Transmission Electron Microscopy Observations on the Copper Doped Samples

Chapter 4 Characterization Results and Discussion

4.1 Copper Doping

4.1.3 Transmission Electron Microscopy Observations on the Copper Doped Samples

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4.1.3 Transmission Electron Microscopy Observations on the Copper

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Figure 4.4: TEM images of undoped TiO2 (a), 2% Cu TiO2 (b), 3% Cu TiO2 (c), 4% Cu TiO2

(d) and 5% Cu TiO2 (e).

Figure 4.5 show higher magnification images of the undoped TiO₂ and the copper doped catalysts. It appears in these higher magnification images that the particles in Figure 4.4 are made up of smaller particles. This means that these samples are agglomerations. These small particles that make up the agglomerates are difficult to measure due to overlap and poor definition of boundaries.

500nm

200nm 200nm

500nm

200nm

(e) (a)

(d) (c)

(b)

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Figure 4.5: Higher magnification TEM images of undoped TiO2 (a), 2% Cu TiO2 (b), 3% Cu TiO₂ (c), 4% Cu TiO₂ (d) and 5% Cu TiO₂ (e).

4.1.4 XRD

Figure 4.6 presents the XRD diffractograms of copper doped catalysts. When comparing Figure 4.6 to the JCPDS (00-021-1272) file for anatase phase of titanium dioxide it can be

100nm 100nm

100nm 100nm

100nm

(a) (b)

(c) (d)

(e)

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seen that the materials are in the anatase form of titania. There are no diffraction peaks that indicate that there is any rutile (JCPDS 00-021-1276) phase present, this is expected as the temperature of calcination was below that required to effect a phase change [3].

Figure 4.6: XRD diffractogram patterns of undoped and copper doped titanium dioxide materials. Inset magnified view of the 101 peak.

There are no diffraction peaks representing copper or any copper oxides in the diffractograms shown in Figure 4.6. Considering that the copper is expected to be doping the TiO2 as in either the case of copper doping or copper being on the surface of the catalyst the amount of copper used is below the detection limit of XRD [4]. Inset in Figure 4.6 can be seen a magnified image of the 101 diffraction peak. It can be seen that there is broadening of this diffraction peak with doping and that there is also a decrease in the intensity of this peak.

This indicates that there is a loss of structure quality and/or loss of crystallinity caused by

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24 25 26 27

Intensity/ a.u

2 Θ/ ο

Undoped TiO

2

2% Cu TiO₂ 3% Cu TiO

2

4% Cu TiO₂ 5% Cu TiO₂

(004)

Int ens it y / a .u

2 Θ / °

Undoped TiO

2

2% Cu-TiO

₂

3% Cu-TiO

₂

4% Cu-TiO

₂

5% Cu-TiO

2

(101) (211) (105)

(200) (116) (224) (215) (107) (220)

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doping [5]. The 4% Cu TiO2 catalyst has a very similar pattern to the 5% Cu TiO2 catalyst the 101 diffraction peak for these catalysts appear overlaid.

The d-spacing of undoped catalyst is shown which was calculated from the 101 diffraction peak in Figure 4.6, using the Bragg’s law equation where n = 1 and λ = 0.15405 nm.

From the results in Table 4.2 it can be seen that all of the catalysts have the same d-spacing value of 0.352 nm. This d-spacing value matches that of d-spacing for the 101 diffraction peak give in the JCPDS file for anatase.

Table 4.2: Lattice parameters and cell volume for copper doped photocatalysts.

Catalyst a (nm) c (nm) a²*c (nm³) Crystallite size (nm) d-spacing (nm)

Undoped 0.3784 0.9501 0.1360 17.75 0.352

2% Cu-TiO2 0.3787 0.9486 0.1360 9.34 0.352

3% Cu-TiO2 0.3785 0.9480 0.1358 10.42 0.352

4% Cu-TiO2 0.3786 0.9470 0.1358 10.75 0.352

5% Cu-TiO₂ 0.3786 0.9470 0.1358 10.85 0.352

Table 4.2 lists the lattice parameters and crystallite sizes of the undoped and copper doped titanium dioxide catalysts. The ‘a’ and ‘c’ parameters seen in Table 4.2 were calculated from the 2θ values of the 200 and 004 diffraction peaks respectively, using the Bragg’s law equation with miller indices, for a tetragonal system [6]. The d-spacing was calculated as shown above but for either the 200 or 004 diffraction peak. Below is an example of the ‘c’

parameter calculation for the undoped sample.

The ‘a’ parameter was calculated in the same way using the 200 diffraction peak.

From the values in Table 4.2, the ‘a’ lattice parameter is relatively unchanged when comparing all the catalysts. The ‘c’ parameter on the other hand shows a slight decrease with increased doping with the undoped catalyst having a ‘c’ parameter of 0.9501 nm which decreases to 0.9486 for the 2% Cu catalyst. However comparing the cell volume of the

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catalysts it can be seen that there is a very small decrease in cell volume for the doped catalysts but the all doped catalysts have the same cell volume of 0.1358 nm³. There is very little change seen with the lattice parameters due to the similarity in size of the metal ions.

Prekajski et al doped TiO₂ with Cr³⁺ and found that the lattice parameters of the doped materials showed little change compared to the undoped material, even to high doping levels [7]. The lack of change in lattice parameters was attributed to the similar size of the Ti⁴⁺ ion and Cr³⁺ ion. A similar finding was made by Trenczek-Zajac et al, who also doped TiO₂ with Cr³⁺ and found that the lattice parameters did not change again the reasoning was a similar ionic radius [8]. On the other hand Bartlett et al found that doping TiO₂ with Nb resulted in a linear increase in the ‘a’ parameter with increased doping [9].

Crystallite sizes were calculated from the FWHM of the 101 diffraction peak in Figure 4.6 using the Scherrer equation.

The crystallite sizes in Table 4.2 show that all the copper doped catalysts have lower crystallite sizes than the undoped TiO2 catalyst which has a crystallite size of 17.75 nm.

Initially the doping causes a decrease in the crystallite size of the catalysts, as the 2% Cu- TiO2 catalyst has a crystallite size of 9.24 nm which is smaller than that of the undoped TiO2

catalyst. However thereafter with increasing doping there is an increase in crystallite size in comparison to the 2% Cu-TiO2 catalyst. Navas et al doped in range of 2.5-7.5 mol % of TiO2

and reported a linear decrease in the crystallite size [10]. Rajamannan et al reported that doping TiO₂ in range of 4-16% Cu (not stated if molar, atom or weight percentage) increases crystallite size compared to that of the undoped catalyst [11]. Considering both these results it could be that after a certain doping level, under the given reaction conditions crystallite growth is promoted.

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