Análisis de la interacción cuando inicia la secuencia didáctica

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Titanium dioxide has been o f interest for a num ber o f years due to its photocatalytic properties. For this reason bulk TiOz has been used to successfully rem ove pollutants, such as chlorobenzene from w aste water*"^'^^, and has also been used in conjunction w ith detergents in the clearing up o f oil spills. There is a great deal o f interest in the production o f TiO ] thin film s on substrates due to their high surface area and the possible uses to w hich they could be put. For exam ple, bathroom tiles are produced w ith a TiO ] coating that helps to keep the surface o f the tile clean by breaking dow n organic m aterial on the tile surface^^.

This chapter describes the form ation o f titanium dioxide films by the A PCVD reaction o f titanium tetrachloride w ith a selection o f co-reactants. The co-reactants chosen w ere the sam e as in previous chapters, w ater, ethanol, m ethanol and ethyl acetate. D epositions w ere carried out and the properties o f the films w ere analysed by a variety o f techniques.

The purpose o f the w ork w as to produce thin transparent films that could be deposited onto a glass substrate and then to investigate their possible use as photocatalytic coatings, (previously described in C hapter 1).

In order for the coatings to be transparent two criteria have to be satisfied. Firstly, the coatings m ust be thin < 2 0 0 nm , because, as thickness increases a golden colouration is observed to appear in the films. Secondly, no gas phase or particulate form ation can be allow ed to take place as this results in the form ation o f a w hite hazy TiO ] film.

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D ue to the difficulty o f studying very thin film s, firstly, a study o f thicker films was carried out as this allow ed a feel for the system s to be developed and m ade the analysis b y techniques, such as glancing angle X RD , easier.

D eposition studies w ere carried out b y altering the follow ing experim ental param eters: substrate deposition tem perature, deposition time, precursor tem perature, precursor carrier gas flow rates and type o f co-reactant. O ther studies w ere also carried out such as varying the tim e o f m ixing before the two precursors entered the reaction chamber.

5.1 Deposition o f thick TiÛ

thin films

The set o f depositions w as carried out w ith a deposition tim e o f 90 seconds to enable a film to be produced that w as thick enough to b e easily seen b y the naked eye. This allow ed a very quick judgem ent as to the suitability o f a co-reactant to b e m ade, i.e. i f no film or a poor quality film was produced then there w ould b e no point attem pting to grow thin film s w ith the same co-reactant.

The gas flows w ere set to the follow ing levels, table 5.1, as these w ere found to give good carry over o f the TiCU vapour w hen the bubbler w as set to tem perature o f 90 °C.

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G as line Flow r a te / dm ^m ln’^

Plain line 1 0

Bubbler line 1 0.5

Bubbler line 2 N ot U sed

D ilutent line 2

Syringe line 1 0

T a b le 5.1 experim en tal co n d itio n s for d ep o sitio n tem perature stu d ies fo r titanium d io x id e

As TiCU contains no direct Ti - O bonds a co-reactant w as needed and so no ‘d ry ’ depositions w ithout the use o f a co-reactant w ere conducted.

5.1.1 Reaction with water

A series o f depositions w ere carried out starting at 600 °C using an injection rate o f 2.5 cm^ o f w ater per m inute, dow n to a tem perature o f 400 °C. It w as found that although at high tem peratures a film was produced it w as patchy over the surface o f the glass substrate. The resultant coatings w ere reasonably clear, w ith only small am ounts o f haze being present. This showed that little gas phase reaction had taken place and the lack o f colours o f refraction suggested that the coating w as o f sim ilar thickness. An SEM o f a sam ple is shown below in figure 5.1.

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F igure 5.1 sh o w in g SEM m icrograph o f a TiO? film produced from TiCU and w ater

The film is made up of irregularly shaped islands of growth. The films did have a