Photocatalytic Degradation of Isopropanol Over PbSnO3Nanostructures Under Visible Light Irradiation

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.


Introduction
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][2][3][4][5][6][7][8][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 lightdriven 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][11][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 SrSnO 3 nanostructures [13] from a facile hydrothermal method. Compared with the catalyst from the traditional solid state route, nanostructured SrSnO 3 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, PbSnO 3 nanostructures including particulate and tubular shapes. Experimental results confirmed that these nanostructures show distinguished photocatalytic oxidation activity upon mineralizing isopropanol (IPA) into CO 2 in the visible light region.

Synthesis of PbSnO 3 Nanostructures
For the synthesis of tubular PbSnO 3 nanostructures, two same surfactant-water solutions were first prepared by dissolving 0.2 g poly(vinyl pyrrolidone) (PVP) surfactant in 25 mL distilled water, respectively. Then, equivalent amounts of Pb(AC) 2 and Na 2 SnO 3 (2 mmol) were dissolved in the above surfactant-water solution at room temperature, separately. After stirred for 30 min, the solutions were mixed together and kept stirring for another 30 min, which were then transferred into a Teflon-lined stainless steel autoclave and subsequently heated at 180°C for 16 h in an oven. After cooling to room temperature, the yellow precipitate was filtered and washed for several times with distilled water and ethanol, respectively, then dried in air at 70°C. PbSnO 3 nanoparticles were also synthesized in this work using a similar process without the use of surfactant PVP. Brief flowcharts illustrating the formation of PbSnO 3 nanostructures are shown in Scheme 1.

Synthesis of Bulk PbSnO 3 from SSR
To compare the photocatalytic properties, bulk PbSnO 3 was also synthesized by selecting optimal experimental parameters including calcinations temperature and time. For the synthesis of PbSnO 3 bulk material, we first dissolved equivalent amounts of Pb(AC) 2 and Na 2 SnO 3 into 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 PbSnO 3 at high temperature.

Characterization
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 PbSnO 3 nanostructures were evaluated by decomposition of gaseous IPA under visible light irradiation. Typically, 0.1 g PbSnO 3 catalyst was spread uniformly in a quartz-made vessel with an irradiation area of 7.8 cm 2 . Prior to light irradiation, the vessel was kept in dark for 2 h until an adsorptiondesorption equilibrium was finally established. The visible light with light intensity of about 1.8 mW/cm 2 was 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 The crystal structure of both as-synthesized PbSnO 3 nanostructures from the hydrothermal process and bulk material from the solid-state route were characterized by XRD and the results are shown in Fig. 1. In these patterns, all peaks can be indexed as cubic phase PbSnO 3 with pyrochlore-type structure (space group: Fd3m). The calculated lattice constant a = 10.67 Å is in agreement with previously reported value (JCPDS 17-060). From the XRD patterns, it can be clearly seen that the PbSnO 3 nanostructures are of better crystallinity than the bulk material, which might be one of the reasons why nanostructured PbSnO 3 show higher photocatalytic activities (detailed contents in the part of discussion). Inset in Fig. 1 is a typical SEM image of the product from the SSR. Scheme 2 shows the crystal structure of pyrochlore-type PbSnO 3 , an anion-deficient three-dimensional framework consisting of corner-sharing SnO 6 octahedra. Figure 2a shows a TEM image of as-prepared PbSnO 3 nanoparticles from the hydrothermal process. Obviously, the products are consisted of many small nanoparticles with dimensions in the range of 10-15 nm. The corresponding selected-area electron diffraction (SAED) pattern (Fig. 2b) can be readily indexed as cubic phase PbSnO 3 , which is in agreement with the XRD result. An EDS spectrum in Fig. 2c depicts the presence of Pb, Sn, and O elements, indicating the formation of PbSnO 3 . In this spectrum, the signals corresponding to Cu arise from the TEM grid. The microstructures of the produced PbSnO 3 nanoparticles were investigated using high-resolution TEM. As indicated in Fig. 2d, the nanoparticles are well-crystallized and of good crystallinity. The marked lattice fringes of 0.32 and 0.25 nm correspond well to the (311) and (331) crystalline planes of cubic PbSnO 3 .
In the presence of surfactant PVP, polycrystalline PnSnO 3 nanotubes were obtained instead of nanoparticles. Panels (a) and (b) of Fig. 3 are typical TEM images of as-obtained PnSnO 3 nanotubes, which reveal that the nanotubes are polycrystalline with typical diameters of 300-340 nm and wall thickness of 40-80 nm. Figure 3c is the corresponding SAED pattern taken from a single PbSnO 3 nanotube, confirming the formation of polycrystalline nanotube. The three polycrystalline rings are in accordance with those of (311), (400), and (533) of cubic phase PbSnO 3 . Typical HRTEM images of the nanotubes are shown in Fig. 3d and e. It can be seen that the polycrystalline PbSnO 3 nanotubes are composed of numerous nanoparticles with diameters of several to ten nanometers. The interplanar spacing was calculated to be about 0.32 nm, corresponding to the (311) plane of cubic PbSnO 3 , in accordance with the SAED result.
UV-Vis spectra of all three PbSnO 3 samples were checked and the spectra are displayed in Fig. 4. It is evident that PbSnO 3 nanostructures could absorb much more visible light than bulk sample at the present condition. Corresponding band gaps of PbSnO 3 are determined to be 2.8 eV for bulk material, 2.8 eV for nanotubes, and 2.7 eV for nanoparticles from the absorption edges, respectively (as shown in Table 1).

