TiO2 nanoparticles and their ... - CyberLeninka

Materials Research Bulletin 48 (2013) 4283–4286

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Synthesis of CdTe/TiO2 nanoparticles and their photocatalytic activity§ Deliang Li *, Shijun Wang, Jing Wang, Xiaodan Zhang, Shanhu Liu Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Center for Environment and Health Engineering, Henan University, Kaifeng 475004, Henan, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 31 January 2013 Received in revised form 27 May 2013 Accepted 26 June 2013 Available online 4 July 2013

Visible-light-induced CdTe/TiO2 photocatalysts were synthesized under multibubble sonoluminescence (MBSL) condition. The morphology, phase and optical properties of the products have been characterized by X-ray powder diffraction, field emission scan electron microscope, UV–vis absorption spectroscopy and photoluminescence spectroscopy. The results showed that as-prepared nanoparticles are wellcrystallized with a better size distribution. In addition, the photocatalytic activities of products were evaluated using the photocatalytic degradation of rhodamine B as a probe reaction. The results showed that the products synthesized under MBSL condition exhibited higher photocatalytic activities than those without MBSL irradiation. ß 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Keywords: Multibubble sonoluminescence (MBSL) CdTe/TiO2 nanocomposite Photocatalytic activities

1. Introduction During the past decades, the photocatalytic applications of semiconductor material have received a great deal of attention [1– 5]. Among various photocatalysts, TiO2 has been extensively studied due to its unique physical and chemical properties, such as superior photoreactivity, non-toxicity, long-term stability and low price [6–9]. However, the intrinsic band gap of TiO2 (3.2 eV) and high recombination rate of photogenerated electron–hole pairs are the main drawbacks limiting the future improvement of photochemical activity. Various attempts such as substitutional element doping [10–12] and surface modification with noble metal and semiconductor [13–15] have been successfully tried to overcome the two obstacles. Coupling a second narrow band-gap semiconductors with a more negative conduction band level can result in the vectorial transfer of conduction band electrons and valence band holes from one semiconductor to another. This gives rise to charge separation and a decrease in the pair recombination rate, i.e. an increase of their lifetime, making it more suitable in photovoltaic or photoconduction applications [16–20]. Among the most widely used narrow band-gap semiconductor sensitizers, CdTe has received much attention because of its high absorption coefficient and nearly optimum band-gap energy for the efficient absorption and conversion of solar energy.

§ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Fax: +86 03783881589. E-mail address: [email protected] (D. Li).

As one of the most important II–VI semiconductor nanomaterials, CdTe appeared to have attracted vast attention for both fundamental studies and various promising applications. The direct optical band gap of 1.44 eV that is nearly optimally matched to solar spectrum and the conduction band level of CdTe is more negative than that of TiO2, so that the conduction band electrons of CdTe are allowed to be injected to TiO2 conduction band while the TiO2 valence band holes are injected to CdTe valence band. Thus, a wide electron–hole separation is achieved. Recently, a serious of approaches have been suggested for the fabrication of CdTe/TiO2 compound semiconductors. Li et al. synthesized CdTe quantum dots doped TiO2 photocatalysts were synthesized by a facile sol–gel method at room temperature and their photocatalytic activities were investigated by degrading malachite green (MG) in aqueous solution under halogen– tungsten lamp irradiation. The results revealed that the catalysts exhibited much higher photocatalytic activities than both controlled TiO2 and P25 [21]. Feng et al. reported the photocatalytically oxidative decomposition of p-nitrophenol (PNP) with the CdTe nanoparticles-modified TiO2 nanotube arrays (CdTe/TiO2 NTAs) as catalyst under visible light irradiation. The CdTe/TiO2 NTAs were prepared through pulse electrodeposition method and showed much higher degradation rate than the unmodified TiO2 NTAs [22]. Wang et al. prepared CdTe quantum dots sensitized TiO2 nanotube array photoelectrodes by depositing CdTe on the tube walls of self-organized TiO2 NTs using successive ionic layer adsorption and reaction technique. The results indicated that TiO2 NTs photoelectrodes sensitized by CdTe quantum dots exhibited excellent photoelectrochemical properties, with an open-circuit voltage of 0.95 V and a short circuit current density of 11.15 mA cm 2 [23]. However, most of their synthetic methods

0025-5408/$ – see front matter ß 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.06.052

