Synthesis, Characterization, and Photocatalytic Properties of SnO2

Article pubs.acs.org/JPCC

Synthesis, Characterization, and Photocatalytic Properties of SnO2/ Rutile TiO2/Anatase TiO2 Heterojunctions Modified by Pt Weirong Zhao,* Meng Zhang, Zhuyu Ai, Yanan Yang, Haiping Xi, Qiaomeng Shi, Xinhua Xu, and Huixiang Shi Department of Environmental Engineering, Zhejiang University, 866 Yu Hang Tang Road, Hangzhou 310058, China S Supporting Information *

ABSTRACT: To improve the separation rate of photogenerated electrons and holes, a SnO2/rutile TiO2 (RTiO2)/anatase TiO2 (A-TiO2) photocatalyst modified by Pt nanoparticles with three pairs of heterojunctions was fabricated by a facile hydrothermal method. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) clearly illustrated the structure of the three pairs of heterojunctions connected to each other. Heteronanostructure photocatalysts with increased specific surface area could offer more active sites when contacting pollutants, resulting in improved photocatalytic activity. The red shift in UV−vis diffuse reflectance spectra (DRS) indicated the utilization of visible-light. Photoluminescence (PL) and photoelectrochemical (PEC) measurements suggested the enhancement of electron and hole separation, in accordance with the results obtained for the photocatalytic oxidation of decomposing toluene over 4 h. A catalyst containing 1 wt % Pt/10 at. % SnO2/R-TiO2/A-TiO2 exhibited the best photocatalytic mineralization rates of toluene: 40.9% and 72.3% under visible-light and UV-light irradiation, respectively. A proposed mechanism was elaborated to reveal the effective photocatalytic progress of charge transfer along multiple pathways along the three pairs of heterojunctions doped with Pt.

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INTRODUCTION Photocatalysis applications for addressing environmental issues such as air and water pollution and energy crises have attracted considerable attention in recent years.1−8 Conventional homogeneous photocatalysts have inherent drawbacks such as the easy recombination of photoexcited electron−hole pairs and the absorption of light only at ultraviolet (UV) wavelengths (λ < 380 nm). Developing heterogeneous photocatalysts by surface modification has been an effective strategy for charge carrier transfer in enhancing the separation of photogenerated electrons and holes. A wide range of heterogeneous photocatalysts, including metal/semiconductor (Au/TiO2,8 Pt/ TiO29,10), semiconductor/semiconductor (CdS/TiO2,1 SnO2/ TiO211), molecule/semiconductor,12 and multiheteronanostructure systems (SnO2/Pd/TiO2,13 CdS/Pt/TiO214), have been successfully explored for promoting charge separation and transportation. TiO2-based multiheteronanostructure photocatalysts have triggered research interest because of their unique photocatalytic activity and high chemical stability.2 Among the multiheteronanostructure systems that have been developed, SnO2 has attracted much interest due to the similarity between its tetragonal structure and lattice parameters compared with those of TiO2.3,15 In the heterojunction between TiO2 and SnO2, photogenerated electrons transfer to the conduction band (CB) of SnO2 and holes move in the opposite direction, which can prevent the recombination of electrons and holes. Furthermore, doping photocatalysts with Pt can enhance the number of surface sites and assist the © XXXX American Chemical Society

