ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2012, том 46, № 2, с. 176-180
НАНОСТРУКТУРИРОВАННЫЕ СИСТЕМЫ И МАТЕРИАЛЫ
INFLUENCE OF PT PROMOTER ON THE PHOTO-OXIDATIVE DEGRADATION BY VISIBLE-LIGHT PLASMONIC Pt-TITANIA CATALYST
© 2012 г. Dongfang Zhang
College of Science, Huazhong Agricultural University Wuhan 430070, PR China E-mail: zdfbb@yahoo.cn Поступила в редакцию 22.06.2011 г. В окончательном виде 18.09.2011 г.
Nanometer platinum-deposited titania particles were prepared through a soft chemical reduction method. The physico-chemical properties of the obtained products are analyzed by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, and photoluminescence spectra. The as-prepared metal-TiO2 nanohybrid was found to show excellent photocatalytic reactivity toward rhodamine-B decomposition. In the current case, chemical reduction of hydrogen hexachloroplatinate by glucose results in the intercalation of metallic platinum into the titania matrix. The deposited platinum atoms were featured by surface-plasmon resonance under visible-light excitation and electrons from platinum nanoparticles would transfer to the conduction band of titania and accelerate the
formation O 2 to degrade dye molecule. As a consequence, platinum deposition onto TiO2 surface is confirmed to yield a superior photocatalytic performance over the naked titania.
Environmental pollution has increased public concern nowadays and the photocatalytic degradation of organic pollutants in waste water has been the subject of numerous investigations in the last couple of decades [1—3]. In this field, TiO2 photocatalyst has led the relevant applications to the stage of commercialization due to its photochemical stability, low cost and non-toxicity [4—6]. Nevertheless, there are still two major issues to be resolved which limit the application of titania as a photocatalyst. One issue is that TiO2 can only be activated by ultraviolet (UV) light source since its band-gap energies are large (about 3.2 eV), which leads to a limited success for TiO2 UV based technologies. It is thus of great interest to make titania visible light sensitive, which will increase the efficiency of solar energy utilization. Another important issue is that TiO2 presents a relatively high electron—hole recombination rate, which is harmful to its photocata-lytic activity. As such, it is necessary to improve the quantum efficiency or the efficiency of utilization of the photoexcited state of photocatalysts, since electronic excited states of titania are deactivated by recombination of electrons and holes to lower quantum efficiency [7]. Addition of noble metals (as surface modifiers) to titania to fabricate a semiconductor-noble metal heterostructure can modify the photocatalytic properties by changing the distribution of electrons on the TiO2 surface and resulting in an increase in the quantum yield of surface photoreactions [8]. It is thought that in the semiconductor-metal composites, the metal acts as a sink for the electrons while the holes are scavenged by suitable species at the semiconductor-electrolyte interface. Therefore, the deposition of noble metals on
semiconductor oxide surfaces facilitates the enhancement of photocatalytic reaction efficiency by promoting charge separation and transfer. Noble-metal-loaded titania photoreaction systems appear to have some superior functions for visible-light-driven photocatalytic reactions. In addition, noble-metal deposits are robust and stable even under photoirradiation in the presence of oxygen. As an example, supporting platinum over surface of titania is thought to improve charge separation inside the semiconductor and to inhibit the recombination chances of electron-hole pairs. Metallic platinum nanoparticles can also enhance the absorption constants for organic compounds [9]. Besides, platinization of TiO2 usually shows high photocatalytic activity for a range of photocatalytic oxidation reactions since the deposited platinum not only accelerate the photoinduced electron transfer rate at the interface but also provide catalytic sites [10, 11]. Therefore, the modification of titania with platinum appears to be one of the most effective techniques to tailor the photocatalytic properties of pristine TiO2 material.
The present study demonstrates the advantage of semiconductor/metal composite in photocatalytic application. The photocatalytic properties of pristine TiO2 were tailored via platinum metallization and the mechanism of photocatalytic reaction upon platinum-TiO2 system under visible-light activation was examined. Using Rhodamine-B dye degradation as a probe reaction, we investigated the photocatalytic activity of the as-prepared hybrid material under visible-light illumination. Our preliminary results show the great potential of the platinum-TiO2 catalyst in pollution removal.
