Hydrothermal Synthesis of Rare-Earth Modified Titania: Influence on Phase Composition, Optical Properties, and Photocatalytic Activity

Abstract

In order to expand the use of titania indoor as well as to increase its overall performance, narrowing the band gap is one of the possibilities to achieve this. Modifying with rare earths (REs) has been relatively unexplored, especially the modification of rutile with rare earth cations. The aim of this study was to find the influence of the modification of TiO₂ with rare earths on its structural, optical, morphological, and photocatalytic properties. Titania was synthesized using TiOSO₄ as the source of titanium via hydrothermal synthesis procedure at low temperature (200 °C) and modified with selected rare earth elements, namely, Ce, La, and Gd. Structural properties of samples were determined by X-ray powder diffraction (XRD), and the phase ratio was calculated using the Rietveld method. Optical properties were analyzed by ultraviolet and visible light (UV-Vis) spectroscopy. Field emission scanning electron microscope (FE-SEM) was used to determine the morphological properties of samples and to estimate the size of primary crystals. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical bonding properties of samples. Photocatalytic activity of the prepared photocatalysts as well as the titania available on the market (P25) was measured in three different setups, assessing volatile organic compound (VOC) degradation, NO_x abatement, and water purification. It was found out that modification with rare earth elements slows down the transformation of anatase and brookite to rutile. Whereas the unmodified sample was composed of only rutile, La- and Gd-modified samples contained anatase and rutile, and Ce-modified samples consisted of anatase, brookite, and rutile. Modification with rare earth metals has turned out to be detrimental to photocatalytic activity. In all cases, pure TiO₂ outperformed the modified samples. Cerium-modified TiO₂ was the least active sample, despite having a light absorption tail up to 585 nm wavelength. La- and Gd-modified samples did not show a significant shift in light absorption when compared to the pure TiO₂ sample. The reason for the lower activity of modified samples was attributed to a greater Ti³⁺/Ti⁴⁺ ratio and a large amount of hydroxyl oxygen found in pure TiO₂. All the modified samples had a smaller Ti³⁺/Ti⁴⁺ ratio and less hydroxyl oxygen.

Keywords: TiO2; modification; photocatalytic activity; rare earths; visible light activity.

Figure 1

Schematic diagram of the synthesis procedure.

Figure 2

Graphical representation of the reactor system used to measure volatile organic compound (VOC) degradation over titania samples.

Figure 3

Graphic output of Rietveld refinement of sample Ce-TiO₂. The red line represents the calculated pattern, the black open circles represents the observed pattern, and the blue line represents the difference between the observed and calculated pattern. The small vertical bars represent the positions of reflection of the three phases: black for anatase, red for rutile, and orange for brookite. The powder diffraction file numbers used were 21-1272, 21-1276, and 29-1360 for anatase, rutile, and brookite, respectively.

Figure 4

Kubelka-Munk transformation of the ultraviolet and visible light (UV-Vis) spectra of the samples. The inset in the upper right corner is a magnified graph that shows the extent of light absorption of the samples more clearly. Ce-TiO₂ absorption tail extends to approximately 585 nm. Other samples show similar absorption characteristics. Their absorption does not seem to extend much beyond 400 nm.

Figure 5

Field emission scanning electron microscope (FE-SEM) micrographs of the synthesized samples: (a) TiO₂, (b) Ce-TiO₂, (c) La-TiO₂, and (d) Gd-TiO₂. From Figure 5a we can see that rutile is composed of elongated particles. Given the prevalence of anatase in Ce-TiO₂, we assume that the small round particles are anatase. Figure 5c,d shows samples La-TiO₂ and Gd-TiO₂, respectively, where both rutile and anatase can be observed.

Figure 6

X-ray photoelectron spectroscopy (XPS) analysis results: (a) Ti2p de-convolution, (b) O1s de-convolution. We can observe that sample TiO₂ has a larger amount of Ti³⁺ ions present, compared to other samples. The results also show that in sample TiO₂ a significant amount of oxygen is in the form of –OH groups, whereas in other samples it exists in lattice oxygen.

Figure 7

Photocatalytic activity of samples in liquid-solid methylene blue (MB) removal. Sample TiO₂ shows the highest activity, followed by Gd-TiO₂, La-TiO₂, and Ce-TiO₂.

Figure 8

Photocatalytic activity of samples measured by NO_x conversion (%) under solar lamp. Again, sample TiO₂ exhibits the highest activity and Ce-TiO₂ the lowest. Here, La-TiO₂ is more active than Gd-TiO₂, but the difference is minor.

Figure 9

Histograms of photocatalytic activity of samples under UV and solar irradiation. The kinetic constants are normalized to the P25 constant and are presented as percentages of the P25 constant. P25 is therefore by definition always equal to 1.

Figure 10

Histograms of photocatalytic activity of samples under visible light irradiation. The kinetic constants are normalized to the P25 constant and are presented as percentages of the P25 constant. P25 is therefore by definition always equal to 1.