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Open AccessArticle

Enhanced UV-Visible Light Photocatalytic Activity by Constructing Appropriate Heterostructures between Mesopore TiO2 Nanospheres and Sn3O4 Nanoparticles

Department of Physics, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 10083, China
*
Author to whom correspondence should be addressed.
Nanomaterials 2017, 7(10), 336; https://doi.org/10.3390/nano7100336
Received: 25 September 2017 / Revised: 12 October 2017 / Accepted: 13 October 2017 / Published: 19 October 2017
(This article belongs to the Special Issue ZnO and TiO2 Based Nanostructures)

Abstract

Novel TiO2/Sn3O4 heterostructure photocatalysts were ingeniously synthesized via a scalable two-step method. The impressive photocatalytic abilities of the TiO2/Sn3O4 sphere nanocomposites were validated by the degradation test of methyl orange and •OH trapping photoluminescence experiments under ultraviolet (UV) and visible light irradiation, respectively. Especially under the visible light, the TiO2/Sn3O4 nanocomposites demonstrated a superb photocatalytic activity, with 81.2% of methyl orange (MO) decomposed at 30 min after irradiation, which greatly exceeded that of the P25 (13.4%), TiO2 (0.5%) and pure Sn3O4 (59.1%) nanostructures. This enhanced photocatalytic performance could be attributed to the mesopore induced by the monodispersed TiO2 cores that supply sufficient surface areas and accessibility to reactant molecules. This exquisite hetero-architecture facilitates extended UV-visible absorption and efficient photoexcited charge carrier separation.
Keywords: photocatalyst; heterostructures; TiO2; Sn3O4 photocatalyst; heterostructures; TiO2; Sn3O4

1. Introduction

As a stable, low-cost and environmentally benign material, nanoscaled titanium dioxide (TiO2) with unique structural and functional properties has become a widely used semiconductor photocatalyst for various solar-driven clean energy technologies [1]. Tailoring the morphology of TiO2 photoanode is a preferred route to achieving high performance in solar cells due to its enhanced properties, such as high surface area, faster electron transport, lower electron-hole recombination rate and good light-harvesting features [2,3]. Nevertheless, the wide optical bandgap of TiO2, which seriously limits its light harvesting capability, leaving about 96% of the solar light energy wasted [4]. Compared with the solution of generating donor or acceptor states in the band gap by adding impurities, rationally designing and constructing the surface heterostructures would be a more efficient strategy for achieving an excellent photocatalyst [5].
Recently, Sn3O4, a novel non-stoichiometric oxide, has raised particular interest in the field of photocatalysis—especially in terms of its catalytic behavior under visible light irradiation—due to a suitable band-gap inside visible light (2.2–2.9 eV) and a distinct surface structure composed of both valences of tin [6,7]. Several studies have shown its great potential as an auspicious photocatalyst under visible light, both for generating hydrogen and degrading dyes [8]. However, some drawbacks have hindered its performance in practice. As a semiconductor with a relatively narrow band gap, pure Sn3O4 generally leads to a fast recombination of photoexcited electron-hole pairs, which ultimately decreases its degradation rate [9].
Discussing these problems together, it proposes an intriguing idea that Sn3O4, as the second component, attaches to the surface of TiO2 nanostructures, for an exquisite TiO2/Sn3O4 heterostructure. On the one hand, a theoretical analysis indicates that the interface between TiO2 and Sn3O4 is to be a perfect type-II heterojunction (both the potentials of valence band (VB) and conduction band (CB) of Sn3O4 are higher than that of TiO2) [10], which is actually conducive to the separation of photoexcited electron-hole pairs. Furthermore, latest reports have exhibited the superiority of the heterogeneous composite of this kind [11,12]. On the other hand, increased photoactive facets can effectively facilitate the efficiency of photo-absorption and oxygen chemisorption, and bring about a fast rate of surface reactions [13]. Therefore, highly dispersive anatase TiO2 mesopore nanospheres, which possess a large number of active surfaces, would likely be an amazing matrix in TiO2/Sn3O4 nanocomposites.
The two-step self-assembly approach is a feasible strategy for the refined design of hierarchical nanostructures with complex morphologies, and has been proven to be an effective way to design multiscale nanostructures, since the morphology and composition obtained from the first step can be further tuned and adjusted by a subsequent second process. Moreover, this approach also allows the combination of multiple synthetic techniques, and the synthesis of complex nanostructures with hierarchical multiscale structures compared with the conventional one-step self-assembling method [14]. Recently, a lot of complex nanostructures with high photocatalytic performance for both visible light and ultraviolet has been acquired by the two-step synthesis method. Usually, these methods can be classified into two categories: (1) synthesis under two continuous identical methods, such as in [9,15,16]; and (2) synthesis under two different methods, such as in [17,18]. In 2015, TiO2/Sn3O4 nanobelts [9] were successfully produced by first synthesizing the TiO2 nanobelts, and then assembling Sn3O4 onto the TiO2 nanobelts in a subsequent hydrothermal procedure; in the same year, hierarchical Sn3O4/N-TiO2 nanotubes [17] were synthesized by first weaving N-doped TiO2 nanotube via electro-spinning, and then modifying them with Sn3O4 via hydrothermal reaction. In 2017, a range of heterojunction WO3/TiO2 thin films were deposited via a two-step process using chemical vapor deposition (CVD) methods [15]. Generally, electro-spinning and chemical vapor deposition is associated with at least one of the following factors: expensive equipment, high voltage, hazardous by-products, or toxic chemicals, rendering the method less environmental friendly and much more complicated than the hydrothermal method or sol-gel synthesis. Hence, synthesizing complex nanostructures via a combination of sol-gel and hydrothermal methods would be a low-cost, scalable, easy to control, and eco-friendly strategy in terms of preparing high-quality, uniform, catalysts.
Herein, we developed a scalable two-step route, combining the sol-gel method and hydrothermal progress to achieve excellent visible and ultraviolet photocatalytic activity by uniformly synthesizing the Sn3O4 nanoparticles on the surface of TiO2 nanospheres. As expected, an enormous enhancement of photocatalytic efficiency was achieved by the distinctive TiO2/Sn3O4 nanocomposites.

