TNT Heterojunction Nanocomposite as a High

Article pubs.acs.org/JPCC

Synthesis of a SnO2/TNT Heterojunction Nanocomposite as a HighPerformance Photocatalyst Cheng-Yen Tsai,† Chen-Wuing Liu,*,† Chihhao Fan,† Hsing-Cheng Hsi,‡ and Ting-Yu Chang† †

Department of Bioenvironmental Systems Engineering, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 106, Taiwan ‡ Graduate Institute of Environmental Engineering, National Taiwan University, No. 71, Chou-Shan Road, Taipei 106, Taiwan ABSTRACT: In this study, high-performance SnO2/TiO2 nanotube (TNT) nanocomposites were prepared through a facile one-step hydrothermal method. The samples retained a high BET surface area because they were calcination-free. The BET surface area of all of the samples was over 300 m2/g, and the length was within 100−200 nm. During the hydrothermal reaction, a heterojunction between the SnO2 and TNTs through oriented attachment was achieved. Additionally, we demonstrate that the introduction of cassiterite SnO2 as a precursor in the hydrothermal process can induce the growth of anatase TiO2. A SnO2/TNT nanocomposite photocatalyst not only facilitated dye adsorption at a high concentration around the surface of the nanocomposites but also exhibited efficient charge separation properties and achieved outstanding photocatalytic performance. This study proposes a simple, cost-effective, and environmentally friendly synthetic strategy for fabrication of TNT-based heterostructures as high-performance photocatalysts through adding precursors in the proper amounts.

1. INTRODUCTION Titanium dioxide (TiO2) photocatalysts possess a high potential for decomposing various pollutants in gaseous and aqueous phases. Increasing the interface concentrations of pollutants on the TiO2 surface can markedly enhance both adsorption and photocatalytic reactions. TiO2 nanotubes (TNTs) with high specific surface areas could exhibit a better photocatalytic performance superior to those of TiO2 nanoparticles.1 Following an initial report on the alkaline hydrothermal fabrication of titania nanotubes,2 subsequent studies have utilized this process to develop various TNT composites. The benefits of this process include easy operation, low cost, a simpler single-stage procedure, and low synthesis temperatures. Due to the relatively low processing temperature of the hydrothermal method, much research supports utilizing the phase transition of titanate nanotubes to anatase crystallites through additional calcination to enhance photocatalytic activity.3−6 A multistep fabrication procedure, namely, sample synthesis followed by subsequent calcination, is thus necessary to transform the obtained titanate nanotubes into anatase or rutile. Additionally, the phase transition of TNTs through calcination usually results in the conversion of the nanotubular shape into nanoparticles, which reduces surface area. This invalidates the purpose of fabricating a nanotubular structure by using raw TiO2 nanoparticles. Therefore, it is beneficial to develop a TNT or TNT composite that does not require calcination or avoids a multistep fabrication procedure. Improved photocatalytic activity of TNTs may be attributable to enhanced charge separation efficiency. Developing heterogeneous photocatalysts through surface modification was © 2017 American Chemical Society

reported as an effective strategy for charge carrier transfer in enhancing the separation of photogenerated electrons and holes.7 Recently, coupling SnO2 with TiO2 as a complex system (SnO2/TiO2) has attracted substantial attention.8−10 The coupling of TiO2 with SnO2 affects the electronic structure, and is thus used to control and enhance the surface chemical and physical properties of SnO2/TiO2 composites.11 Both materials exhibit similar crystal properties (the rutile-type tetragonal structure), electronic properties, and ionic radii. When SnO2 heterojunction or doping occurs with TiO2, they can easily form a substitutional composite. Tin compounds, such as SnCl4·5H2O9,12−17 and SnCl2·2H2O,7,18,19 have been extensively investigated as SnO2/TiO2 composite precursors. For example, Hou et al. used anatase TiO2 as a raw material to manufacture nanotubes and followed by mixing in SnCl4·5H2O, ethanol, and TNTs to synthesize a SnO2/TNT composite.12 Additionally, a SnO2/TiO2 coupled photocatalyst was fabricated using TiCl4 and SnCl4·5H2O.9,13 Wang et al. and Wu et al. used P-25 and SnCl4·5H2O to prepare SnO2 nanoparticles with a TiO2 nanobelt14 and nanosheet15 composites. Another study manufactured SnO2 nanostructure/TiO2 nanofiber heterostructures with TiO2 nanofibers and a SnCl4−H2O− C2H5OH solution.16 Huang et al. synthesized TiO2/SnO2 nanocomposites with Ti(OC4H9)4 and SnCl4·5H2O by using the sol-hydrothermal method.17 Moreover, Mourão et al. mixed commercial TiO2 and SnCl2·2H2O in an aqueous KOH Received: November 2, 2016 Revised: March 6, 2017 Published: March 7, 2017 6050

