Photocatalytic Degradation of Isopropanol Over PbSnO3Nanostructures Under Visible Light Irradiation
© to the authors 2009
Received: 12 November 2008
Accepted: 17 December 2008
Published: 7 January 2009
Nanostructured PbSnO3photocatalysts with particulate and tubular morphologies have been synthesized from a simple hydrothermal process. As-prepared samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area, transmission electron microscopy, and diffraction spectroscopy. The photoactivities of the PbSnO3nanostructures for isopropanol (IPA) degradation under visible light irradiation were investigated systematically, and the results revealed that these nanostructures show much higher photocatalytic properties than bulk PbSnO3material. The possible growth mechanism of tubular PbSnO3catalyst was also investigated briefly.
Since the Honda–Fujishima effect was reported in 1972, considerable efforts have been paid to develop semiconductor photocatalysts for water splitting and degradation of organic pollutants in order to solve the urgent energy and environmental issues [1–9]. However, to date, most of the photocatalysts reported only respond to UV light irradiation (<420 nm). For visible light accounts for about 43% of the solar spectrum, the utilization of visible light is more significant than UV light and thus developing visible light-driven photocatalyst is one of the most important and meaningful subjects in this field. The fundamental steps for photocatalytic reaction of oxide semiconductor mainly include the following processes: (i) the generation of photoexited charges in the semiconductor materials, (ii) the separation and migration of the generated charges without recombination, and (iii) the redox reaction on the surface of the semiconductor. The first and second steps are associated with the electronic structures of the oxide semiconductor, while the third step is strongly relevant to the surface properties of the catalyst [10–12].
Generally, the improvement of surface area always contributes to more reaction sites, which is beneficial to the photocatalytic reaction. With particular microstructures, nanomaterials have recently gained much attention to be used as high-performance photocatalysts with enhanced photocatalytic activities. For example, in our previous work, we reported the synthesis of perovskite SrSnO3 nanostructures  from a facile hydrothermal method. Compared with the catalyst from the traditional solid state route, nanostructured SrSnO3 catalysts with larger surface areas showed higher photocatalytic activities for water splitting under UV light irradiation. Undoubtedly, the enhanced photocatalytic activities are mainly attributed to the increased surface areas, which are believed to be one of the efficient approaches to enhance the activity of catalysts. From a similar hydrothermal process, we reported here the preparation of a new visible light-responded photocatalyst, PbSnO3 nanostructures including particulate and tubular shapes. Experimental results confirmed that these nanostructures show distinguished photocatalytic oxidation activity upon mineralizing isopropanol (IPA) into CO2 in the visible light region.
Synthesis of PbSnO3Nanostructures
Synthesis of Bulk PbSnO3from SSR
To compare the photocatalytic properties, bulk PbSnO3was also synthesized by selecting optimal experimental parameters including calcinations temperature and time. For the synthesis of PbSnO3bulk material, we first dissolved equivalent amounts of Pb(AC)2and Na2SnO3into distilled water under stirring, and then mixed them to obtain the white precursor. Heating the white precursor at 500 °C for 5 h in a quartz tube under Ar flow resulted in yellow powders. In this process, temperature is very important for the formation of yellow powders due to the instability of PbSnO3at high temperature.
The crystal structure of the as-prepared sample was confirmed by the X-ray diffraction pattern (JEOL JDX-3500 Tokyo, Japan). The morphology and size of the sample were characterized by transmission electron microscope (HRTEM, JEM-3000F) equipped with an X-ray dispersive spectrometer (EDS). UV–Vis diffuse reflectance spectra were recorded on a UV/Vis spectrometer (UV-2500, Shimadzu) and were converted from reflection to absorbance by the standard Kubelka–Munk method. The surface area of the sample was measured by the BET method (Shimadsu Gemini Micromeritics).