2. Experimental Section

2.1. Chemicals

The chemicals used in this study were of analytic grade, and were used without further purification. Tetrabutyl titanate was purchased from Beijing Xingjin Chemical Factory, Beijing, China. Methyl orange (MO) was obtained from Tianjin Jinke Fine Chemical Industry Research Institute, Tianjin, China. Tin dichloride dihydrate (SnCl2∙2H2O) and trisodium citrate dihydrate (Na3C6H5O7∙2H2O) were purchased from Xilong Chemical Industry Co., Ltd., Guangdong, China. Terephthalic acid was purchased from Alfa Aesar (Tianjin, China). All the other organic solvents and salts, including ethylene glycol, acetone, NaOH, were purchased from Sinopharm Chemical Reagent Beijing Co. P25 (nanoscale TiO2 powder, surface area 50 m2∙g−1) was purchased from Degussa AG (Hanau, Germany).

2.2. Synthesis of Samples

In this research, we propose a two-step synthesis method to obtain TiO2/Sn3O4 nanocomposites by preparing TiO2 core via sol-gel route first and then synthesizing Sn3O4 on the surface of TiO2. (1) Synthesis of core TiO2 nanospheres [19]: 3.5 mL tetrabutyl titanate was dissolved in the 50 mL ethylene glycol while stirring vigorously for 10 h. Then, the mixture was immediately poured into a solution containing 170 mL acetone and 2.7 mL deionized water under constant stirring, until white precipitation appeared. The acquired precipitate was calcined in air at 500 °C for 1 h to produce the TiO2 powders; (2) Coating TiO2 with Sn3O4: 0.2 g of the TiO2 product described above and 5.0 mmol SnCl2∙2H2O were mixed with 25 mL deionized water, followed by the addition of 12.5 mmol Na3C6H5O7∙2H2O and 2.5 mmol NaOH under magnetic stirring. During this process, Sn(II) ions were attached to the surface of hydroxyl-rich TiO2 spherical colloids through inorganic grafting. The resulting precursor was then transferred to a 50 mL Teflon-lined stainless autoclave and maintained at 180 °C for 12 h. Finally, the collected powder was washed several times with deionized water and ethanol, and dried at 60 °C for 12 h.