separation of photogenerated electrons and holes due to the Schottky barrier formed at the Pt-semiconductor interface.9,16 Lee et al. prepared anatase TiO2 (A-TiO2)/rutile TiO2 (RTiO2)/SnO2 nanofibers containing two pairs of heterojunctions showing highly efficient photocatalytic H2 evolution.4 Recently, Chang et al. synthesized A-TiO2/SnO2/Pd and R-TiO2/SnO2/ Pd nanoparticle heterostuctures that exhibited higher efficiency in the degradation of methylene blue than pristine TiO2 nanotubes.13 A combined mechanism involving two electron−hole transfer pathways, such as by charge carrier transfer in heterojunctions and cocatalysts, has been accordingly proposed. However, to the best of our knowledge, there is no literature focusing on the preparation of multiheteronanostructure systems other than two pairs of heterojunctions to promote photocatalytic activity further. In this study, P25 and SnCl2, both easily obtained materials, were used as precursors to synthesize three pairs of heterojunction photocatalysts composed of A-TiO2, R-TiO2, and SnO2 nanoparticles via a facile hydrothermal method. The photocatalysts were doped with metallic Pt nanoparticles to further enhance their photocatalytic performance under UV and visible-light irradiation. The simple synthesis process can be scaled up for commercialization easily. A series of characterization techniques were employed to determine the Received: June 30, 2014 Revised: September 13, 2014

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dx.doi.org/10.1021/jp506495a | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Scheme of formation of Pt/SnO2/R-TiO2/A-TiO2 photocatalyst.

of formation of the Pt/SnO2/R-TiO2/A-TiO2 photocatalyst is shown in Figure 1. Characterization. The crystal phases of the photocatalysts were analyzed by X-ray diffraction (XRD) with Cu Kα radiation (model D/max RA, Rigaku, Japan) over the 2θ range 15−80°. The accelerating voltage and the applied current were set to 40 kV and 150 mA, respectively. The morphological and lattice structure of the photocatalysts were investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEM-2010, Jeol, Japan) at an accelerating voltage of 200 kV. The chemical status and the composition of the photocatalysts’ surfaces were investigated by X-ray photoelectron spectroscopy (XPS) using an Al X-ray source (Al Kα150 W, hv = 1486.6 eV, Thermo ESCALAB 250). All binding energies were calibrated by using contaminant carbon (C 1s = 284.8 eV) as a reference. The Brunauer−Emmett−Teller (BET) surface areas of the photocatalysts were measured with an automatic analyzer (Autosorb-iQ-MP, Quantachrome) using nitrogen as the adsorbate, and the pore size distributions were determined by using desorption curves and the Barrett− Joyner−Halenda (BJH) method. UV−vis diffuse reflectance spectra (DRS) were measured using a UV−vis spectrophotometer (TU-1901, Pgeneral, China) equipped with an integrating sphere at room temperature. Photoluminescence (PL) emission spectra were measured at room temperature with a fluorescence spectrophotometer (Fluorolog-3-Tau, France) using a Xe lamp as the excitation source. The timeresolved PL decay (TRPL) spectroscopy was detected by a fluorescence spectrometer (FLSP920, England) with the excitation wavelength of 405 nm and the measurement wavelength of 520 nm. The incident visible and UV-light intensities were detected by a radiometer (FZ-A, Handy, China) and a UV light meter (ST-512, Sentry, China), respectively. All photoelectrochemical measurements (linear sweep voltammograms (LSVs), Tafel analysis, and electrochemical impedance spectra (EIS)) were carried out with an electrochemical workstation (CH Instruments 650D, Shanghai, China) in a standard three-electrode quartz cell as illustrated in our previous work.20 Photocatalytic Activity Tests. The photocatalytic oxidation of toluene was conducted in a 1.2-L top-irradiation quartz photoreactor with a 12.5 cm-diameter dish, on which 0.1 g assynthesized photocatalyst was homogeneously dispersed with ethanol. Before toluene was injected, ultrapure CO2 free air was

properties of the photocatalysts and further illustrate the catalysts’ charge-carrier transfer behavior. The influence of three pairs of heterojunctions on promoting photocatalytic performance was discussed with the combination of the timeresolved PL (TRPL) decay curves and the photoelectrochemical (PEC) characterization. The results confirmed that the introduction of multiple pathways of electron transfer played an important role in suppressing the recombination of photoelectrons and holes in the multiheteronanostructure photocatalysts system.