DOI: 10.1021/acs.jpcc.6b11005 J. Phys. Chem. C 2017, 121, 6050−6059

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The Journal of Physical Chemistry C solution, generating TiO2/SnO2 heterostructures.18 Tsai et al. prepared Sn2+-incorporated TNTs by washing a layered sodium titanate with a SnCl2 solution.19 Zhao et al. prepared a SnO2/ TNT composite by combining P-25 and 10 M NaOH and then mixing TNTs and SnCl2 into a 0.1 M HCl solution.7 The aforementioned approaches are suitable for synthesizing SnO2/TNTs nanocomposites. However, a multistep process involving TNT preparation, mixing the TNTs with SnCl4· 5H2O or SnCl2·2H2O aqueous, and subsequent calcination may be required. Numerous attempts to prepare differently shaped photocatalysts (i.e., nanotubes, nanobelts, nanofibers, and nanosheets) with SnO2 have been performed; however, there are no reports on the use of cassiterite SnO2 as a crude material to prepare a SnO2/TNT nanocomposite. Hence, we develop a simple process for synthesizing SnO2/TNT nanocomposites by utilizing the hydrothermal method. To the best of our knowledge, this is the first study to use Degussa P-25 and cassiterite SnO2 as direct precursors for synthesis of SnO2/ TNT nanocomposites that possess an anatase (TiO2)/rutile (SnO2) mixed phase, even without the calcination step. In contrast to composites prepared using the traditional synthesis method, the resulting composites have a large Brunauer− Emmett−Teller (BET) surface area, tubular shape, and surface functionality. Results obtained from this study are practical because the proposed synthesis procedure can easily fabricate high-performance photocatalysts.

Fluoro Max-4) under excitation with 330 nm irradiation. The diffuse reflectance UV−visible spectra (UV−vis) of samples were measured from 300 to 900 nm using a spectrophotometer (JASCO V-650). The composition and Ti 2p, O 1s, and Sn 3d bonding patterns were examined using an X-ray photoelectron spectroscope (XPS; VG Scientific ESCALAB 250). The obtained XPS spectra were deconvoluted with the XPSPEAK software. All binding energies (BEs) were referred to the C 1s peak at 285 eV. The Cl content of the sample was characterized using an X-ray energy dispersive spectrometer (EDS, JEOL JSM-7000F). 2.3. Photocatalytic Activity Evaluation. The tests were carried out in a 100 mL photochemical reactor containing 15 ppm of methylene blue (MB) aqueous solution and 10 mg of sample. Two UV lamps (80 W, Raceahead Co., Taiwan) with a major sharp peak at 254 nm (Rainbow Light RLS-1000, Taiwan) were used and located 20 cm away from the photochemical reactor. The UV intensity was measured (Sentry UVC-ST512) to be 3.0 ± 0.1 mW/cm2. A quartz flake was placed on a photochemical reactor to avoid any contact between the solution and the atmosphere. The determination of the decolorization efficiency was calculated by the following equation Decolorization efficiency (%) =

C0 − C i × 100% C0

(1)

where C0 is the initial concentration of the dye and Ci is the dye final concentration after illumination by UV-light for 60 min. The methylene blue adsorption measurement used 15 ppm MB aqueous solution and 10 mg of sample under dark conditions for 60 min. After photodegradation or adsorption test, the solution was treated by a centrifuge at 8000 rpm for 20 min to separate the TNT sample and MB solution. The solution MB concentration was determined by a UV−vis spectrophotometer (Thermo Genesys-20) at λ = 670 nm. All photodegradation and adsorption tests were performed at room temperature.

