Implications of heat treatment on the properties of a magnetic iron oxide–titanium dioxide photocatalyst

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Abstract

Magnetic iron oxide–titania photocatalysts (Fe3O4–TiO2) were prepared using a coating technique in which the photoactive titanium dioxide was deposited onto the surface of a magnetic iron oxide core. These particles had a core–shell structure. The prepared particles were heat treated at high temperature in order to transform the amorphous titanium dioxide into a photoactive crystalline phase. The heat treatment temperature and duration were varied, and the correlation between the heat treatment and the observed photoactivities was investigated. An increase in the applied heat treatment, either by increasing the temperature or increasing the heat treatment duration, led to a decrease in the activities of the catalyst particles. A decrease in surface area due to sintering, along with the diffusion of Fe ions into the titanium dioxide coating are seen as contributing factors to the decline in photoactivity which accompanied an increase in the heat treatment. Differential scanning calorimetry analysis (DSC) results confirmed that the presence of the iron oxide core did not have an effect on the phase transformation of titania under the experimental conditions investigated. In this study we also present surface charge measurements which show that the surface of the particles became more positive as the heat treatment was increased. This is an indication of changing surface properties as heat treatment is applied. For single-phase TiO2 powders, this is postulated to be due to a decrease in the surface hydroxyl (OH) groups and/or residual organics (OR) groups. For the Fe3O4–TiO2 powders, in addition to the loss of OH and OR groups, the diffusion of the Fe into the titania shell is postulated to also play a role in the changing surface properties with applied heat treatment. Experiments aimed at reducing the duration of the heat treatment revealed that a heat treatment duration of 20 min at 450 °C was sufficient to transform amorphous titanium dioxide into a photoactive crystalline phase. This does not only minimise loss of surface hydroxyl groups but it also has the potential to limit the oxidation of the magnetic core, which occurs due to the porosity of the coating. This has practical implications in terms of separating the magnetic particles from the treated waste waters under the application of an external magnetic field. It also presents an opportunity to produce photoactive composite particles while limiting the interactions between the core and the shell during the heat treatment.

Introduction

Titanium dioxide mediated photocatalytic oxidation offers potentially a facile and cheap method for removing inorganic and organic pollutants from waste waters. This technology is at a sufficiently advanced stage now to present a possible solution to the current environmental crisis in both developed and developing countries with regard to the supply of high quality water for drinking and commercial use.

In the presented work, the particles being studied are mixed iron oxide–titanium dioxide systems. These particles were synthesised with an aim to produce a magnetic photocatalyst, in which the titanium dioxide material was deposited onto the surface of an iron oxide core. Thus, the iron oxide–titania systems produced in this work have a core–shell structure. The magnetic core is useful for enhancing the separation properties of suspended particles from solution, whereas the photocatalytic properties of the outer titanium dioxide shell are used to destroy organic contaminants in wastewaters.

The titanium dioxide deposited during the coating procedure adopted in this study is amorphous in nature. X-ray amorphous titanium dioxide is known to be almost inactive as a photocatalyst [1], [2]. Hence an essential step in the preparation of the photocatalyst samples is the calcination at high temperature which leads to the crystallisation of the hydrous titanium dioxide.

The crystalline titanium dioxide has three polymorphs, rutile, brookite and anatase. Thermodynamically, brookite and anatase are metastable forms, which transform exothermically and irreversibly to the stable rutile form upon heating. The transformation of the amorphous titanium dioxide to anatase titanium dioxide has been reported to commence at temperatures as low as 100–150 °C [1], [3], while rutile has been reported to form over the temperature range of 550–800 °C [4], [5]. Anatase has been reported to be the most effective phase for photocatalysis [2], [6], [7], [8]. Pure rutile had been reported to have poorer activity [2].

The anatase to rutile phase transformation has been extensively studied. The main conclusion of such studies was that the temperature at which the anatase to rutile phase transformation occurs is highly influenced by the synthetic conditions of the titanium dioxide particles [9]. The phase transition behaviour of TiO2 has also been found to be strongly influenced by the presence of foreign ions in the lattice, even in small amounts [10], [11], [12], [13], [14]. The anatase to rutile phase transformation involves the rupture of TiO bonds. Thus impurities and reaction conditions which create oxygen vacancies tend to accelerate the phase transformation, while conditions which increase the concentration of interstitial titanium tend to inhibit the transformation [10], [12].