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EXPERIMENTAL METHODS Preparation of Titania Nanotube Photocatalysts. Titania nanotubes (TNT) were prepared via a hydrothermal method as described in previous reports.17,18 Briefly, 70 mL of a 10 M NaOH solution containing 1.0 g of P25 powder was magnetically stirred for 2 h. Then, the mixture was submitted to thermal treatment at 150 °C for 24 h. After cooling, the obtained sample was filtered, washed with distilled water several times until the rinsing solution was neutral, and dried at 80 °C for 5 h; the sample was denoted as S0. Preparation of SnO2/R-TiO2/A-TiO2 Photocatalysts. In a typical SnO2/R-TiO2/A-TiO2 photocatalyst synthesis procedure,19 0.5 g TNT powder and the desired amount of SnCl2 were added to a 0.1 M HCl solution followed by stirring for 48 h. The mixture was then dispersed in 40 mL of distilled water under ultrasonication for 0.5 h. After filtration, the precipitate was washed thoroughly, dried at 80 °C for 5 h, and calcined at 300 °C for 1 h. Sn4+ existed on the surface of TNT and its laminated layers, respectively. SnO2/R-TiO2/A-TiO2 photocatalysts containing Sn/Ti ratios of 2 at. %, 6 at. %, 10 at. %, and 15 at. % were designated as S1, S2, S3, and S4. Preparation of Pt-Doped Photocatalysts. SnO2/RTiO2/A-TiO2 photocatalysts synthesized as indicated above without calcinations were added to a certain amount of H2PtCl6·6H2O solution and then magnetically stirred for 48 h. In this step, Sn2+ acted as the reductant to reduce Pt4+ on-site. Then, 10 mL of 0.75 M HCOONa solution was added to the mixture for the sake of further facilitating Pt4+ reduction and then stirred for 1 h. A black precipitate was obtained by washing with distilled water, drying at 80 °C for 5 h, and calcining at 300 °C for 1 h. S3 doped with Pt (1 wt %) was labeled as P1 (S3 exhibited the best photocatalytic activity among the SnO2/R-TiO2/A-TiO2 photocatalysts). The scheme B



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(215) planes of A-TiO2 (JCPDS card 21-1272), respectively. For S3, S4, and P1, the characteristic diffraction peaks of the phases of R-TiO2 and SnO2 appear in the XRD patterns. Four main diffraction peaks at 2θ values of 27.41°, 36.12°, 41.24°, and 54.26° can be observed, corresponding to the (110), (101), (111), and (211) planes of R-TiO2 (JCPDS card 21-1276), respectively. Furthermore, the 2θ values of 26.61°, 38.97°, and 51.78° are identified as cassiterite SnO2 peaks associated with the (110), (111), and (211) planes (JCPDS card 41-1445), suggesting the formation of crystalline SnO2. The results clearly show that the peak intensities of A-TiO2 decrease with an increase in SnO2 content. Also, the peak intensities of R-TiO2 become increasingly intense. It can be ascribed to the rather similar lattice constants between SnO2 and R-TiO2 and the lattice distortion induced by the substitution of Sn ions for Ti.7,13 However, no typical peaks of R-TiO2 and SnO2 can be observed in S1 and S2 due to the low content of these components. There is no distinct change in the diffraction peak positions of TiO2 and SnO2, indicating that the loaded Pt metal did not affect the crystalline structures. Because the content of Pt in P1 is low (1 wt %), no diffraction peaks corresponding to Pt can be identified. TEM analysis afforded detailed information regarding the nanostructure of the photocatalysts. Figure 3a shows the TEM image obtained for S0, which was composed of nanotubes measuring several hundred nanometers in length, with an outer diameter of 8−12 nm and an inner diameter of 5−7 nm. Figure 3b shows an HRTEM image of the detailed structure of P1. It is clearly shown that A-TiO2, R-TiO2, and SnO2 were closely connected to each other, forming three well-mixed pairs of heterojunctions. The interplanar distances of 0.238 and 0.325 nm correspond to the lattice spacings of the A-TiO2 (004) plane and R-TiO2 (110) plane, respectively. The observed

purged into the reactor for 30 min to remove CO2. Then, saturated toluene vapor was injected into the reactor. The gas in the reactor was analyzed every 30 min over a typical run time of 4 h by a gas chromatograph/flame ionization detector (GC/ FID, FULI 9790, China) after absorption equilibrium.