2. EXPERIMENTAL SECTION 2.1. Preparation of SnO2/TNT Nanocomposite. TNTs and SnO2/TNT nanocomposites were synthesized by a hydrothermal method from TiO2 nanoparticles (Degussa P25 TiO2) and cassiterite SnO2 (Alfa Aesar 99.9%). The hydrothermal process was similar to that described by a previous report.20 All chemicals used in the experiment were analytical reagent grade. To prepare TNTs, 5 g of P-25 TiO2 was added into 200 mL of 10 M NaOH aqueous solution in a Teflon-lined autoclave at 130 °C for 24 h. After that, the product was washed with 0.5 N HCl and distilled water several times until the pH value of the washed solution reached around 3.5. Subsequently, the samples were dried at 100 °C for 24 h. After arid treatment, the sample was cooled to room temperature for subsequent analysis and photocatalytic activity tests. The formed TiO2 nanotube was designated as TNTs. SnO2/TNT nanocomposites were synthesized by a similar process by changing the added amount of SnO2 to synthesize the SnO2/TiO2 nanotube composites with 5, 10, and 20 wt % SnO2 loading; the products were designated as Sn5−TNTs, Sn10−TNTs, and Sn20−TNTs, respectively. 2.2. Catalyst Characterization. The image of the synthesized samples was determined with a transmission electron microscope (TEM; Hitachi H-7100) and high resolution transmission electron microscope (HR-TEM; Philips/FEI Tecnai 20 G2) by visually counting. The Brunauer−Emmett−Teller (BET) specific surface area was analyzed by using a Micromeritics ASAP 2020 based on the N2 adsorption isotherm obtained at 77 K. The phase compositions of samples were analyzed with X-ray powder diffraction (XRD; Bruker, D2 Phaser) with Cu Kα radiation (λ = 1.5405 Å). The crystalline phases were identified with the JCPDS database. The surface functional groups of raw and resulting samples were examined using a Fourier transform infrared spectrometer (FTIR; Bruker Vertex 80v). Photoluminescence spectroscopy (PL) was carried out using a fluorescence spectrometer (Horiba

3. RESULTS AND DISCUSSION 3.1. Characterization of TNTs and SnO2/TNT Composites. Table 1 lists the BET surface area (SBET), total pore Table 1. BET Surface Area, Total Pore Volume, and Pore Size of Sn−TNT and TNT Samples sample

SBET (m2/g)

Vtotal (cm3/g)

pore size (nm)

TNTs Sn5−TNTs Sn10−TNTs Sn20−TNTs

392.0 370.9 354.2 312.9

1.13 1.19 0.94 0.89

11.5 12.8 10.6 11.4

volume (Vtotal), and average pore size of the TNT samples and SnO2/TNT composites. The SBET of raw P-25 was approximately 55 m2/g, and the Vtotal was 0.15 cm3/g. After hydrothermal treatment, the SBET and Vtotal of the TNT sample (392 m2/g and 1.13 cm3/g, respectively) increased by a factor of 7, compared with those of the raw P-25. In contrast, adding SnO2 as a precursor reduced the SBET and Vtotal of the TNT samples, but the average pore sizes for all samples were approximately 10−12 nm. The BET surface areas for samples with a SnO2/P-25 mass ratio of 5, 10, and 20 wt % were 370.87, 354.2, and 312.9 m2/g, respectively, indicating that the surface area of the SnO2/TNT composite was affected by increasing the SnO2 content. This observation suggests that the addition 6051


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Figure 4. UV−visible absorption spectra of synthesized TNTs and Sn−TNTs.