2.3. Characterization of Samples

In order to obtain the physical and chemical properties of as-prepared samples, several characterizations were conducted. X-ray diffraction (XRD) patterns were recorded by a Rigaku D/MAX-2500 diffractometer with Cu Κα radiation (Rigaku, Tokyo, Japan). Raman spectra were obtained using a HORIBA HR800 spectrometer with an Nd:YAG laser at a wavelength of 532 nm (Horiba Yvon, Paris, France). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were acquired from the ZEISS SUPRA 55 (Zeiss, Oberkochen, Germany)and JEOL JEM-2010 (JEOL, Tokyo, Japan), respectively. X-ray photoelectron spectra (XPS) were recorded on a scanning X-ray microprobe PHI Quantera II (Ulvac-PHI, Chigasaki, Japan). The nitrogen adsorption-desorption isotherm was measured at 77 K on an Autosorb-iQ2-MP analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The absorption spectra were carried out by UV-visible spectrophotometer (Lambda 950, Perkin-Elmer, Shelton, WA, USA) and the hydroxyl radicals (•OH) trapping photoluminescence spectra were examined by a fluorescence spectrophotometer (Hitachi F-4500, Hitachi, Tokyo, Japan) using excitation a wavelength of 315 nm.

2.4. Photocatalytic Experiments

The photocatalytic activity of TiO2/Sn3O4 nanocomposites was evaluated via methyl orange (MO) degradation rate. 80 mL aqueous suspension of MO (20 mg/L) and 80 mg of photocatalyst powder were placed in a 100 mL beaker. Prior to irradiating, the suspensions were magnetically stirred in the dark for 40 min to establish adsorption-desorption equilibrium. A 250 W mercury lamp and a 500 W Halogen lamp with a 420 nm cut-off filter were used as the UV and visible light sources, respectively. After given irradiation time intervals, aliquots of the mixed solution were collected and centrifuged to remove the catalyst particulates for analysis. Four consecutive cycles were tested. The samples were washed thoroughly with water and dried after each cycle.
Using terephthalic acid as a probe molecule, the hydroxyl radicals (•OH) at the photo-illuminated sample/water interface were examined by a special photoluminescence (PL) technique. Terephthalic acid reacts readily with •OH, producing 2-hydroxyterephthalic acid, a great fluorescent material with a unique photoluminescence peak at 426 nm [20], which makes it easy to detect by fluorescence spectrum (excitation wavelength: 315 nm, fluorescence peak: 426 nm). In a typical experiment, 80 mL 0.5 mM terephthalic acid and 2 mM NaOH aqueous solution were completely mixed and then transferred into a 100 mL beaker. The rest of steps are the same as for the degradation of MO.