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RESULTS AND DISCUSSION Figure 2 presents the XRD patterns of S0, S1, S2, S3, S4, and P1. The anatase phase of TiO2 is observed for all photo-

Figure 2. XRD patterns of S0, S1, S2, S3, S4, and P1. Asterisks, hollow triangles, and rhombi indicate A-TiO2, R-TiO2, and SnO2, respectively.

catalysts. Briefly, the diffraction peaks at 2θ of 25.20°, 37.76°, 48.00°, 53.76°, 54.96°, 62.69°, 68.72°, and 75.03° correspond to the (101), (004), (200), (105), (211), (204), (116), and

Figure 3. (a) TEM image of S0, (b, c) HRTEM images of the heterostructures and Pt particles in P1, and (d) EDX spectrum of P1. C



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spacing of 0.335 nm is indicative of the SnO2 (110) facet. Pt nanoparticles are shown in Figure 3c. Energy-dispersive X-ray (EDX) spectroscopy verified the existence of Ti, O, Sn, and Pt in P1, as shown in Figure 3d. XPS was used to analyze the chemical nature and composition of surface elements in the photocatalysts. Figure 4a shows the fully scanned spectra of S0, S3, and P1. The C 1s peak is located at 284.8 eV, which was induced by the adventitious carbon-based contaminant. Only Ti 2p and O 1s peaks are detected in S0. The spectrum of S3 shows the typical peaks of Ti, O, and Sn. Related emissions of Ti, O, Sn, and Pt are clearly observed for P1. Figure 4b shows the high-resolution XPS spectra of Ti 2p in the three photocatalysts. The peaks located at 464.2 and 458.5 eV correspond to Ti 2p1/2 and Ti 2p3/2, respectively, which indicates the normal state of Ti4+ in S0, S3, and P1.11,21−23 Figure 4c displays the Sn 3d XPS spectra of S3 and P1. As shown, the Sn 3d5/2 peak is located at 486.5 eV, and the peak located at 495.0 eV is assigned to Sn 3d3/2. The peak associated with the spin−orbit splitting between Sn 3d5/2 and Sn 3d3/2 occurs at 8.5 eV, which is a typical value for the state of Sn4+ in S3 and P1.11 As depicted in Figure 4d, the binding energy of the Pt 4f7/2 peak is shifted to 72.6 eV, which is slightly higher than the value of 71.3 eV observed for Pt0.24,25 The discrepancy can be attributed to the presence of a small amount of Pt(II) as an impurity in the photocatalyst.26 Figure 4e shows the O 1s XPS spectra of S0, S3, and P1 fitted to three different chemical states. The dominant peaks located at 529.8 eV can be ascribed to OTi−O, whereas the peaks centered at 531.1 eV likely arise from OSn−O.27 The peaks of S0, S3, and P1 centered at 532.2 eV are ascribed to surface hydroxyl groups, OH−O.28 The textural properties of the photocatalysts were studied by N2 sorption measurements. S0, S3, and P1 show similar isotherm patterns displaying type IV hysteresis (Supporting Information Figure S1), which is associated with slit-shaped pores or the space between parallel plates.29 The parameters of the curves (Supporting Information Table S1) show an increase in specific surface area in the order S0 < S3 < P1, demonstrating that the number of mesoporous structures existing in the photocatalysts increase the specific surface area.5 All photocatalysts show relatively narrow pore size distributions in the range 3−8 nm (inset of Supporting Information Figure S1), which demonstrates that the pores are uniform in size. S3 and P1 show larger pore volumes and pore diameters than S0, resulting in better adsorption of contaminants and more active sites. The pore diameter of P1 is smaller than that of S3, which is attributed to the coating of Pt on the TiO2 nanotubes. The UV−vis DRS of the photocatalysts are displayed in Supporting Information Figure S2. As shown, S0 only exhibits strong UV absorption due to a band-to-band transition.6 Compared to the spectrum of S0, the spectra of the photocatalysts present extended absorption into the visible region, which can be ascribed to the formation of R-TiO2 (the energy band gaps Eg = 3.0 eV),13 as confirmed by XRD and HRTEM characterizations. The narrower Eg values (Supporting Information Figure S2) can lead to better photocatalytic behavior.30 PL emission, which is the result of the recombination of free carriers, was employed to reveal the surface states of charge transfer and the recombination of charge carriers and sequentially to understand the fate of electron−hole pairs in semiconductors.31 The high PL intensity indicates a high recombination rate of photogenerated charge carriers, resulting