Figure 1. XRD patterns of Sn−TNTs synthesized with 0−20 wt % SnO2 addition.

g (TNTs) to 312.9 m2/g (Sn20−TNTs). This decrease may have been caused by the coverage of SnO2 on the surface of the TNTs. Similar physical property changes have been observed in other studies.16 The X-ray diffraction (XRD) powder patterns of the TNT samples and SnO2/TNT composites are illustrated in Figure 1. All of the diffraction peaks exhibited anatase and rutile phases. The peaks located at 25.27, 37.8, 48.0, 55.0, and 62.6° corresponded to the (101), (004), (200), (211), and (204) planes of the anatase phase of TiO2 (JCPDS No. 21-1272), and the peaks located at 27.5, 36.1, and 54.4° corresponded to the (110), (101), and (211) planes of the rutile phase of SnO2 (JCPDS No. 41-1445). The ratio of anatase to rutile phases in Degussa P-25 TiO2 is approximately 3:1.21 The XRD result of the TNT sample reveals that most of the rutile structure in raw P-25 TiO2 had been destroyed following the hydrothermal process. By contrast, using SnO2 and P-25 as precursors after the hydrothermal process produced both anatase (TiO2) and rutile (SnO2) crystalline phases. Due to the extremely low solubility of cassiterite SnO2, mixed-phase TNTs can be generated without the calcination or a multistep process. Additionally, the rutile intensity was increased by elevating the SnO2 amount, suggesting that the rutile phase was contributed by the addition of SnO2. Moreover, increasing the SnO2 contents caused the (004), (211), and (204) peaks of anatase to become sharp. It is important to note that the TNT sample did not appear in anatase (004), (211), and (204) crystal peaks. These observations suggest that pure cassiterite SnO2 as precursors also can be rutile crystal seeds, which can induce the anatase TiO2 growth units during the hydrothermal process. It is important that photocatalytic performance is improved by an anatase phase. Notably, Sn−TNTs produced a mixed crystalline phase superior to that of TNTs after hydrothermal treatment, and retained a high SBET because the calcination treatment was not applied. The catalytic property of a material is greatly affected by its surface morphology, surface −OH content, surface electronic states, and defects.22 Fourier transform infrared (FTIR) spectra were used to characterize the surface OH groups and water adsorption on the surface of the synthesized TNT samples (Figure 2). The peak at 650 cm−1 was attributed to the vibration of Ti−O−Ti bonds in the TNTs.23 Two peaks at 1635 and 2900−3800 cm−1 were noted in the IR data of the

Figure 2. FTIR spectra of prepared TNTs and Sn−TNTs.

Figure 3. Photoluminescence spectra of prepared samples.

of SnO2 during SnO2/TNT synthesis through hydrothermal treatment produced another pattern, which could not retain a tubular shape. Consequently, the SBET decreased from 392 m2/ 6052


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Figure 5. TEM images of precursor SnO2, hydrothermal treatment SnO2, and Sn−TNT samples.

TNT samples. The first peak corresponds to the OH group on the surface, and the second broader peak is caused by the adsorbed water molecules.24 The intensity of two peaks decreased with a reduction in the SBET of the sample, indicating that the amount of OH groups and preadsorbed water molecules depends on the sample surface area. Jain et al. and Xiao et al. have reported that the main oxidizing species, OH radicals, were generated through the oxidation of water molecules adsorbed on TiO2, and the hydroxyl radicals then oxidized the adsorbed pollutant molecules.25,26 Moreover, defect sites are prone to affect the electron−hole (e−/h+) recombination in photocatalysts, causing a change in chemical rates dependent on the charge transfer from either electrons or holes.22 The rate of recombination of e−/h+ pairs is also a key factor affecting the photoactivity of titania.27 The photoluminescence (PL) spectroscopy was thus applied to investigate the e−/h+ separation efficiency (Figure 3). The emission peak at around 376 nm can be attributed to the existence of self-trapped excitations from TiO68− octahedrons.28 The emission peak at approximately 466 nm was attributed to the presence of surface defects (i.e., oxygen vacancies), which provide acceptor levels below the conduction band edge,