3. Results and Discussion

Figure 1a illustrates the XRD pattern of the TiO2/Sn3O4 nanocomposites. All the diffraction peaks can be indexed to the anatase TiO2 (JCPDS 21-1272, marked with black •) and triclinic Sn3O4 (JCPDS 16-0737, marked with red ☆), validating the high purity of the synthesized TiO2/Sn3O4 composite phase. Compared with the single-phase TiO2 and Sn3O4, all the diffraction peaks are broad and weak, which indicates that the crystallinities are slightly reduced [21]. This result may be attributed to lattice distortion induced by interfacial strain because of different lattice parameters between Sn3O4 and TiO2 [22]. In addition, Raman spectroscopy results (Figure 1b) further confirm the purity of the synthesized TiO2/Sn3O4 composite phase. Specifically, Raman activities of 144, 196, 396, 520 and 638 cm−1 were assigned to the anatase TiO2 [23], and the 133, 143, 170 and 238 cm−1 Raman peaks could be attributed to the Sn3O4, in accordance with previous reports [7,24]. The textures of the as-synthesized TiO2/Sn3O4, TiO2, Sn3O4 and P25 were characterized by N2 physisorption experiments. The Brunauer–Emmett–Teller (BET) surface area data of samples are provided in Table 1. The N2 adsorption-desorption isotherm and pore-size distribution of TiO2/Sn3O4 nanocomposites are shown in Figure 1c. The results display that the TiO2/Sn3O4 nanocomposites possess an average pore diameter of 2.733 nm and a larger surface area of 68.1 m2/g than the as-prepared TiO2 (0.04 m2/g), Sn3O4 (35.2 m2/g), and the reported TiO2/Sn3O4 nanobelt heterostructure (51.5 m2/g) [9]. Such a high surface-to-volume ratio for the TiO2/Sn3O4 nanocomposites might be of extreme good value in photocatalytic processes, as they would provide more active sites for the adsorption of reactant molecules, and their optical absorbance would increase at visible wavelengths [25]. Figure 1d shows the UV-visible diffusion reflectance spectra (DRS) and plots of (F(R)hv)1/2 versus photo energy (hv) of the TiO2/Sn3O4 along with spectra of the pristine TiO2 and Sn3O4 for comparison. The absorption spectra of the TiO2/Sn3O4 nanocomposites exhibit the mixed absorption properties of both the components. In particular, the absorption edge for TiO2/Sn3O4 nanocomposites is clearly shifted towards visible region (near 505 nm). The optical band gap determined from the plot of the Kubelka-Munk function was found to be 2.46 eV, compared to the observed values of 3.22 eV and 2.61 eV for TiO2 and Sn3O4, respectively. These data reveal the Sn3O4/TiO2 nanocomposites have a lower band gap than the pure Sn3O4 and TiO2 nanoparticles, which is consistent with the published literature [9], and can be explained by the reduced crystallinity of both materials [1], as shown by XRD analysis.
The chemical composition and valence state were characterized by X-ray photoelectron spectroscopy (XPS). The full range of XPS spectra, ranging from 0 to 1000 eV, of TiO2/Sn3O4 nanocomposites are shown in Figure 2a. No impurities were observed in the spectra, which is consistent with the results of XRD and Raman. Figure 2b shows the curve fitting data of the Sn 3d core-level spectra. Moreover, the Sn 3d doublet characterized by Sn 3d3/2–Sn 3d5/2 splitting peak can be clearly observed. The prominent peak of Sn 3d5/2 level is dissolved into two peaks centered at 486.77 and 486.15 eV, which can be attributed to Sn(IV) and Sn(II) configurations [26], respectively. The Sn 3d3/2 spectra exhibit two peaks at 495.14 and 494.51 eV, which are assigned to Sn2+ and Sn4+ [26]. As shown in Figure 2c, the binding energies (BE) of Ti 2p3/2 and Ti 2p1/2 are 458.5 and 464.2 eV respectively, which are ascribed to the Ti4+ oxidation states [27]. On the basis of the above discussion, it can be concluded that the TiO2/Sn3O4 sample is composed of Ti(IV), Sn(II and IV), and O, which is in good agreement with the XRD and Raman results. In addition, the calculated Ti/Sn ratio is 0.20, indicating that most of the surface of the TiO2 nanocrystals is covered by Sn3O4 nanocrystals (SEM and TEM experiments further confirm this result, and will be discussed later).