Figure 4. (a) XPS fully scanned spectra of S0, S3, and P1, and (b−e) high-resolution XPS spectra of Ti 2p for S0, S3, and P1; Sn 3d for S3 and P1; Pt 4f for P1; and O 1s for S0, S3, and P1. D



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in low photocatalytic activity.32 The intensities of the PL signals (Supporting Information Figure S3) follow the order S0 > S1 > S2 > S4 > S3 > P1. Obviously, the typical peak around 396 nm (3.13 eV) is observed in all the curves due to emission associated with the bandgap transition.31 The peak centered at around 520 nm of S0 can be ascribed to the irradiative transition from oxygen vacancies with one trapped electron to the valence band for A-TiO2.32 When coupling with SnO2, the PL intensities for S1 to S4 decrease dramatically, which can be ascribed to the charge carrier transfer among the heterojunctions of A-TiO2, R-TiO2, and SnO2; this transfer suppresses the direct recombination of charge carriers. The weakest intensity of P1 can be explained by the heterojunctions and Schottky barrier synergistically hindering the recombination of charge carriers.9,25 TRPL curves for the photocatalysts are shown in Supporting Information Figure S4. It infers that the PL decay lifetimes of these photocatalysts are in the order S0 < S1 < S2 < S4 < S3 < P1. Each decay curve is fitted into a triexponential equation. The three decay time values and relative proportions for each photocatalyst are shown in Supporting Information Table S2. The different values of decay time can be elucidated such that the smaller value is attributed to the radiative recombination, while the larger value is related to the trap emission.33 This result is in good agreement with the charge carrier trapping point of John T. Yates, Jr., and co-workers.34 The fast decay component (τ1) is prolonged to 312 ps (P1), which is 2.2 times that of the fastest one of S0 (139 ps). The slow decay components (τ2 and τ3) of S1, S2, S3, S4, and P1 with heterojunctions are longer than those of S0 (τ2 = 971 ps, τ3 = 3.89 ns). There are no apparent changes in relative proportion of three components for all of photocatalysts. Therefore, heterojunctions prolong the decay time and thus suppress the recombination of the electrons and holes, which is consistent with the photocatalytic performance. The photocurrent densities of S0, S3, and P1 were measured using LSVs to evaluate the charge separation capability of the heterojunctions.25 Figure 5 demonstrates the different effects of bias potentials on the photocurrent density under UV (Figure 5a) and visible-light (Figure 5b) irradiation. All of the corresponding curves of the photocatalysts show similar trends in both figures. The photocurrent density of S0 increases with the applied potential because the external electric field can promote the separation rate of photoelectrons and holes.35 However, it increases slowly in the range 0.25−0.35 V that can be attributed to the significant recombination of photogenerated electrons and holes simultaneously. Moreover, the photocurrent densities of S3 and P1 with heterojunctions under UV illumination increase gradually from 0.25 to 0.41 V until reaching their maximum, 0.45 and 0.47 mA/cm2, respectively, after which they begin to decline. The results obtained for the heterojunction photocatalysts indicate that electrons and holes are separated by the interior electric field of the space charge layer, which becomes thinner with the increase in the bias potential, resulting in weaker separation of electrons and holes.9 The photocurrent densities of S3 and P1 under visible-light irradiation increase up to 0.18 and 0.19 mA/cm2, respectively, which are lower than those of the corresponding photocatalysts under UV irradiation, after which they begin to decrease. The lower photocurrent values are ascribed to the fact that only RTiO2 can be irradiated under visible-light, thus inducing a lower extent of charge carrier transfer in the heterojunctions. Tafel analysis was performed to determine the current density values and further investigate the PEC properties of the