inhibiting the transfer of photoinduced electrons from the conduction band to the valence band.27 Several reports have indicated that Ti−O−Ti linkages of TiO68− octahedrons were broken in strong NaOH solutions, thereby leaving some oxygen vacancies in the composition of TNTs.20,29,30 The peak observed at approximately 562 nm can be attributed to the irradiative recombination of charge carriers.31 Because PL emission is the consequence of the recombination of excited electrons and holes, the lower PL intensity of the sample indicated a lower recombination rate.28 The PL analysis results suggest that e−/h+ recombination could decrease with elevation of the SnO2 content. Two explanations for this phenomenon are feasible. First, the excitonic PL intensity of TNTs decreased as the surface area increased; this decrease is possibly caused by an increase in surface oxygen vacancies and defects. However, the SBET of TNTs was larger than the Sn−TNT surface area. Therefore, the amount of oxygen vacancies on the Sn−TNT surface is unlikely to have been the only factor producing the reduction recombination of e−/h+ pairs. Secondarily, some species on the TNT surface, such as metal or anion species with only a small resistance, can conduct electrons;32−34 consequently, e−/h+ recombination is inhibited. Reducing the PL 6053


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Figure 6. Morphological and structural information on the Sn5−TNT sample. (a) TEM image of the resulting sample synthesized with 5 wt % SnO2 and 95 wt % P-25. (b) HR-TEM image of the nanotubules. (c) HR-TEM image of the SnO2 incorporated by a nanotube. (d−g) Heterojunction of SnO2 and TNTs through the oriented attachment mechanism. (h) The proposed dislocation drawing in the interface of SnO2 and TNTs.

intensity verifies that the SnO2 content exerted a major influence on the decreasing recombination of e−/h+ pairs. Hence, it is theorized that OH groups, oxygen vacancies, and SnO2 on the surface of TNTs promote photocatalyst activity. The absorption edges of the synthesized TNT samples and Sn−TNT composite were examined using a UV−visible spectrophotometer, and the result is illustrated in Figure 4. Predictably, the TNTs lacked appreciable absorption above the basic absorption sharp edge at approximately 350 nm. However, the synthesized Sn−TNT composite exhibited a

red shift of the absorption edge. Additionally, the absorption spectra of the Sn−TNT composite shifting from 350 to 360 nm upon an increase in the SnO2 content can be attributed to the band gap narrowing in relation to the interstitial SnO2 species on the TNT surface. This observation suggests that the junction between SnO2 and TNTs does not appreciably modify the band structure of the TNTs. The morphology of the as-prepared Sn−TNT samples, raw SnO2, and SnO2 subjected to hydrothermal treatment was observed through transmission electron microscopy (TEM) 6054


