Preparation, Characterization, and Photocatalytic Property of Cu_𝟐O-TiO_𝟐 Nanocomposites

Abstract

The Cu₂O-TiO₂ nanocomposites were successfully synthesized by the homogeneous hydrolysation, followed by the solvothermal crystallization and ethylene glycol-thermal reduction process, respectively. The obtained products were characterized by means of X-ray diffraction(XRD), Uv-vis diffuse reflectance spectroscopy, laser particle size analysis, and scanning electron microscopy (SEM), respectively. The photocatalytic performance of Cu₂O-TiO₂ nanocomposites was evaluated by the degradation of methyl orange (MO) as a model compound. The experimental results showed that the prepared Cu₂O-TiO₂ nanocomposite exhibited higher photocatalytic activity for the decomposition of MO than the pure Cu₂O and the commercial Degussa P25 TiO₂ under visible light irradiation.

1. Introduction

The semiconductor photocatalysis has been used to mineralize the organics via a series of intermediates into inorganic substances such as H₂O and CO₂ in the presence of light. To date, many semiconductors have been found to be good photocatalysts to decompose various organics. Among various semiconducting materials, much attention has been given to TiO₂ [1–4] because of its high photocatalytic activity, resistance to photocorrosion, chemical and biological inertness, commercial availability, and inexpensiveness. However, the photoinduced charge carrier in single bare semiconductor particles like TiO₂ has a very short lifetime owing to the high-recombination rate of the photogenerated electron/hole pairs, which reduces photocatalytic efficiency. On the other hand, Titania has a large band gap (about 3.2 eV for anatase phase) and only a small fraction of solar light can be absorbed. These hinder the wide-scale engineering applications of pure titanium dioxide. Therefore, in order to improve the photocatalytic activity of TiO_2, it is important to prevent the photoelectron/hole recombination until a designated chemical reaction takes place on the surface of semiconductor particles as well as to extend the light absorbing property of TiO₂. Some previous researches have found that the coupling of two semiconductors can improve the photoexcited charge separation and enhance the photocatalytic activity [5–11]. On the other hand, Cu₂O is a p-type semiconductor with direct band gap of 2.0 eV and has a noticeable light absorption capability in the visible-light region [12–14]. Accordingly, it is expected to prepare Cu₂O-TiO₂ nanocomposite with the highly efficient photoexcited charge separation, the enhanced photocatalytic efficiency and the remarkable visible-light photoresponse by coupling TiO₂ with Cu₂O, which has been reported in the few previous studies [15–18].

In this study, using the ethylene glycol as the solvent and the reducing agent, and titanium tetrabutoxide and Cu (II) acetate as precursors, the Cu₂O-TiO₂ nanocomposite was successfully synthesized by the homogeneous hydrolysation, followed by the solvothermal crystallization treatment and the ethylene glycol-thermal reduction reaction, respectively. The process would develop a new method for preparing Cu₂O-TiO₂ nanocomposite with the visible-light photocatalytic activity under mild conditions.

2. Experimental

2.1. Materials and Apparatus

The reagents (titanium tetrabutoxide, copper (II) acetate monohydrate, glacial acetic acid, ethylene glycol, and methyl orange) are all analytic reagent grade. A Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (𝜆=0.15418 nm), the accelerating voltage of 40 kV, emission current of 40 mA, and the scanning speed of 8°/min was used to determine the crystal phase composition and the crystallite size of the coupled oxides prepared. And a scanning electron microscope (LEO1530VP) was used to observe the shape and size of the prepared products. The UV-Vis diffuse reflectance spectroscopy was obtained using a UV-visible spectrophotometer (TU-1901, Beijing Purkinje General Instrumental Co., China), and the particle size analysis was performed on a Zetasizer Nano ZS90.