Figure 5. Photocurrent density versus potential plotted for S0, S3, and P1 electrodes in 0.1 M Na2SO4 solution at a scan rate of 100 mA/s under (a) UV and (b) visible-light irradiation.

photocatalysts. The corrosion current density (j), which is obtained by extrapolating the linear portions of the anodic and cathodic curves to the crossing point,20 and anodic slope follow the order P1 > S3 > S4 > S2 >S1 > S0 under UV and visiblelight irradiation (Supporting Information Figure S4). Compared with S0, the other photocatalysts with heterojunctions exhibit higher j values and anodic slopes, which are attributed to the fact that the heterojunction structure can accelerate charge separation and improve the transfer rate of electrons and holes. To further understand the effects of the improved performance of the electrochemical process on charge transport and recombination under irradiation, EIS was carried out (Figure 6). The impedance spectra of the photocatalysts were measured from 100 Hz to 1000 kHz under a UV-light intensity of 600 μW/cm2 (Figure 6a) and visible-light intensity of 40 mW/cm2 (Figure 6b). The suggested equivalent circuit model (Figure 6c) contains solution resistance (Rs), charge transfer resistance (Rct), double-layer capacitance (Cdl), and Warburg impedance (Zw) elements. The EIS parameters of the photocatalysts were determined using the ZSimpWin 3.20d program for data fitting and are summarized in Table 1. As shown, the EIS Nyquist plots are well-represented by high-frequency semicircles and low-frequency lines in both cases. All of the photocatalysts show semicircles of different diameters, which correspond to a kinetically controlled process attributed to Rct.36 The chemical capacitance at the hybrid interface is defined as Cdl. The Rct values of the photocatalysts increase, and the Cdl values decrease in the order of P1, S3, S4, S2, S1, and S0 under UV and visible-light irradiation. Compared with S0, the other photocatalysts exhibit much smaller Rct values and larger Cdl values, which may be related to the easier charge carrier transfer in the heterojunction structure and better photocatalytic performance. The Rs values, which can be associated with differences at the electrode/electrolyte interface,37 vary from E