Growth Mechanism
One-dimensional micro-or nanosized tubular materials with hollow interior structure have attracted extraordinary attention owing to their unique properties and potential applications [14][15][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 [17], NiCl 2 [18], Nb 2 O 5 [19], Se [20], etc. During the growth of PbSnO 3 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 PbSnO 3 nanotubes may involve three following distinctive stages: (i) the generation of PbSnO 3 particles, (ii) the adsorption of PVP molecules on the surface of particles and subsequently selfassembly into tubular microstructure, and (iii) the formation of uniform PbSnO 3 nanotubes. In the initial stage, cubic PbSnO 3 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 selfassembled into tubular structures with the help of PVP template molecules. As a result, the growth of PbSnO 3 nanotubes would form eventually by a typical oriented  Fig. 5. It was clear that the concentration of IPA in the reaction system almost decreased from the initial concentration to zero; the concentration of acetone also decreased continually while the concentration of CO 2 increased with the long-term irradiation. Inset in Fig. 5 shows that almost no additional CO 2 gas was detected under dark test, suggesting that degradation of IPA over the catalyst was driven by light irradiation. Figure 6 further displays the concentration changes of evolved acetone over different PbSnO 3 nanostructures and bulk material with the increasing of irradiation time. Clearly, acetone was detected over all these catalysts when light was turned on. Among them, particulate PnSnO 3 performs the best activity for degradation of IPA under the present conditions.
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, PbSnO 3 nanostructures with larger surface areas as 68 m 2 /g for nanoparticles and 50 m 2 /g for nanotubes, respectively, resulted in enhanced photocatalytic activities than bulk material with 10 m 2 /g of surface area. Meanwhile, the improved crystallinity of PbSnO 3 nanostructures (shown in XRD patterns) resulted in the increase of photocatalytic activity since it could reduce electron-hole recombination rate.
The wavelength dependence of the rate of acetone evolution from IPA degradation over PbSnO 3 nanoparticles was investigated by using different cutoff filters, as shown in Fig. 7. The intensity variation of the incident light with different cutoff filters is given as an inset figure for reference. It is notable that the rate of acetone evolution decreased with increasing cutoff wavelength, which is in good agreement with the UV-Vis diffuse reflectance There was no detectable change between the spectra of PbSnO 3 before and after the photodegradation of IPA gas, suggesting that the catalyst was fairly stable for the degradation of organic compounds. For many p-block metal oxides photocatalysts with d 10 configuration, the VB and CB are the 2p orbital of the oxygen atom and the lowest unoccupied molecular orbital (LUMO) of p-block metal center, respectively [22][23][24]. Meanwhile, for the lead-containing compounds, it was found that an additional hybridization of the occupied Pb 6s and O 2p orbitals seems to push up the position of the valence band and result in a narrower band gap [25]. Based on the above depiction, we assumed that the VB of PbSnO 3 is composed of hybridized Pb 6s and O 2p orbitals, whereas the CB is composed of Sn 5s orbitals, and these bands meet the potential requirements of organic oxidation.

Conclusion
In summary, we have successfully synthesized pure phase PbSnO 3 nanoparticles 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 PbSnO 3 structure. PbSnO 3 nanostructures 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.