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are harmful and energy consuming. Therefore, it is necessary to develop a facile and convenient method for the preparation of CdTe/TiO2 nanocomposite under mild conditions. Sonochemical process is an application of sonoluminescence (SL), a light emission phenomenon associated with the catastrophic collapse of acoustic cavitation bubbles under the influence of ultrasound. During the sonication process, the collapse inside the bubbles and liquid adjacent to the bubble produces intense localized heating and high pressures, with very short lifetime. Over recent years, sonochemistry has been proven to be a useful technique for performing a wide range of chemical reactions and processes including chemical synthesis, materials production and water treatment [24–26]. For a typical sonochemical reaction under multibubble sonoluminescence (MBSL) conditions in this study, the hot-spots occur in clouds of acoustic cavitation bubbles have equivalent temperature of roughly 5000 K, pressure of about 1000 atmospheres, heating and cooling rates above 1010 K/S [27,28]. The energy provided by the MBSL irradiation is high enough to permit access to a range of chemical reactions such as oxidation, reduction, dissolution and decomposition, which have been explored for producing novel materials with unusual properties [29–33]. By this process, homogeneous composite semiconductor such as CdS/TiO2, CdSe/TiO2 with a core/shell structure in nano scale were prepared in our previous research. Not only coating process but the nucleation of nanocrystals also occurred in ultrasonic field [34,35]. To our best knowledge, there is no report about CdTe sensitized TiO2 nanoparticles through MBSL method. In this study, CdTe and CdTe/TiO2 nanoparticles were synthesized under MBSL condition, and the photochemical properties of the prepared photocatalysts are systematically studied. The result showed that the products were well formed and its photocatalytic activity was improved by coupling with TiO2. Further more, the possible mechanism is described below. 2. Experimental 2.1. Preparation of CdTe/TiO2 nanocomposites Cadmium chloride (CdCl2), sodium borohydride (NaBH4), thioglycollic acid (TGA) and rhodamine B (RB) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tellurium (Te) powder was purchased from Beijing Chemical Works. Titanium dioxide (TiO2) powders were purchased from Zhoushan Mingri Nanometer Material Co., Ltd. All the chemicals were of analytical grade and used as received without purification. For the synthesis of sodium hydrogen telluride (NaHTe), a modified literature method was adopted [36]. Briefly, 37.8 mg (1 mmol) of NaBH4 and 25.5 mg (0.2 mmol) of Te powder were loaded in a 25 ml three-necked flask, pumping off the air in the system for 30 min and flowing with N2. 29 mg (0.1 mmol) of CdCl2H2O and 25 mL (0.2 mmol) of TGA was mixed in 25 mL of water and the pH of the solution was adjusted to 10.0 by dropwise addition of 1.0 M NaOH solution with stirring. After the addition of 1.6 mmol TiO2, an oxygen-free NaHTe solution was injected into the Cd precursor solution at room temperature. Then, the reaction mixture was sonicated at the aforementioned MBSL conditions with a high intensity ultrasonic horn (20 kHz, 55 W). The temperature was maintained at approximately 55 8C by external constant temperature system. Finally, the brownish red CdTecapped TiO2 nanocomposites were centrifuged, washed and dried at vacuum oven at 55 8C for 12 h.

instrument using Ni-filtered Cu Ka1 irradiation (40 kV, 40 mA). The morphologies of the prepared photocatalysts were investigated by using a field emission scanning electron microscope (FESEM, Hitachi, model S-4800, Japan). Diffuse reflectance absorption spectrum (DRS) for evaluation of photophysical properties were measured on Hitachi U-3010 apparatus equipped with an integration sphere, BaSO4 was used as a reference. Room temperature PL spectra were recorded on a Hitachi F-7000 luminescence spectrometer with Xenon lamp over a range of 400–800 nm. 2.3. Photocatalytic activity measurement The photocatalytic experiments were carried out in order to evaluate the photocatalytic activity of the CdTe/TiO2 nanocomposites synthesized under MBSL conditions. The influence of initial substrate concentration and photocatalyst amount was investigated. In the case of photocatalysis degeneration, 200 mg/L of catalysts were introduced into the aqueous RB (15 mg/L) solution, and the suspension was stirred for 30 min in the dark to ensure complete equilibration of absorption and desorption of RB on the catalysts’ surface. A 350 W Xenon lamp with a 420 nm cut-off filter was used as a visible-light source. The change in concentration of RB was monitored using UV–vis spectrometer during photocatalytic degradation. For comparison, identical control experiments were carried out by using non-ultrasonic CdTe as catalyst instead. 3. Results and discussion 3.1. X-ray diffraction experiments In order to study the structure properties of the as-prepared nanoparticles, i.e. to investigate the cryptographic phase, the overall crystalline quality, and the possible texture, X-ray diffraction experiments have been carried out. Fig. 1 presents the typical XRD patterns of the as-prepared CdTe (Fig. 1a) and CdTe/TiO2 (Fig. 1b). The characteristic zinc blend planes of (1 1 1), (2 2 0), and (3 1 1) locating at 24.408, 41.608, and 47.908 for CdTe have been obviously observed. They can be well distinguished as a cubic structure of CdTe (JCPDS card no. 15-0770). Meanwhile, the remaining peaks at 25.48, 37.98, 48.18 and 55.28 can be indexed to the reflection from (1 0 1), (0 0 4), (2 0 0) and (2 1 1) planes of the anatase phase of TiO2. With further introduction of TiO2, the diffraction peak positions have no significant shift. Therefore,

2.2. Characterization The synthesized nanocomposite has been studied by various apparatus. The XRD patterns were recorded by Philips X’Pert pro

Fig. 1. XRD spectra of the products synthesized under MBSL condition: (a) CdTe nanoparticles and (b) CdTe/TiO2 nanocomposite.


