Materials synthesis

2.1.1

Sn-doped TiO2

Sn-doped-rutile, TiO2 with selected compositions given by the general formula

SnxTi1-xO2 (0 x 0.2) and (0.80 x 1.0) were synthesised by conventional solid

state reaction (SSR). The reagents were dried at specific temperature, Table 2-1. Reagents were weighed in the desired ratio, mixed and ground using an agate mortar and pestle in an acetone slurry, dried, pelleted, transferred to an alumina crucible and fired in a muffle furnace at 1200C for 72 hours and then quenched by rapid cooling to avoid spinodal decomposition that can occur during slow cooling. The samples were quenched by removal from the furnace at 1200C into air followed by cooling on a brass metal disc (235) and phase identification results monitored by XRD.

2.1.2

Cu-M doped TiO2, M= Nb and Ta

Compositions of formula: CuxTa2xTi1-3xO2 with x=0.05, 0.10, 0.12 and 0.15 and

CuxNb2xTi1-3xO2 with x=0.05, 0.10, 0.15 and 0.20; were synthesised. All the

chemical reagents were obtained from Sigma-Aldrich and dried overnight at a specific temperature, Table 2-1. The desired amounts of the reagents were mixed and ground using an agate mortar and pestle in acetone media. The slurry was dried then calcined in Pt boats at 935°C (236) for 2-3 days with intermittent grinding; the results were monitored by XRD.

Table ‎2-1: Drying temperature and purity of reagents.

Reagent Drying Temperature, C Purity, %

Rutile TiO2 900 99.9 SnO2 1100 99.9 CuO 700 99.0 Nb2O5 900 99.9 Ta2O5 900 99.8

2.2

Structural characterization

2.2.1

Powder X-ray diffraction

Powder X-Ray Diffraction (XRD) is one of the most versatile techniques available for phase determination structural characterization. The position (d- spacing) and intensities of reflections in the diffraction pattern provides a fingerprint for most crystalline solids (237). This technique is useful to identify new or unknown materials as each crystalline solid has a unique X-ray diffraction pattern. It is also used to analyse the phase purity of known materials. XRD can provide additional information about unit cell type, space group, lattice parameters, type and distribution of atom in the crystal structure, crystallite size and microstrain.

2.2.2

XRD Principle and Bragg’s‎Law

In an XRD experiment, an X-ray beam is reflected from repeating lattice planes of atoms in a crystalline structure when the wavelengths of the scattered X-rays interfere constructively. Constructive interference is at a maximum when the differences in the travel path are equal to integral multiples of the wavelength. When this occurs, a diffracted beam of X-rays will leave the crystal at an angle equal to that of the incident beam and the intensity of the reflected beam is measured by a detector (237,238).

The reflection from two layers of a sample is shown schematically in Figure 2-1, where the path difference between the two reflected x-rays is 2d sin θ. The position of the diffracted beam depends on the wavelengths of the incident X-rays and

spacing between the crystal lattice planes of atoms and the general relationship between them is known as Bragg's Law and expressed as:

n  = 2 d sin  (1) where n is any integer,

λ is the wavelength of the incident X-rays d is the inter-planar spacing, and

θ is the diffraction angle

Peaks appear in the diffraction pattern at certain 2 values from which d-spacings can be calculated. The d- spacings of the observed peaks are related to the size and shape of the unit cell while the intensities of the peaks are related to the atomic arrangement within the unit cell. Furthermore, the number of observed peaks is related to the symmetry of the unit cell. The sharpness of the peaks is a sign of the degree of ordering of the crystal structure while very broad peaks are a sign of disordered or less crystalline regions such as amorphous regions within the sample.

Figure ‎2-1: X-ray reflection from different parallel planes in the structure and derivation of Bragg's Law of diffraction.

2.2.3

Experimental

The XRD technique was used for phase analysis and lattice parameter measurements. The diffraction patterns were recorded using a STOE STADI P diffractometer (239) with position sensitive detector (PSD) and Mo K radiation (=0.7093 Å). The angular scan range was 5  40 2 with step size 0.1. The sample holder was rotated to avoid preferred orientation effects. Accurate 2 values were obtained by an angular correction using either external or internal silicon standard. The reasons for the use of Mo-K1 radiation over Cu radiation are

high intensity and less absorption in the case of titanium containing samples. A small amount of sample powder was ground and glued in between the centre of two circular acetate films, dried and placed in the circular holder. The sample holder was inserted in the rotation stage of the diffractometer system. In the case of the electrochemically treated samples, the powder was inserted between the two acetate films inside an argon-box and sealed temporarily and tightly to prevent any contact with atmospheric air and moisture before taking out.

Collected data were processed using the WinXPOW software package version 1.06 (239,240). Phase analysis was determined by comparing diffraction data against a database maintained by the International Centre for Diffraction Data (ICDD) and the lattice parameters were calculated.

2.2.4

Crystallite size

Different methods can be use to estimate the crystallite size and strain such as; Scherrer equation (average size, neglects strain) and integral breadth method (provide average values of size and strain)(238).

2.2.4.1 Scherrer equation

The particle size (D) can be estimated from the broadening of the X-ray diffraction peaks using the Scherrer equation (241):

  cos FWHM K D (2)

where K = Scherrer constant,

FWHM = full width at half maximum of the reflection peak, λ = wavelength of x-rays, and

θ = diffraction angle of x-rays

The Scherrer constant (K) depends on the shape of the particle and is generally taken to have the value 0.9 but K actually varies in the range 0.62  2.08 (242). The position of the highest intensity peak can be determined, along with the width of this peak at half maximum, and the d-spacing. The Scherrer equation gives a rough estimate of particle size and the size obtained yields the average particle- size for a material. The Scherrer equation may give an unreliable value of

crystallite size due to the fact that it does not take into account the effect of lattice strain and instrumental factors on peak broadening.

2.2.4.2 Integral breadth

The integrated intensity of diffraction peaks can be measured as the ratio of area under a peak to the maximum height of the peak.

In the Scherrer equation, FWHM of the diffraction peak is considered in the calculation while the microstrain, which can induce a greater broadening in the diffraction peak, is neglected. To overcome this limitation, the integral breadth was suggested to be used along with Scherrer equation to reduce large errors in crystallite size estimation.

In this work, crystallite size was determined from peak widths using the ‘Size/Strain' option in the WinXPOW software using the integral breadth as a measure of the peak width (240). The contribution of peak width from the instrument was corrected by using a standard material without peak broadening.