2.2. Synthesis Procedure

All of the chemical reagents used in the experiments were analytical grade without further purification and treatment. The synthesis procedures of Cu₂O-TiO₂ were as follows: 0.01 mol of titanium tetrabutoxide and 0.04 mol of glacial acetic acid were dissolved in 50 mL of ethylene glycol. The solution is taken in Teflon-lined stainless-steel autoclave, heated to 120°C at a rate of 5°C/min, and kept under the temperature for 2 hr. Uniform hydrolysis of Ti(OBu)₄ was accomplished via in situ homogeneous generation of water, which is formed by the esterification reaction of ethylene glycol with acetic acid. As Result, the amorphous hydrous titanium oxide precursor TiO₂⋅nH₂O was obtained, and then the autoclave was heated up to 200°C and held at this temperature for 10 hr, and the amorphous precursor was transformed into the nanosized TiO₂ with stable crystal structure. After the autoclave was allowed cooling to room temperature, the solution of copper (II) acetate in ethylene glycol was added into the autoclave in certain mole ratio of Ti/Cu under stirring condition. The reaction mixture was heated in the temperature range of 150–190°C for 6 hr in order to load Cu₂O onto the surface of TiO₂ by ethylene glycol-thermal reduction process. Subsequently, the product Cu₂O-TiO₂ was separated from the solid-liquid mixture by low pressure distillation at 150°C and grinned in agate mortar to obtain the powder samples.

2.3. Photocatalytic Activity Measurement

The photocatalytic activity tests of the obtained Cu₂O-TiO₂, the pure Cu₂O, and the commercial Degussa P25 TiO₂ were performed at ca. 30°C in a 250 mL glass reactor, respectively. A 36-W fluorescent lamp used as a visible light source was placed above the reaction mixture approximately 10 cm away from solution surface. The initial concentrations of MO and photocatalyst powders were 0.02 and 3 g·L⁻¹, respectively. Prior to irradiation, the suspension was stirred in a dark to establish adsorption-desorption equilibrium. Once the concentration of MO had stabilized, the reaction mixture was irradiated, signaling the start of photocatalysis. At given time intervals, sample was collected, centrifuged, and filtered through a 0.2 μm millipore filter. Then the filtrate was analyzed on a 722 visible spectrophotometer at 464 nm.

3. Results and Discussion

3.1. Influence of Thermal Reduction Temperature on Phase Composition and Crystal Size

To determine the crystal phase composition and crystal size of the prepared photocatalyst, X-ray diffraction(XRD) measurements were carried out at room temperature over the diffraction angle (2θ) 20°–80°. Figure 1 showed the XRD patterns of Cu₂O-TiO₂ powders prepared by ethylene glycol-thermal reduction at 150, 160, 170, 180, and 190°C for 6 hr, respectively. From Figure 1, we can observe that there is a continuous sharpening and intensifying of the diffraction peaks for Cu₂O in Cu₂O-TiO₂ with increasing thermal reduction temperature, indicating that the crystal size of Cu₂O increased as the thermal reduction temperature went up. The crystal size of Cu₂O in different reduction temperature can be calculated according to the Scherrer equation. The results showed that the mean sizes of Cu₂O in Cu₂O-TiO₂ were 12.7, 23.9, 35.1, 50.1, and 79.7 nm at 150, 160, 170, 180, and 190°C, respectively. On the other hand, we can see the diffraction peaks of Cu in the Cu₂O-TiO₂ samples as the thermal reduction temperature rising to 180°C and above indicated that part Cu₂O was reduced further to metallic copper.

Figure 1 

XRD patterns of the Cu2O-TiO2 samples.

3.2. Morphology and Size Distribution of Couple Oxides

In order to study the morphology of the prepared Cu₂O-TiO₂, scanning electron microscopy (SEM) was used. Figure 2 showed the TEM image of the Cu₂O-TiO₂ sample prepared at the thermal reduction temperatures of 160°C. SEM micrograph indicated that the obtained Cu₂O-TiO₂ sample was homogeneously distributed nanocomposite particles, and the particle size of them was in the range of 40−60 nm.