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Information Figure S6. The frequency peak at the highfrequency region can be ascribed to the charge transfer at the surface of photocatalysts in the electrolyte solution. The lifetime (τ) of the electrons (shown in the inset table of Supporting Information Figure S6) can be calculated by the expression of τ = 1/2πf, where f is the maximum frequency of the line. The trend of τ under UV radiation is the same as that under visible-light irradiation, i.e., S0 < S1 < S2 < S4 < S3 < P1. For UV radiation, τ increases from 0.61 μs (S0) to 1.6 μs (P1), which is 2.6 times of that of S0. It can be seen that the values of τ under visible-light irradiation are a little lower than those of UV radiation for the same photocatalyst. The largest value (1.1 μs for P1) is 2.2 times that of the smallest one (0.50 μs for S0) under visible-light irradiation. The longer lifetime of electrons may be in favor of photocatalytic reaction, which is in good accordance with the result in the next part. The increased ability to separate photoexcited electrons and holes confirmed by PEC analysis benefits from the heterojunction and Schottky barrier of the photocatalysts. We evaluated the photocatalytic activity through toluene degradation conducted under UV and visible-light irradiation in a quartz reactor, the results of which are shown in Figure 7. A control experiment was performed without any photocatalysts; the results show that approximately 3% CO2 was produced by direct photolysis after 4 h of UV irradiation (Figure 7a), which demonstrates that the photocatalysis is the primary process. The CO2 evolution is clearly observed to increase in the order S0 < S1 < S2 < S3 (corresponding to 20.43, 41.44, 53.63, and 113.81 ppm/h, respectively). The elevated CO2 evolution rates are attributed to the heterojunction structure of SnO2/R-TiO2/ A-TiO2 in S1, S2, and S3. At 15 at. % Sn/Ti, the CO2 evolution rate decreases to 76.53 ppm/h, which can be attributed to the reduced number of active sites inducing lower photocatalytic efficiency mainly caused by two reasons: one is extra SnO2 on the surface of photocatalyst covering the sites for photon absorption;11,13 the other is the bigger Eg resulting in less light absorption, which leads to fewer photoelectrons and holes and subsequently fewer active sites for catalysis. P1 can further improve the photocatalysis efficiency and decompose toluene to CO2 at a rate of 120.15 ppm/h by inducing the separation of photogenerated charge carriers due to the effect of the Schottky barrier. The results demonstrate that 72.3% toluene is decomposed to CO2 (eq 1) after 4 h of UV irradiation. The CO2 evolution rate under visible-light irradiation (Figure 7b) increases in the order P1 > S3 > S4 > S2 > S1 > S0, in accordance with the order observed under UV radiation. However, the concentration of CO2 is lower than that under UV irradiation, which can be attributed to two factors: a high

Figure 6. EIS Nyquist plots of photocatalysts in 0.1 M Na2SO4 solution obtained at an applied voltage between −0.3 and 0.3 V under (a) UV and (b) visible-light irradiation and (c) the suggested equivalent circuit model. Rs, Rct, Cdl, and Zw denote solution resistance, charge transfer resistance, double-layer capacitance, and Warburg impedance elements, respectively.

9.23 to 3.64 Ω/cm2. The line segment corresponds to a diffusion-limited process related to Zw.36 There is no apparent difference among the Zw values of the photocatalysts under the two conditions. The lower Rct values and higher Cdl values of these photocatalysts under UV radiation compared to those obtained under visible-light irradiation indicate that the photocatalysts with heterojunctions exhibit stronger charge carrier transfer under UV radiation, as expected. Bode phase plots of the photocatalysts are shown in Supporting

Table 1. Model Parameters of the Photocatalysts Based on the EIS Results S0

S1

Rs (Ω/cm2) Rct (Ω/cm2) Cdl × 108 (F/cm2) Zw × 104 (S0.5/Ω cm2)

9.23 103 3.59 5.56

± ± ± ±

0.50 0.80 0.052 0.18

4.97 63.7 4.52 5.61

± ± ± ±

0.49 1.10 0.13 0.34

Rs (Ω/cm2) Rct (Ω/cm2) Cdl × 108 (F/cm2) Zw × 104 (S0.5/Ω cm2)