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Fig. 2. FE-SEM images of CdTe/TiO2 synthesized under MBSL conditions.

CdTe/TiO2 have the same crystal structure as CdTe. It should be note that no CdTe peaks are observed when the experiment is carried out without MBSL irradiation, indicating that the MBSL irradiation is the prerequisite for the formation of the CdTe/TiO2 nanocomposite. The important influence of MBSL irradiation perhaps derives from cavitation phenomenon involving the nucleation, growth and violence collapse of gas/vapor filled microbubbles in a liquid. 3.2. FE-SEM micrograph analysis The morphologies of as-prepared CdTe/TiO2 nanocomposites were characterized by FE-SEM and the image was presented in Fig. 2. As can be seen that it consisted of solid microspheres with diameters in the range of several tens nanometers to several micrometers. The micrograph of CdTe/TiO2 nanocomposites illustrates the better size distribution and the growth of particles was occurred more uniformly.

Fig. 3. DRS and corresponding PL emission spectra of the products synthesized under MBSL conditions: (a) CdTe nanocrystals, (b) CdTe/TiO2 nanocomposites, and (c) TiO2 nanoparticles.

3.3. DRS and PL emission spectra analysis Fig. 3 shows the temporal evolution of DRS and corresponding PL emission spectra of the resulting CdTe/TiO2, CdTe and commercial TiO2 nanoparticles. It reveals that the absorption intensity of TiO2 nanoparticles is much higher than the other two in the ultraviolet light range, and quickly drops to the lowest in the visible light range. After CdTe sensitization, a great increase in absorbance and broadening of light spectrum absorption region is achieved in the CdTe/TiO2 nanostructure. For the PL spectra of CdTe, it exhibits two main types of emissions located at 535 nm and 640 nm, which correspond to the defect and energy band, respectively. Since the introduction of TiO2, a red-shift of the emission peak in PL spectra was observed. 3.4. Effect of initial concentration of RB The photocatalytic experiments were carried out at different initial concentration for a catalyst loading of 200 mg/L in order to evaluate the initial concentration of RB. As shown in Fig. 4, the photocatalytic degradation was found to be inversely affected by the initial concentration of RB and decreased with an increase of the concentration. This could be attributed to the fact that the active oxygen species formed on the surface of catalysts is limited with an increase of the initial concentration of RB. Further, increased concentration of RB interfered the light absorption of

Fig. 4. Effect of initial RB concentration on photocatalytic degradation at 200 mg/L catalyst loading.

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heterostructure leads to a smaller effective bandgap than each one of the constituted components, producing a red-shifted absorption, and then, the energy gradient that exists at the interfaces can spatially separate electrons and holes to different components as opposed to charge collection [37]. Thus, a photoexcited electron in CdTe can quickly transfer to the conduction band of TiO2. 4. Conclusion

Fig. 5. Effect of catalyst loading on RB sonophotocatalytic degradation at 15 mg/L initial dye concentration.

In this work CdTe and CdTe/TiO2 nanoparticles were prepared under MBSL condition using CdCl2, NaBH4, Te, and/or TiO2. Compared with the other preparation methods, this synthetic methodology has advantages of brief reaction steps, convenient manipulation and environmentally friendly. The crystal structure of product was in cubic phase and there were no other appreciable impurities in the preparation process. This special nanostructure effectively broadens the absorption spectra, enhances the charge extraction efficiency by the electron and hole separation and provides a fast transfer channel for charge carriers. Therefore, it is expected that these CdTe/TiO2 nanoparticle will be great potential in the biolabeling and other applications. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11] [12] [13] [14] Fig. 6. Photocatalytic activity of the catalysts obtained under different condition.

catalysts in some degree and then resulted in the decrease of photocatalytic degradation.

[15] [16] [17] [18] [19]

3.5. Effect of catalysts load [20]

The photocatalytic degradation of RB at different quantities of catalysts for a initial concentration of 15 mg/L was investigated. As indicated in Fig. 5, the rate of photocatalytic degradation increased with the increase of catalysts up to 200 mg/L and stood still. Therefore, we concluded that excessive growth in catalysts supply is unfavorable for the increase of photocatalytic degradation as a result of a lower light penetration. 3.6. Photocatalytic activity analysis The concentration changes of RB solutions versus irradiation time were shown in Fig. 6. As indicated in Fig. 6, the rate of photocatalytic degradation of RB has made a remarkable improvement by introduction of TiO2 (from 40% to 80%). In the case of CdTe/ TiO2 nanoparticles, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of CdTe lie more positive than the conduction and valence band levels of TiO2, respectively. And the staggered band alignment of the

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