Figure 2 

SEM pattern of Cu2O-TiO2.

In the present study, the particle size and size distribution of the Cu₂O-TiO₂ sample prepared at the thermal reduction temperatures of 160°C also was measured on a Zetasizer Nano ZS90 (Malvern Instrument, Worcestershire, UK). As shown in Figure 3, the Cu₂O-TiO₂ particles size was found in the range of 35–105 nm, and the average particle size was about 64 nm. The particle size measured by the zetasizer was larger than that observed in XRD and SEM. Because Malvern Instruments' Zetasizer used light scattering techniques to measure hydrodynamic size of nanoparticles, a increase in particle size can be the result of particle agglomeration.

Figure 3 

Particle size distribution of Cu2O-TiO2.

3.3. UV-Vis Diffuse Reflectance Spectra

The absorption spectra of Cu₂O, P25 TiO₂, and Cu₂O-TiO₂ prepared at the thermal reduction temperatures of 160°C were given in Figure 4. Figure 4 showed that all the samples had a strong absorption at the wavelength range from 230 to 380 nm. In addition, it can be also observed from Figure 4 that the absorption spectroscopy of the Cu₂O-TiO₂ sample was red-shifted compared to that of TiO₂, and the Cu₂O-TiO₂ sample had obvious absorption in the visible region (>400 nm). The absorption wavelength of the Cu₂O-TiO₂ nanocomposites was extended to a visible region due to absorption of visible light by Cu₂O.

Figure 4 

UV-Vis diffuse reflection spectra.

3.4. Photocatalytic Activities of Samples

The photocatalytic activities of the Cu₂O-TiO₂ samples through the photodegradation of MO under the visible light irradiation for 3 hours were evaluated and were also compared with that of the commercial Degussa P25 TiO₂ powder and the pure Cu₂O powder. The experimental results were illustrated in Figure 5. Obviously, the synthesized Cu₂O-TiO₂ showed higher photocatalytic activity than the pure TiO₂ and Cu₂O under UV-vis light irradiation. The high photocatalytic activity of Cu₂O-TiO₂ can be attributed to the more efficient separation of photoinduced hole-electron (h-e) pairs in the Cu₂O-TiO₂ composite, that is, to say that the photogenerated holes migrate towards the interface while the electrons migrate towards the bulk due to Cu₂O-TiO₂ p-n heterojunction. Meanwhile, the excited electrons on Cu₂O can also transfer to TiO₂ because the conduction band of TiO₂ lies more positive than that of the Cu₂O. Therefore, the Cu₂O-TiO₂ composite exhibited much higher photocatalytic activity than the pure TiO₂ and Cu₂O.

Figure 5 

Photocatalytic activities of different photocatalysts under visible-light irradiation (the Cu2O-TiO2 sample was prepared at a thermal reduction temperature of 160°C).

4. Conclusions

The nanoscale Cu₂O-TiO₂ couple oxide photocatalyst was successfully prepared and was characterized by X-ray diffraction, laser particle size analysis, and scanning electron microscopy, respectively. The characterization results indicated that the couple oxide samples consisted of the nanosized Cu₂O and TiO₂ phases when the thermal reduction temperature was not more than 180°C. The results also showed that the crystal size of Cu₂O was obviously affected by the thermal reduction temperatures, that is, the Cu₂O particle size increased with increasing thermal reduction temperature. Besides, there was a phase change from Cu₂O to Cu in the obtained samples when the thermal reduction temperatures were over 180°C. In addition, the photocatalytic activity experiment results showed that the couple oxide Cu₂O-TiO₂ exhibited much higher photocatalytic activity than the pure TiO₂ and Cu₂O.

Acknowledgments

The authors thank anonymous reviewers very much. This work was supported by the Nature Science Foundation of Anhui Provincial Education Committee (No. KJ2010A302).