The morphology and microstructure of the as-prepared TiO2/Sn3O4 nanocomposites were carefully analyzed by microscopy. Generally, lots of interleaved Sn3O4 nanoplates are able to self-assemble into an ordinary flower-like nanostructure, as shown in Figure 3a. Similarly, by introducing highly monodispersed TiO2 nanospheres of ~130 nm diameter (Figure 3b) into the growth environment, the homogeneous Sn3O4 nanoparticles started to grow on the surface of each individual TiO2 core with intimate contact, thus forming an interface of two different semiconductors that would facilitate photo-excited electron transfer and photon-generated carrier separation (Figure 3c,d). Noticeably, the composites inherit a favorable dispersion in the solution, and fully contact with the absorbate, which is positive for the outstanding photocatalytic performance. However, it should be noted that the TiO2/Sn3O4 nanocomposites are not completely covered by Sn3O4 nanocrystals. Furthermore, these advantageous heterojunctions with 10–20 nm sizes wrapping uniformly onto the surface of TiO2 nanospheres were verified by TEM observation, as shown in Figure 1e,f. The well-resolved lattice fringes from the core and shell regions manifestly correspond to the (101) planes of anatase TiO2 and the ( 2 ¯ 10) planes of Sn3O4, respectively, clearly revealing the phase distribution of the TiO2/Sn3O4 nanocomposites again.
The photocatalytic activities of P25, TiO2, Sn3O4 and TiO2/Sn3O4 heterostructures were evaluated by the degradation of MO in water under UV- and visible-light irradiation (Figure 4a,b).The degradation of the MO solution under identical experimental conditions, but with no photocatalyst, is provided for comparison. The degradation efficiency of the as-synthesized TiO2/Sn3O4 heterostructures was defined as C/C0, where C0 is the initial concentration of MO after equilibrium adsorption, and C is the concentration during the reaction. Both blank experiment results showed that MO could not be decomposed without photocatalyst under UV- or visible-light irradiation. In contrast, the photodegradation efficiency of TiO2/Sn3O4 nanocomposites was 95% within 30 min under UV-light irradiation, which is superior to the as-prepared TiO2 nanospheres (62%) and Sn3O4 nanoplates (70%). Furthermore, the MO decomposition efficiency found for the TiO2/Sn3O4 photocatalyst was comparable to that determined under the same experimental conditions for the reference P25 catalyst; that is, 99% after 10 min. Additionally, in the visible-light irradiation experiment (Figure 4b), the TiO2/Sn3O4 nanocomposites (81%) exhibited significantly higher photocatalytic activity than P25 (9%), TiO2 (0.4%) and Sn3O4 (60%) at 30 min. Finally, MO was completely degraded within 80 min. The results show that the TiO2/Sn3O4 heterostructures exhibited improved photocatalytic activity.
For a better understanding, the photocatalytic kinetics of the samples was analyzed using the Langmuir-Hinshelwood model, as shown in Figure 4c,d. All of the data follow a first-order reaction model, and the calculated apparent kinetic rate constants (κ) are summarized in Table 1. We found that, under UV irradiation, the TiO2/Sn3O4 exhibited a much faster photo-decomposition activity (κ = 0.24 min−1) than the pure TiO2 (0.028 min−1) and Sn3O4 (0.064 min−1), and was as fast as the P25 (0.24 min−1). Furthermore, under visible-light irradiation, the calculated value of κ for the TiO2/Sn3O4 sample (κ = 0.052 min−1) was twice as high as that for the neat Sn3O4 (0.024 min−1), and more than a dozen times higher than that for the single TiO2 (0.0010 min−1) and the P25 (κ = 0.0023 min−1). In addition, the TiO2/Sn3O4 heterostructures could be recycled and reused at least four times without significant loss of efficiency (Figure 4e,f), which demonstrates its great potential as an efficient and stable photocatalytic material. These remarkably good performances can be attributed to the improved UV- and visible-light absorption efficiency, and the high photo-excited carrier-separation rate resulting from the novel TiO2/Sn3O 4 heterostructures.