Evolution of Photocatalytic Property
The photoactivities of the obtained PbSnO3nanostructures were evaluated by decomposition of gaseous IPA under visible light irradiation. Typically, 0.1 g PbSnO3catalyst was spread uniformly in a quartz-made vessel with an irradiation area of 7.8 cm2. Prior to light irradiation, the vessel was kept in dark for 2 h until an adsorption–desorption equilibrium was finally established. The visible light with light intensity of about 1.8 mW/cm2was obtained by using a 300 W Xe lamp with a set of combined filters (L42 + B390 + HA30) and a water filter. The products in the gas phase were analyzed with a gas chromatograph system (GC-14B, Shimadzu, Japan), using a flame ionization detector (FID) for organic compounds determination.
Results and Discussion
Crystal Structure and Morphology
Physical and photocatalytic properties of PbSnO3samples
Band gap (eV)
Rate of acetone (ppm/h)
One-dimensional micro- or nanosized tubular materials with hollow interior structure have attracted extraordinary attention owing to their unique properties and potential applications [14–16]. Many kinds of growth mechanisms have been proposed for the formation of nanotubes. For example, the rolling mechanism and template-assisted mechanism have been reported to explain the formation of tubular structure with layered or pseudo-layered structures such as BN , NiCl2, Nb2O5, Se , etc. During the growth of PbSnO3 nanotubes, surfactant PVP was used and was found to be the key issue for nanotube growth. Thus, the surfactant-assisted growth process can be used to explain the formation of these nanotubes. The possible formation process of PbSnO3 nanotubes may involve three following distinctive stages: (i) the generation of PbSnO3 particles, (ii) the adsorption of PVP molecules on the surface of particles and subsequently self-assembly into tubular microstructure, and (iii) the formation of uniform PbSnO3 nanotubes. In the initial stage, cubic PbSnO3 tiny nuclei could easily crystallize and serve as the seeds for the growth of nanotubes. Meanwhile, PVP molecules in the solution would strongly and rapidly adsorb on the surfaces of these nascent nuclei, which confined the crystal growth and efficiently controlled the dimension and morphology of the final products. Then, these particles with high free energy aggregated and self-assembled into tubular structures with the help of PVP template molecules. As a result, the growth of PbSnO3 nanotubes would form eventually by a typical oriented attachment process under the hydrothermal conditions. Meanwhile, the existence of PVP in this solution can alter the surface energies of various crystallographic surfaces to promote selective anisotropic growth of nanocrystals .
Photocatalytic Degradation of IPA
In this case, the photocatalytic activities for IPA degradation over these catalysts were in the order of nanoparticle > nanotube > bulk material, which was in consistent with that of BET surface areas. As mentioned earlier, BET surface area of catalyst is closely related to its photoactivity. Usually, larger surface area means much more active sites, at which the photocatalytic reaction occurs. Thus, as shown in Table 1, PbSnO3nanostructures with larger surface areas as 68 m2/g for nanoparticles and 50 m2/g for nanotubes, respectively, resulted in enhanced photocatalytic activities than bulk material with 10 m2/g of surface area. Meanwhile, the improved crystallinity of PbSnO3nanostructures (shown in XRD patterns) resulted in the increase of photocatalytic activity since it could reduce electron-hole recombination rate.
In summary, we have successfully synthesized pure phase PbSnO3nanoparticles and nanotubes from the facile hydrothermal process at low temperature. The surfactant PVP used as the capping reagent plays a crucial role in the formation of tubular PbSnO3structure. PbSnO3nanostructures with better crystallinity and larger surface areas show enhanced photocatalytic activity for the decomposition of organic pollutant isopropanol under the visible light irradiation than the catalyst prepared by the solid-sate method.
This work was partially supported by the Global Environment Research Fund from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. This work was also supported the World Premier International Research Center Initiative (WPI Initiative) on Materials Nanoarchitectonics, MEXT, Japan and the Strategic International Cooperative Program, Japan Science and Technology Agency (JST).
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