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and polygonal objects. The size of the polygonal objects decreased after reduction of the SnO2 precursor. In accordance with thermodynamic theory, syntheses were performed using differing amounts of SnO2 and P-25 as precursors under the same conditions; at 5 wt %, the SnO2 precursor could receive more energy than it could at 20 wt %. Consequently, 5 wt % SnO2 easily broke into nanoparticles. This suggests that, at a high SnO2 loading, the 10 M NaOH hydrothermal treatment is less effective in converting the SnO2 into pieces. A SnO2/TNT sample with a 5 wt % SnO2 loading (Sn5− TNTs) was observed through high-resolution (HR) TEM (Figure 6). Figure 6a illustrates that SnO2 was erratically connected with the TNTs, and did not exist independently. The HR-TEM analysis indicated that the d-spacing of the nanotube was 0.35 nm, which corresponded to an anatase (101) phase (Figure 6b). Additionally, the nanotube was observed to have multiple layers, potentially because of the rolling up of the nanosheet.35 Moreover, some small pieces of SnO2 may have been incorporated into the nanotube while the nanosheet was rolled up (asterisks in Figure 6c). Furthermore, some SnO2 may have attached to the surface of the TNTs (triangles in Figure 6d,e). The fringe pattern with a d-spacing of 0.26 nm, marked by arrows, illustrates that the (101) plane of SnO2 is in the rutile phase and attached to the nanotube surface (Figure 6e,f). For here, we assume that SnO2 can be tightened by attaching to the surface of the TNTs through an oriented attachment mechanism. Penn and Banfield reported that the oriented attachment can give rise to a homogeneous single crystal or to crystal separation by twin boundaries or other planar defects.36 We theorize that, in a strong alkaline solution, planar defects develop when the Ti−O−Ti linkages of TiO68− octahedrons break. Moreover, another study reported that imperfectly oriented attachment of nanocrystals can generate dislocations with edges.37 The triangles in Figure 6g show SnO2 attached to the surface of the TNTs by a line of edge dislocation in the interface. The proposed dislocation drawing is shown in Figure 6h. Ribeiro et al. used cassiterite SnO2 and rutile TiO2 as precursors and found that the oriented attachment mechanism could be carried out in hydrothermal conditions.37 Penn and Banfield indicated that oriented attachment involved spontaneous self-organization of adjacent particles such that they shared a common crystallographic orientation, followed by joining of these particles at a planar interface.36 In addition, Ribeiro et al. proposed that oriented attachment may occur through two mechanisms in colloids: (1) nonaligned particles surface crystallographic rotates until a favorable geometry is obtained and coalescence occurs, and (2) coalescence occurs through effective collision between particles with the same crystallographic orientation.38 Here, we postulate that SnO2 oriented along the (101) plane undergoes crystallographic rotation until it is in contact with TNTs oriented along the (101) plane and thus achieves oriented attachment during hydrothermal treatment. The amount of tin in the Sn5−TNT lattice was relatively low, and could not be measured through X-ray photoelectron spectroscopy (XPS) in this study. Nevertheless, the existence of SnO2 could be evidenced from the XPS examinations on Ti 2p and O 1s of the TNTs and Sn5−TNT samples (Figure 7). The peaks Ti4+ 2p1/2, Ti3+ 2p1/2, Ti4+ 2p3/2, and Ti3+ 2p3/2 indicate the presence of TiO2 and Ti2O3, respectively.39,40 The reduction of Ti4+ to Ti3+ mainly resulted from the presence of oxygen vacancy.40 There were negligible changes in the Ti 2p peaks of the TNTs and Sn5−TNT samples, implying that SnO2

Figure 7. XPS spectra of TNTs and Sn5−TNTs.

(Figure 5). TEM images showed that TNTs had a diameter of approximately 5 nm and a length within 100−200 nm. The raw SnO2 was a polygonal shape. Additionally, a nanotubular shape did not form following a hydrothermal process and acid washing treatment identical to those performed for TNT production. The results indicate that the as-prepared SnO2 sample possesses a similar shape to that of the raw SnO2. However, a size reduction was observed following the hydrothermal treatment and acid washing. Moreover, TEM images of the Sn−TNT samples depicted some nanotubular 6055


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Figure 8. Photocatalytic degradation and adsorption of MB for prepared samples.

