Effects of pH on the microstructures and photocatalytic activity of mesoporous nanocrystalline titania powders prepared via hydrothermal method

doi:10.1016/j.molcata.2006.05.036

Journal of Molecular Catalysis A: Chemical

Volume 258, Issues 1–2, 2 October 2006, Pages 104-112

https://doi.org/10.1016/j.molcata.2006.05.036 Get rights and content

Abstract

Highly photoactive nano-sized TiO₂ powder photocatalyst was prepared by a hydrothermal method at 180 °C for 5 h using tetrabutyl titanate (TBOT) as the precursor. The pH values of the starting suspensions were adjusted from 1 to 11 using a 1.0 M HCl or 1.0 M NH₃·H₂O solution. The obtained TiO₂ powders were characterized with X-ray diffraction (XRD), N₂ adsorption–desorption measurement, transmission electron microscopy (TEM), X-ray photoelectron spectra, Fourier transform infrared (FTIR), Raman spectra and UV–vis spectrophotometry. The photocatalytic activity was evaluated by photocatalytic oxidation decomposition of acetone in air. The results showed that the pH values of the solutions obviously influenced the microstructures and photocatalytic activity of the as-prepared TiO₂ powders. With increasing pH values, the crystallization enhanced, crystallite size increased and BET specific surface areas decreased. The photocatalytic activity of TiO₂ powders prepared at a pH range of 6–9 exceeded that of Degussa P-25 by a factor of more than two times.

Graphical abstract

Effects of pH on the microstructures and photocatalytic activity of mesoporous nanocrystalline titania powders prepared via hydrothermal method.

Introduction

To solve the increasingly serious problems of environmental pollutions, a great deal of effort has been made in recent years, and various catalytic techniques are being applied in the fields of environmental protection. Photocatalysis is one technique that has great potential to control aqueous organic contaminates or air pollutants. It is believed to have several advantages over conventional oxidation processes, such as: (1) complete mineralization of the pollutants; (2) use of the near-UV or solar light; (3) no addition of other chemicals; (4) operation at near room temperature [1], [2], [3], [4], [5], [6]. Although photocatalytic degradations of trace toxic organic compounds in water or air have been investigated intensively in the past decade, there still remain some problems in practical applications [5]. Fundamental research regarding the preparation of photocatalyst with highly photocatalytic activity, the immobilization of powder photocatalyst, and the improvement of photocatalyst performance are priorities to be considered [5], [6], [7].

Among various oxide semiconductor photocatalysts, titania has been proven to be the most suitable for widespread environmental applications for its strong oxidizing power, non-toxicity, long-term photostability and cost effectiveness. Titania has three crystalline phases: rutile, anatase and brookite, among which, rutile is a thermodynamic stable state while the latter two phases are metastable state [8], and anatase is generally recognized to be the most active phase as opposed to the rutile and brookite forms. Usually, the photocatalytic activity of titania depends on its crystal phase, crystallite size, crystallization, specific surface areas, pore structures and so on. For example, many studies have confirmed that the anatase phase of titania is a good photocatalytic material due to its low recombination rate of photogenerated electrons and holes [9], [10]. Moreover, we found that the composite of two phases of titania was more beneficial for suppressing the recombination of photo-generated electrons and holes and thus enhanced the photocatalytic activity [9].

The chemical and physical properties of titania photocatalysts depend also on the procedures and conditions of preparation. A few methods, such as hydrolysis (chemical precipitation) [11], reverse micelles (microemulsion) [12], [13], sol–gel [14], [15], and liquid phase deposition [16], have been used to prepare TiO₂ nanocrystalline photocatalyst. Comparing with these methods, the hydrothermal method has more advantages [12], [13]: (1) Crystallization temperature for anatase phase is below 200 °C. (2) By changing hydrothermal conditions (such as temperature, time, reactant concentration, additives, etc.), various crystalline products with different composition, structure and morphology could be obtained. (3) Low energy consumption and environmentally friendly process. (4) The equipment and processing required are simpler, and the control of reaction conditions is easier, etc. So the hydrothermal synthesis should be a good method for the preparation of semiconductor photocatalyst and other oxide ceramic fine powders.

In this work, highly photoactive mesoporous nanocrystalline TiO₂ powder photocatalysts were prepared by a simple hydrothermal method at 180 °C for 5 h at different pH. The effects of initial pH values on the microstructures and photocatalytic activity of mesoporous titania powders were investigated and discussed.

Section snippets

Preparation

All chemicals used in this study were used as received from Shanghai Chemical Regent Factory of China without further purification. Distilled water was used in all experiments.

Tetrabutyl titanate (Ti(OC₄H₉)₄, TBOT) was used as a titanium source. The pH values of the distilled water were adjusted to pH 1, 3, 6, 9 and 11 using a 1.0 M HCl or 1.0 M NH₃·H₂O aqueous solution. Then, 13.2 ml of TBOT was added dropwise into 135 ml of the above solution under vigorous stirring for 2 h. The obtained slurry

Crystal structure

XRD was used to investigate the changes of phase structure of the as-prepared TiO₂ powders at various pH values. Fig. 1 shows XRD patterns of the as-synthesized TiO₂ powders at different pH value. At pH ≤ 6, TiO₂ powders contained two phases. The anatase was main phase and brookite was minor phase. A small peak at 2θ = 30.7° corresponded to the (1 2 1) diffraction peak of brookite phase. The presence of this phase caused the slight shift to a higher angle of the anatase (1 0 1) peak since there was an

Conclusions

(1)
The pH values of the starting solution had significant effect on the crystallinity, crystallite size, phase structure, and photocatalytic activity of the prepared TiO₂ powders by hydrothermal method. Weak acid conditions of the starting solution were in favor of the formation of brookite phase while basic conditions were favorable for the formation of pure anatase phase. With increasing the pH value, the crystallization enhanced, crystallite size increased and BET specific surface areas

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (no. 20473059). This work was also financially supported by the Key Research Project of the Ministry of Education (no. 106114).

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Effects of pH on the microstructures and photocatalytic activity of mesoporous nanocrystalline titania powders prepared via hydrothermal method

Abstract

Graphical abstract

Introduction

Section snippets

Preparation

Crystal structure

Conclusions

Acknowledgements

J. Photochem. Photobiol. C

Appl. Catal. B

J. Catal.

Thin Solid Films

Appl. Catal. A

J. Solid State Chem.

Appl. Catal. B

Mater. Chem. Phys.

J. Catal.

Appl. Catal. B

J. Mol. Catal. A

J. Coll. Interf. Sci.

Carbon

Appl. Catal. B

Thin Solid Films

J. Solid State Chem.

Chem. Rev.

Chem. Rev.

Langmuir

J. Phys. Chem. B

New J. Chem.

J. Mater. Sci.