Many studies have been carried out regarding processes involved during the heat treatment of Fe-doped and iron oxide-doped titania [15], [16], [17], [18], [19], [20]. These systems are often encountered in photocatalysis where Fe-doped TiO2 are used to extend the photocatalyst's response into the visible region or enhance its photoactivity. The commonly studied Fe-doped and iron oxide-doped TiO2 catalysts are prepared using a number of different methods. These include the coprecipitation and the sol–gel method [21] the wet impregnation method [5], [22] and spray pyrolysis [22]. All of these methods result in either Fe-doped or iron oxide doped TiO2 (depending on the nominal concentration of the iron salt), in which there is an almost homogeneous distribution of iron in the TiO2. Fe-doped TiO2 catalyst have been shown to have enhanced activities in particular for the photocatalytic reduction of nitrite ions [21], [23], [5].

The heat treatment applied during the synthesis of such particles has been found to be a critical parameter in determining the photoactivity of these Fe-doped TiO2 catalysts. Thermal behaviour studies on composite Fe–Ti oxide systems have revealed the complexity of the solid state chemistry and the intricate microstructure transformations involved in these systems. The complexity arises due to the strong chemical interactions between the iron oxide phase and the titanium dioxide phase occurring during the heat treatment. These interactions involve the spreading of the iron oxide and the diffusion of foreign Fe ions into titanium dioxide structure [24], [25], [26]. This in turn has an effect on the anatase to rutile phase transformation. The formation of solid solutions, and the existence of solubility limits of Fe ions in the titanium dioxide phase leading to the formation of a separate iron oxide phase at higher dopant concentrations has also been reported. The chemical interactions at elevated temperatures also involve reactions between the iron oxide phase with titanium dioxide with the eventual formation of mixed iron oxide and titanium dioxide phases [23]. The occurrence of the processes listed has been shown to be dependent on factors such as the iron content, the temperature of calcination, and in particular the method of synthesis [27].

We have previously compared the phase transformations and other processes encountered during the heat treatment of core–shell systems with processes encountered in the commonly studied systems in which the iron oxide phase is homogeneously dispersed in the titania phase [28]. There we pointed to the lower extent of interactions occurring between the iron oxide and the titanium dioxide in the core–shell system, when compared with the mixed Fe–Ti systems. In this paper we provide further evidence to support our previous postulation. We also include surface charge measurements as evidence for changing surface properties due to applied heat treatment. Experiments undertaken to minimise the duration of the heat treatment are also presented. Shortening of the duration of heat treatment is envisaged to significantly lower the extent of interaction between the iron oxide core and the titanium dioxide coating, while at the same time limiting the extent of oxidation of the iron oxide core. This is critical for the post treatment magnetic separation of the magnetic photocatalyst particles from the treated wastewater.

Section snippets

Particle synthesis

The iron oxide–titania coated particles were prepared by depositing titanium dioxide onto the surface of a magnetic iron oxide core (magnetite) using the sol–gel process. This involved the hydrolysis of Titanium Butoxide (TBOT) in the presence of magnetite seeds resulting in a structure having an almost core and shell geometry. These particles are referred to as FeTi1. The description of the preparation method has been described elsewhere [28]. Single-phase titanium dioxide samples (Ti2) were

Results and discussion

The direct coating of titanium dioxide onto the surface of the magnetite particles resulted in the formation of an aggregate of magnetite particles surrounded by a matrix of titanium dioxide. As can be seen from Fig. 1, the particles prepared had an almost core–shell geometry in which the iron oxide was predominantly concentrated in the centre of the composite particles.

The titanium dioxide coating deposited using the sol–gel method was amorphous in nature. This amorphous nature was also

Conclusions

A series of Fe3O4–TiO2 samples heat-treated under different conditions were tested for photocatalytic performance. The heat treatment was found to be a critical step in the preparation process, as it had important implications on the photoactivity and the strength of the magnetic properties of the coated particles. The photoactivity of the prepared coated particles was found to decrease with an increase in the heat treatment and was lower than that of single-phase TiO2. The heat treatment step

Acknowledgements

We wish to kindly thank Dr Gary Low and Mr. Steve McEvoy for the discussions with regards to the photoactivity results and for the use of the photoreactor set up. We also wish to thank Professor Robert Burford for generously allowing us to use the DSC apparatus.

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