ABSTRACT

In this work, the crystal structure of titanium dioxide was studied, and the effect of the different synthesis routes on the microstructure and morphology of the nanostructures was analyzed. Samples characterization was carried out by X-ray diffraction by powders (XRD) to determine the different crystalline phases contented in the samples and using scanning electron microscopy (SEM), the morphology and topology of all samples were studied. XRD results were analyzed through Eva provided by Bruker to determine the average crystallite size. The results portrayed here showed that all the synthesis process produced anatase nanostructures with an average crystallite size smaller than 27 nm. Synthesized powders presented similar morphologies in all cases and they were homogeneous in their chemical composition.

1. Introduction

The development of photocatalysis has been the focus of considerable attention in recent years with photocatalysis being used in a variety of products across a broad range of research areas, especially including environmental and energy-related fields [1,2]. According to Mo et al. (2009), the common photocatalysts are primarily metal oxides or sulphides and have been developed for various applications, including the degradation of volatile organic compounds (VOC’s). Metal oxides or sulfur are those listed as TiO₂, ZnO, ZrO₂, SnO₂, WO₃, CeO₂, Fe₂O₃, Al₂O₃, ZnS y CdS [3]. One of the most useful is the TiO₂ because it exhibits excellent optical, electrical, photocatalytic and thermal properties and also poses a relatively low cost [4]. Besides its application as a photocatalyst [1,2,5-7], TiO₂ has been used as solar cells [8] and as functional biomaterials [9].

Regarding its preparation methods, TiO₂ can be obtained through various chemical synthesis routes and different physicochemical properties and controlled morphology have been generated with several advantages. The methods used to obtain this catalyst are: Dip-coating, sol-gel, sputtering, chemical vapor deposition (CVD), hydrothermal [2] and others. In recent years, one modification of the hydrothermal method is the so called hydrothermal microwaves-assisted synthesis methods and its potential applications are been deeply investigated [4,5,7,10], because this new method allows a considerable reduction of the reaction time and as a consequence, an important decrease in the heat provided during the synthesis process. Moreover, the technology and devices required to carry out this synthesis methods is reasonably priced and relatively easy to use. Currently, there is a tendency to improve the synthesis conditions in order to manipulate the morphology, dimensions and crystalline preferential orientation of the synthesized nanomaterials [11-17] which have a direct impact on the properties of the products.

This work has been focused to analyze the effect of different synthesis methods on the crystal structure and morphology of TiO₂ in its anatase phase; specifically, the sol-gel method, sol-gel method assisted by microwaves and the hydrothermal method assisted by microwaves.

2. Experimental Procedure

2.1. Synthesis through Sol-Gel Method

The synthesis of TiO₂ by the sol-gel method was carried out using dissolution of titanium isopropoxide in isopropyl alcohol under an inert nitrogen atmosphere and mechanical stirring to obtain a sol; then, deionized water was added with continued stirring to obtain a suspension. The powders obtained were filtered, dried at room temperature, and then dried at 110˚C for 18 hours in a furnace and finally, calcinated at 550˚C for 4 hours.

2.2. Synthesis through Sol-Gel Method Assisted by Microwaves

The anatase synthes is by this method combines the solgel method with micro waves radiation as follows: first, develops the sol-gel method as mentioned above until the stage in which the suspension was obtained. This suspension was immediately put inside a microwave oven Synthos 3000, and the chemical reaction started. The energy required for the reactions was provided by the microwaves which were inducted with potency of 600 kW from 5 to 20 minutes to obtain anatase powders, and were subsequently dried at room temperature.

2.3. Synthesis through Hydrothermal Method Assisted by Microwaves

Anatase obtained by this means was made from the reaction of TiOCl₂, which was obtained from TiCl₄. TiOCl₂ was mixed with urea in a molar ratio of 5:1; this mixture was exposed to the microwaves radiation using the microwave oven Synthos 3000 with a potency of 950 kW for various reaction times from 2 to 9 minutes. The powders obtained were filtered and washed using double distilled water and finally dried in a furnace for 12 h.

2.4. Labeling of the Samples

According to anatase quality, samples for each method of synthesis were selected and labeled as follows: sol-gel method (SG1), sol-gel method assisted by microwaves (SGMW1), (SGMW2), and hydrothermal method assisted by microwaves (MW6), (MW9).

2.5. Samples Characterization

All the samples obtained were characterized by means of X-ray diffraction by powders (XRD) using a diffractometer Bruker D8 Advance. For all the analyses, an accelerating voltage of 30 kV was used to produce the CuK α radiation of 1.5406 Å. The XRD analyses were performed in the 2θ range from10 to 80 degrees.

The morphology and particle size of the powder was observed by scanning electron microscopy (SEM) Philips model XL30ESEM. Besides, qualitative elemental microanalysis was performed by EDS. The samples were previously glued with a double-faced graphite tape and covered by a thin layer of Au deposited by sputtering.

3. Results and Discussion

The crystal structure of the samples obtained was analyzed using the technique of X-ray diffraction by powders. Diffractograms were obtained for each of the samples and they are portrayed in the figures from 1 through 5. In all cases, titanium oxide in the phase of anatase was identified using de ICDD power diffraction file (PDF) bank. The more relevant signals detected were at 25.3˚, 48.2˚ and 37.8˚ in 2θ related to the (101), (200) and (004) crystal planes respectively.

Figure 1 displays the diffractogram obtained of the sample made by the sol-gel method which presents signals well defined with a high intensity, a relatively flat baseline and narrow XRD signals which are indicative of small crystallites and a good crystalline quality. The determination of the crystallite size was done by the Scherrer formula. The Scherrer formula gives a correspondence between the crystallite size LC and the full width at half maximum FWHM