8.57 102 3.57 5.63

± ± ± ±

0.70 0.96 0.068 0.20

5.17 57.5 4.09 4.20

± ± ± ±

0.27 0.75 0.062 0.54

S2 Ultraviolet 4.63 50.3 4.86 9.54 Visible 4.90 45.7 5.23 4.87 F

S3

S4

P1

± ± ± ±

0.38 0.84 0.15 0.69

3.77 31.5 8.97 15.3

± ± ± ±

0.15 0.63 0.24 1.1

4.36 36.1 8.79 10.2

± ± ± ±

0.35 0.80 0.29 0.66

3.64 31.4 10.9 12.4

± ± ± ±

0.22 0.60 0.32 0.71

± ± ± ±

0.18 0.82 0.15 0.17

3.77 37.6 7.03 5.84

± ± ± ±

0.35 0.76 0.15 0.59

4.29 40.5 6.66 5.49

± ± ± ±

0.33 0.88 0.20 0.24

3.69 34.8 8.71 6.95

± ± ± ±

0.35 0.67 0.29 0.28



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Figure 7. Photocatalytic activities of photocatalysts for CO 2 production mineralized in toluene under (a) UV and (b) visiblelight illumination, respectively. Figure 8. Proposed photocatalytic mechanisms under (a) UV and (b) visible-light irradiation. The Pt deposited on A-TiO2 (UV light condition) and R-TiO2 (visible-light condition) is representative of Pt on semiconductors.

quantum yield under UV irradiation and fewer photogenerated charge carrier transfer routes in the SnO2/R-TiO2 heterojunction under visible-light irradiation.38 % mineralization =

[CO2 ]produced 7[C7H8]converted

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× 100

Figure 8 proposes possible mechanisms for multiple pathways of electron transfer under UV and visible-light irradiation. The figure depicts the valence band (VB) and CB in A-TiO2 (Eg = 3.2 eV), R-TiO2 (Eg = 3.0 eV), and SnO2 (Eg = 3.5 eV).2 The three pairs of heterojunctions in the photocatalysts are formed contacting each other (Figure 8a). The semiconductors are irradiated to generate photoelectrons and holes (eq 2). Under UV irradiation, the photoinduced electrons can be transferred by multiple pathways: from A-TiO2 to RTiO2, A-TiO2 to SnO2, R-TiO2 to SnO2, or the semiconductors to Pt, which were confirmed by the work of peers,39 and holes move from A-TiO2 to R-TiO2, SnO2 to R-TiO2, and SnO2 to ATiO2. However, the charge carriers can be only transferred from R-TiO2 to SnO2 and from the semiconductors to Pt under visible-light irradiation because only R-TiO2 can absorb energy in the visible region, whereas A-TiO2 and SnO2 cannot. The separated charges can migrate to the surface of the photocatalysts, react with O2 and H2O, and generate •O2− and •OH (pathways I and II), respectively, as indicated in eqs 3 and 4. Holes and radicals can mineralize pollutants via pathway III (eq 5). The three pairs of heterojunctions provide multiple pathways of electron transfer to separate electrons and holes efficiently. (2)

e− + O2 → •O2−

(3)

+

h + H 2O → •OH

(5)

CONCLUSIONS In this study, Pt/SnO2/R-TiO2/A-TiO2 nanoparticles with heterostructures were synthesized via a facile hydrothermal method and characterized through various experimental methods, e.g., XRD, TEM, HRTEM, XPS, BET, UV−vis DRS, PL, and PEC analyses. The results clearly indicated that the three pairs of heterojunctions and doping Pt nanoparticles produced can prevent the recombination of electron−hole pairs, extend the absorption range to the visible-light region, and improve the photocatalytic performance. A mechanism was proposed for multiple pathways of electron transfer in the heterojunction structures for effectively enhancing the photocatalytic performance under UV and visible-light irradiation.

(1)

Pt/SnO2 /TiO2 + hv → Pt/SnO2 /TiO2 (e− + h+)

toluene + •OH/•O2− /h+ → CO2 + H 2O

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ASSOCIATED CONTENT

S Supporting Information *

N2 adsorption−desorption isotherms and the corresponding pore-size distribution curves, the UV−vis diffuse reflectance spectra, the PL emission spectra, the time-resolved PL decay curves, Tafel polarization curves, and Bode phase plots. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-571-8898-2032. Fax: +86-571-8898-2032. Notes

The authors declare no competing financial interest.

(4) G



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ACKNOWLEDGMENTS Support from the National Natural Science Foundation of China (Grants 51178412 and 51278456) and the State Science and Technology Support Program (Grant 2013BAC16B01) is gratefully acknowledged.

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Synthesis, Characterization, and Photocatalytic Properties of SnO2

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