adsorption was measured in dark conditions for 60 min. These results from the photodegradation and adsorption of dye for all samples are presented in Figure 8. Generally, the Sn− TNT heterostructures demonstrated enhanced MB removal efficiency in comparison with the TNT samples. The order of the samples according to MB removal efficiency was Sn5− TNTs > Sn10−TNTs > TNTs > Sn20−TNTs. Notably, the SBET of Sn20−TNTs was smaller than that for the TNT sample; this lower value may have contributed to the lower removal efficiency of Sn20−TNTs than that of the TNT sample. Wang et al. reported that higher surface coverage of SnO2 particles reduced the accessibility of the active sites on the surface of TNTs, thereby reducing the photodegradation efficiency.16 Another reason is that SnO2 nanoparticles are not as effective as TNTs in photocatalysis; therefore, the photocatalytic activity of Sn−TNT composites is inversely proportional to the SnO2 content up to a certain level.16 After 60 min of UV irradiation, the decomposition efficiencies of MB were approximately 76.97% (TNTs), 96.18% (Sn5−TNTs), and 58.90% (PTS), respectively. Sn5−TNTs exhibited the highest photodegradation efficiency among all samples in this study. After calcination treatment, the photodegradation removal efficiency displayed a remarkable rise; nevertheless, the adsorption removal efficiency was dramatically reduced. The photodegradation efficiency of the TNT sample calcined at 700 °C (TNTs-700) increased by approximately 10% compared with that of the TNT sample (uncalcination), but the adsorption efficiency decreased by approximately 88%, being only 2.72%. The results indicate that the calcination process increases the crystal phase integrity and thereby enhances the photodegradation efficiency. However, the SBET of the TNTs700 sample was lower than that of the starting material, P-25, due to a loss of the nanotubular structure. By contrast, Sn5− TNTs had extremely high photodegradation and adsorption efficiencies. Notably, the SBET of Sn5−TNTs and Sn10−TNTs was smaller than that for the TNT sample. However, the removal efficiency was higher than that of the TNT sample (Figure 8), which was due to the heterojunction between the SnO2 and TNTs by the oriented attachment mechanism. The Fermi level of SnO2 is lower than that of TNTs,41,42 leading to the easy transfer of photogenerated electrons from TNTs to SnO2. Furthermore, a great number of holes are present on the surface to take part in the reactions for oxidizing H2O into • OH. The MB adsorption results obtained for the Sn5−TNT and Sn10−TNT samples indicate the importance of SnO2 in

Table 2. Adsorption Test of Sample after NaCl Treatment and Cl Content MB adsorption efficiency (%) sample TNTs Sn5−TNTs Sn10−TNTs Sn20−TNTs

without treatment 30.92 42.53 34.74 28.48

± ± ± ±

0.38 1.87 3.58 2.24

Cl content (atom %)

NaCl wash

without treatment

NaCl wash

± ± ± ±

0.61 1.70 1.97 1.83

0.06 n.a. 0.10 0.11

12.80 23.07 20.08 15.89

4.24 0.16 1.21 0.02

addition did not significantly influence the Ti 2p peak shape. However, the Ti 2p binding energy of the Sn5−TNT sample demonstrated a slight shift to a higher level compared with the TNT sample. Since the Fermi level of SnO2 is lower than that of TiO2,41,42 the electrons of TNTs might be transferred to the SnO2 and disperse on the surface of the TNTs, causing alterations in the outer electron cloud density of Ti ions.41 Consequently, the Ti 2p binding energy slightly increased. By contrast, the shapes of the O 1s spectra in both samples are wide and asymmetric, which suggests that there might be two or three chemical states according to the binding energy. Using the XPSPEAK fitting program, the O 1s XPS spectra of the TNTs and Sn5−TNT samples were fitted to two and three chemical states, respectively. The O 1s peak of the TNT samples consisted of two components: The major peak was related to Ti−O bonds in the octahedral coordinated TiO2 crystal lattice, and the other was attributed to the contribution of hydroxyl groups (−OH) attached to the upper layer of the TNT surface.43 By contrast, three O 1s peaks of the Sn5−TNT samples appeared after deconvolution, and were attributed to surface hydroxyl (−OH) oxygen, Ti−O bonds, and the crystal lattice oxygen of SnO2 (Sn−O), respectively. 3.2. Photocatalytic Measurements. For comparisons of photocatalytic activity efficiency, some TNT samples were calcined at 300, 500, and 700 °C for 1 h. Although the anatase peak becomes sharp with increasing calcination temperature (not shown), the large surface area, nanotubular shape, OH groups, and preadsorbed water molecules on the TNT surface can be expected to disappear at the same time. In addition, we physically mixed TNTs and 5 wt % SnO2 that had been treated using the TNT synthesis procedure, and the mixture was labeled PTS. The photocatalytic activities of all obtained samples were studied by analyzing the decomposition of methylene blue (MB) under UV irradiation. The dye 6056


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Figure 9. Proposed reaction mechanisms of SnO2/TNT formation and photocatalytic degradation of MB.