EXPERIMENTAL
All the reagents were analytic grade and used as received without further purification. The high purity water used in all experiments was deionized to a resistivity of 18 Mfi cm. Tetrabutyl titaniate (Ti(OBu)4, TBOT) and chloroplatinic acid (H2PtCl6 • 6H2O) were used as titanium and platinum sources, respectively. Chemical reduction of platinum was carried out by using glucose (C5H11O5CHO) as reducing agent. Typically, 15 mL ofTBOT was added dropwise to 135 mL ethanol under continuous stirring at ambient temperature (AT), named solution I. A mixture consists of 160 mL distilled water (as hydrolysis agent) and a certain amount of H2PtCl6 • 6H2O (33.0 mg) was named solution II. The solution II was added to the solution I under continuous stirring. Then, glucose solution (20 mL C5H11O5CHO/45 mL distilled water) was added to achieve chemical reduction of Pt(IV) ions. The transformation of the precursor to platinum particles could be observed by the change in the solution color from slightly yellow to black. After aging for 24 h, the sols transformed into wet gels. Next, the white precipitate was obtained by centrifugation, followed by washing with distilled water and anhydrous ethanol for several times to remove all remaining chemicals. After that, the wet-gel precursor was heated in an oven to evaporate excess solvent at 70°C to gain xerogel. Finally, the xerogel was calcined at 420°C for 2.0 h to get the platinum-TiO2 nanocomposite. A reference TiO2 sol-gel sample was also prepared similar to the method described above.
X-ray powder diffraction (XRD) data were recorded at room temperature with an X-ray diffractometer (XRD-6000, Shimadzu Corporation) using Cu Ka irradiation (A = 0.15408 nm), operated at 40 kV and 100 mA. The percentage of UV-vis reflectance was measured by diffuse reflectance spectroscopy (DRS) for the powder form of the catalysts using a scanning UV-vis-NIR spectrophotometer (Varian Cary 500) in the region of 200—800 nm. The spectrophotometer was equipped with an integrating sphere assembly. The reflectance data were converted to the absorbance values according to the Kubelka-Munk theory. The photoluminescence (PL) emission spectra of the synthesized samples were measured with a RF-5301 PC spectrofluorophotometer (Shimadzu Corporation) using a Xenon-arc lamp as excitation source at room temperature.
Photocatalytic efficiency of the as-prepared systems was estimated in terms of the degradation of Rohdamine-B (RhB, molecular formula: C28H31ClN2O3) aqueous solution. The artificial light photocatalytic activity was done in a cylindrical quartz photoreactor. Photoirradiation employed a 500-W Xe arc lamp equipped with a UV cutoff filter (emission wavelength A > 400 nm) as visible-light source. All experiments were performed at room temperature. Namely, 25 mg of the sample was dispersed in 150 ml aqueous RhB (1.0 x 10-5 mol/L) solution and magnetically stirred
29, degree
Fig. 1. XRD patterns of the as-prepared photocatalysts.
for 0.5 h to establish the adsorption/desorption equilibrium of RhB solution on the sample surfaces before illumination. Then the suspension solution was irradiated by light and collected at a regular time interval. Finally, the collected solution was centrifuged at a rate of 6000 rpm to remove the solid powders, and UV-vis absorption spectra of the supernatant were measured with a UV-vis spectrophotometer (Spectrumlab 2450, Shimadzu Corporation).
RESULTS AND DISCUSSION
Powder X-ray diffraction (PXRD) patterns were used to investigate the impact ofplatinum modification on the phase structure and the chemical composition. XRD profiles of Pt-deposited TiO2 and pure TiO2 samples are depicted in Figure 1. It was shown that crystalline phase of all samples were almost anatase (PDF # 71-1169, space group: I41/amd) and in both pure TiO2 and platinum modified TiO2 catalyst rutile phase was not detected. Nine characteristic diffraction peaks at around 20 of 25.26°, 37.89°, 47.88°, 53.98°, 55.19°, 62.75°, 68.81°, 70.15° and 75.02°, marked by their Miller indices of (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes, were readily observed in the XRD pattern. The well-defined diffraction patterns confirm that the as-synthesized product was already well crystallized. Compared with the native titania catalyst, no diffraction peaks other than those ofanatase was found in XRD spectra. Besides, the diffraction peaks (20 = = 39.9, 46.1, 67.5°) emerged in the XRD patterns could be assigned to the (111), (200) and (220) planes of metallic platinum (0 oxidation state), which reflects the typical characteristics of crystalline Pt face (centered cubic phase) and confirms the successful deposition of platinum onto the surface of titania. Nonetheless, platinum metal cluster may not have formed on the TiO2 surface since the loaded platinum concentration is low and platinum particles deposited on the surface of titania via soft chemical reduction are usually highly dispersed
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