Based on all of the results above, a possible mechanism for charge transfer and photocatalytic process can be proposed (Scheme 1). As illustrated in Figure 1d, the diffusion reflectance spectra (DRS) and plots of (F(R)hv)1/2 versus photo energy (hv) indicate that the bandgap of Sn3O4 (2.61 eV) is smaller than that of TiO2 (3.22 eV). Additionally, the potentials of the valence band (VB) and conduction band (CB) of Sn3O4 are higher than those of TiO2, so the heterostructure of TiO2/Sn3O4 belongs to typical type-II heterojunction [9]. When Sn3O4 contacts TiO2 cores to form a heterojunction, the difference in chemical potential causes band bending at the interface of the junction [28], which drives photoexcited electrons to transfer from Sn3O4 to TiO2, and photoexcited holes to migrate in the opposite direction, until the Fermi levels of TiO2 and Sn3O4 reach equilibrium. The possible mechanisms for charge transfer and hydroxyl radical (•OH) generation under UV- and visible-light irradiation will be discussed separately. (1) Upon UV illumination, electrons in the VB could be excited to the CB of both oxides, simultaneously forming the same number of holes in the VB. This is due to the fact that the Sn3O4 nanoparticles were not fully coated as a shell onto the TiO2 nanospheres (Figure 3c,d) and that the suitable bandgap of TiO2 and Sn3O4 is lower than the energy of ultraviolet photons. Next, the photo-generated electrons were collected by the TiO2 particles and the holes by the Sn3O4 particles; that is, electrons transferred from Sn3O4 to TiO2, and holes migrated from TiO2 to Sn3O4 (compare Scheme 1a with Figure 1d). The unique behavior that electrons and holes preferentially accumulate on different materials would result in a great separation of photo-generated carriers, and thus reduce the charge recombination rate, ultimately increasing carrier lifetime. As a consequence, the formation efficiency of hydroxyl radicals (•OH)—a strong oxidant for most pollutants [9,20,29]—by the reaction of holes with surface hydroxyl groups or physisorbed water molecules at the Sn3O4 surface and the production rates of •OH and superoxide radicals (O2) radicals resulting from the reactions of electrons with dissolved oxygen molecules and water molecules will be massively enhanced; this will increase the volume of oxidant inside the system. (2) Under visible-light irradiation (Scheme 1b), electrons in the VB could be exclusively excited to the CB of Sn3O4, with a concomitant formation of the same number of holes in the VB. Due to the type II band alignment of the as-prepared sample, the photoexcited electrons in the Sn3O4 CB will be easily injected into the TiO2 CB, where the electrons could reduce surface-absorbed O2 over TiO2 active sites to form superoxide radicals (O2), and the new species can further yield •OH by reacting with water or oxidize MO. On the other hand, holes remaining in Sn3O4 could react with surface-absorbed H2O to generate more •OH. Hydroxyl radicals (•OH) and superoxide radicals (O2) stemming from the above procedure will degrade MO into colorless chemicals, and even CO2 and H2O, which is similar under UV illumination. All in all, the enhanced charge separation related to the TiO2/Sn3O4 heterojunction favors the interfacial charge transfer to physisorbed species, forming •OH radicals and reducing possible back reactions, and therefore accounts for the higher activity of the TiO2/Sn3O4 nanocomposites.
The photocatalytic oxidation of dyes occurs through the reactive species, which came into being after the light absorption and electron-hole formation by the photocatalyst [30]. Terephthalic acid photoluminescence probing technique (TAPL) was employed to examine the generation of active •OH radicals [31]. Figure 5a,b gives the •OH-trapping photoluminescent spectra of TiO2/Sn3O4 nanocomposites in TA solution with UV- and visible-light irradiation, respectively. The increased photoluminescence intensity confirms that the •OH radicals are mainly responsible for the photodegradation process, and it also verifies the photocatalytic activity of the TiO2/Sn3O4 nanocomposites.

