.9)
Where is the offset constant, $%, & and ' are the peak position, width and peak amplitude, respectively. The measured spectral profile of the PL spectrum is analyzed by fitting with Gaussian line-shape function or multiple Gaussian functions using a PeakFit software.
Fig. 2.11. Photograph of the Photoluminescence spectrometer with CCD detector and Cryostat.
D. Time-resolved photoluminescence (TRPL) spectroscopy
Time resolved photo-luminescence (TRPL) is a useful characterization technique that provides the spectral and temporal evolution of the emission of a sample following its illumination by a short pulse of light. More precisely, the short pulse of light generates electron-hole pairs (excitons) that decay to lower energy levels of the sample. These electron- hole pairs can subsequently recombine and emit light. The emitted light is composed of a set of wavelengths corresponding to transition energies of the sample and, as a result, the measurement of the optical spectrum as a function of time provides a means to measure the transition energies and their lifetimes. The measurement basically counts the number of photons of fixed wavelength with time. The emitted photon is analyzed by a spectrometer and detected by a micro-channel-plate photomultiplier tube (MCP-PMT) detector. A Multi Channel Analyzer (MCA) board in the computer analyzes the output pulse voltages into various channels, which correspond to different times. In this way, the MCA can record each photon arriving at the MCP–PMT at a particular time. The number of output pulses from the MCP–PMT is directly proportional to the number of incident photons. Averaging over millions of photons, the measurement creates a histogram which shows how long excitons
“live” after being created by the laser pulse. Thus, the carrier dynamics occurring during these processes provides not only information about the position of the peak emission, as in a normal steady state PL experiment, but also the lifetime of the excitons involved in the recombination.
Fig. 2.12. Photograph of the Time-Resolved Photoluminescence spectrometer (LifeSpec II, Edinburgh Inst., UK).
The photograph of the TRPL spectroscopy (LifeSpec II, Edinburgh Inst., UK) instrument used in the present study is shown in Fig. 2.12. We used adjustable nanosecond pulsed pump laser (EPL series, Edinburgh Inst., UK) of wavelength 405 nm to measure the PL decay of the defects related visible emission in the TiO2 nanostructures. This instrument has a time resolution of ~50 ps. The TRPL data usually follow an exponential decay behavior or combination of such function depending upon the number of decay channels available in the sample. The PL decay equation is expressed as
I(J) = I E$F (J̅/L) + I E$F (J̅/L ) +IB E$F (J̅/LB) + …. …. …. + I (2.10) Where I(J) is the PL count, I, I , IB …. …. are decay channel amplitudes and L, L ,LB ….
…. are the decay time constants and I accounts for the background noise. Measurement of τ provides information on the mechanism of recombination.
E. X-ray photoelectron spectroscopy (XPS)
XPS is a key surface characterization tool which combines surface sensitivity with the ability to quantitatively obtain both elemental and chemical state information for each element detected, through the chemical shift. It is widely used for studies of surface defects and chemical environment, because of its high sensitivity to surface (i.e., up to 10 nm from the sample surface). It also provides useful information about the depth profile (i.e., an evaluation of the variation of composition with depth) and the surface impurities present in the sample. The principle of XPS is based on the photoelectric effect outlined by Einstein in 1905 was developed by Siegbahn and his research group,13 where the concept of the photon was used to describe the ejection of electrons from a sample surface when photons impinge upon it. This process can be expressed by the following equation:
M( = ℎν - N( - ∅, (2.11)
where BE is the binding energy of the electron in the atom, hν is the photon energy of x-ray source, KE is the kinetic energy of the emitted electron that is measured in the XPS spectrometer and ∅ is the spectrometer work function. For XPS, Al Kα (1486.6 eV) or Mg Kα (1253.6 eV) is generally used as the source of x-rays. The photon is absorbed by an atom of the sample, leading to emission of a core (inner shell) electron. The energy of the photoelectrons leaving the sample is determined using an appropriate electron energy analyzer and this gives a spectrum with a series of photoelectron peaks. For each and every
element, there will be a characteristic binding energy associated with each core atomic orbital, i.e., each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies. The peak intensities measure how much of a material is at the surface, while the peak positions indicate the elemental and chemical composition. Other values, such as the full width at half maximum (FWHM) are useful indicators of chemical state changes and physical influences.
In this study, XPS measurements were carried out with a PHI X-Tool automated photoelectron spectrometer (ULVAC-PHI, Inc.) using Al Kα X-ray beam (1486.6 eV) with a beam current of 20 mA. Some of the samples were characterized with ESCALAB 3400 (Shimadzu, Japan) instrument using Mg Kα X-ray beam (1253.6 eV). Carbon 1s spectrum was used for the calibration of the XPS spectra recorded for various samples. Fig. 2.13 shows the photograph of XPS instrument used for this study. The broad peaks with shoulder are fitted with Gaussian line-shape using a PeakFit software followed by the expression
= + ' E$F [−(( !)!)] (2.12)
Where is the offset constant, $%, & and ' are the peak position, width and peak amplitude, respectively.
Fig. 2.13. Photograph of the X-ray photoelectron spectrometer (ULVAC-PHI, Inc.).
F. Fourier transform infrared (FTIR) spectroscopy
FTIR is one of the powerful spectroscopic tools generally used to determine the structural bonding information, impurities and chemical functional groups in the sample. The FTIR is based on the phenomena “Michelson interference” combined with Fourier transformation of the source spectrum.14 FTIR interferogram consists of spectral information of the source along with the transmittance characteristic of the sample. We used FTIR spectroscopy in transmittance mode of operation for our few samples to confirm the formation of pure TiO2 phases and shifting of Ti-O related vibration modes due to oxygen vacancy defects in TiO2 nanostructures. In this study, we used a commercial FTIR spectrometer (Perkin Elmer, Spectrum BX) to obtain the transmission spectrum of the TiO2 nanostructures at room temperature in the range 400- 4000 cm-1 at a resolution of 2 cm-1. All the spectra were taken after background corrections. For the sample preparation, TiO2 powder of very small quantity is mixed with KBr and ground in ceramic mortar, then prepared pellets using KBr press and die.