4. Conclusions

In summary, this study demonstrates a facile route to synthesizing TiO2/Sn3O4 nanocomposites that not only display enhanced photocatalytic performance in UV irradiation, but also allow a significant level of visible light photocatalytic activity. The large surface area derived from the monodispersed mesopore TiO2/Sn3O4 nanospheres and unique TiO2/Sn3O4 heterojunctions are considered to be major contributions to supplying abundant active sites and separating photogenerated carriers, respectively. The strengthened photocatalytic performances will greatly promote the practical application of the TiO2/Sn3O4 nanocomposites in eliminating organic pollutants from wastewater, and producing hydrogen by splitting.

Acknowledgments

We appreciate the financial support of the National Natural Science Foundation of China (Grant No. 61373072).

Author Contributions

F.W. and J.H. conceived and designed the experiments; J.H. and J.T. performed the experiments; J.H., J.T., X.L. and Z.W. analyzed the data; F.W., Q.L. and Y.L. contributed reagents/materials/analysis tools; J.H., J.T. and. X.L. wrote the paper with input from all authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) XRD patterns, (b) Raman spectra of TiO2, Sn3O4 and TiO2/Sn3O4 nanocomposites, (c) N2 adsorption-desorption isotherms of TiO2/Sn3O4 nanocomposites and (d) UV-visible diffuse reflectance spectra and plots of (F(R)hv)1/2 versus photo energy (right insert) of TiO2, Sn3O4 and TiO2/Sn3O4 nanocomposites.
Figure 1. (a) XRD patterns, (b) Raman spectra of TiO2, Sn3O4 and TiO2/Sn3O4 nanocomposites, (c) N2 adsorption-desorption isotherms of TiO2/Sn3O4 nanocomposites and (d) UV-visible diffuse reflectance spectra and plots of (F(R)hv)1/2 versus photo energy (right insert) of TiO2, Sn3O4 and TiO2/Sn3O4 nanocomposites.
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Figure 2. (a) Survey scan of XPS, (b) Sn 3d core level XPS spectra, and (c) Ti 2p core level XPS spectra of the TiO2/Sn3O4 nanocomposites.
Figure 2. (a) Survey scan of XPS, (b) Sn 3d core level XPS spectra, and (c) Ti 2p core level XPS spectra of the TiO2/Sn3O4 nanocomposites.
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Figure 3. SEM images of the (a) Sn3O4, (b) TiO2, and (c,d) TiO2/Sn3O4 nanocomposites; (e) TEM and (f) HRTEM images of the TiO2/Sn3O4 nanocomposites.
Figure 3. SEM images of the (a) Sn3O4, (b) TiO2, and (c,d) TiO2/Sn3O4 nanocomposites; (e) TEM and (f) HRTEM images of the TiO2/Sn3O4 nanocomposites.
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Figure 4. The photocatalytic activity (a,b), plots of -ln[C/C0] versus irradiation time (c,d), and stability for MO photo-degradation (e,f) of TiO2, P25, Sn3O4 and TiO2/Sn3O4 nanocomposites under UV- and visible-light irradiation, respectively. The corresponding curves of MO without photocatalyst under UV and visible light irradiation are provided for comparison.
Figure 4. The photocatalytic activity (a,b), plots of -ln[C/C0] versus irradiation time (c,d), and stability for MO photo-degradation (e,f) of TiO2, P25, Sn3O4 and TiO2/Sn3O4 nanocomposites under UV- and visible-light irradiation, respectively. The corresponding curves of MO without photocatalyst under UV and visible light irradiation are provided for comparison.
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Scheme 1. Illustration of photo-induced charge transfer and separation at the interface of TiO2/Sn3O4 hierarchical hybrid nanostructures under (a) UV- and (b) visible-light irradiation.
Scheme 1. Illustration of photo-induced charge transfer and separation at the interface of TiO2/Sn3O4 hierarchical hybrid nanostructures under (a) UV- and (b) visible-light irradiation.
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Figure 5. The •OH-trapping photoluminescence spectra of TiO2/Sn3O4 nanocomposites under (a) UV- and (b) visible-light irradiation, respectively.
Figure 5. The •OH-trapping photoluminescence spectra of TiO2/Sn3O4 nanocomposites under (a) UV- and (b) visible-light irradiation, respectively.
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Table 1. The specific surface area and apparent reaction rate constants (κ) of TiO2, P25, Sn3O4 TiO2/Sn3O4 samples.
Table 1. The specific surface area and apparent reaction rate constants (κ) of TiO2, P25, Sn3O4 TiO2/Sn3O4 samples.
PhotocatalystTiO2P25Sn3O4TiO2/Sn3O4
κ (min−1)κ (min−1)κ (min−1)κ (min−1)
UV irradiation0.0280.240.0640.24
Visible light0.00100.00230.0240.052
Surface Area (m2∙g−1)0.045035.268.1
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