SnO2 heterojunction. This explains the superior Sn5−TNT photodegradation efficiency, which can be attributed to improved electron transport within TNTs attached to SnO2 caused by the identical lattice arrangement along the (101) plane and the high speed of electron transport from the inside to the surface. On the basis of the reaction processes 2−6, the amount of hydroxyl radicals in the photochemistry reaction system was increased.16,41

MB adsorption. In addition, the anatase crystal structure was more complete due to rutile SnO2 as crystal seeds during the hydrothermal process. Photodecomposition and adsorption efficiencies were different between the Sn5−TNT sample and the physically mixed PTS sample. This is further evidence that a heterojunction between SnO2 and TNTs is achieved during the hydrothermal process in a single step. The Sn5−-TNTs and Sn10−TNTs give better adsorption than TNTs, which may be because the surface of both samples possessed more Cl species than the TNT sample after HCl washing. Cui et al. indicated that Cl− can form ion pairs with MB.44 Although the surface areas of Sn5−TNTs and Sn10− TNTs are smaller than that of the TNT sample, we infer that more Cl species can be retained on the surface of the Sn5− TNT and Sn10−TNT samples during HCl washing. As a result, the Sn5−TNTs and Sn10−TNTs give a better adsorption efficiency than the TNT sample. NaCl solution was used to wash all samples in order to remove Cl species on the surface of the sample.45 Table 2 illustrates the MB adsorption test of the samples and Cl content. The MB removal efficiency of the samples was significantly decreased after NaCl wash. Xiao et al. reported that the electron transfer process was more efficient if the species were preadsorbed on the surface of the TiO2.26 Law et al. proposed that electron transport in a single-crystalline material is several orders of magnitude faster.46 Electrons can be efficiently separated as a result of a

ecb− + O2 → •O2−

(2)

•

O2− + H 2O → •HOO + OH−

(3)

2HOO• → H 2O2 + O2

(4)

H 2O2 + ecb− → •OH + OH−

(5)

h vb+ + H 2O (OH−, H+) → •OH + H+

(6)

According to the preceding analysis results, we suggest that adding the proper amount of SnO2 in the hydrothermal process (5 wt % in this study) enhances photodegradation and adsorption efficiencies. A schematic of the mechanism of SnO2/TNT nanocomposite formation from cassiterite SnO2 and P-25 TiO2 though the hydrothermal process is illustrated in Figure 9. We propose that the SnO2/TNT composite forms in two stages. Initially, nanosheets are formed on the surface of P-25 TiO2 6057


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(2) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Formation of Titanium Oxide Nanotube. Langmuir 1998, 14, 3160− 3163. (3) Yu, J.; Yu, H.; Cheng, B.; Zhao, X.; Zhang, Q. Preparation and Photocatalytic Activity of Mesoporous Anatase TiO2 Nanofibers by a Hydrothermal Method. J. Photochem. Photobiol., A 2006, 182, 121− 127. (4) Yu, J.; Wang, G.; Cheng, B.; Zhou, M. Effects of Hydrothermal Temperature and Time on the Photocatalytic Activity and Microstructures of Bimodal Mesoporous TiO2 Powders. Appl. Catal., B 2007, 69, 171−180. (5) Kuo, H.; Kuo, C.; Liu, C.; Chao, J.; Lin, C. A. Highly Active BiCrystalline Photocatalyst Consisting of TiO2 (B) Nanotube and Anatase Particle for Producing H2 Gas from Neat Ethanol. Catal. Lett. 2007, 113, 7−12. (6) Lin, C.; Chao, J.; Liu, C.; Chang, J.; Wang, F. Effect of Calcination Temperature on the Structure of a Pt/TiO2 (B) Nanofiber and Its Photocatalytic Activity in Generating H2. Langmuir 2008, 24, 9907−9915. (7) Zhao, W.; Zhang, M.; Ai, Z.; Yang, Y.; Xi, H.; Shi, Q.; Xu, X.; Shi, H. Synthesis, Characterization, and Photocatalytic Properties of SnO2/ Rutile TiO2/Anatase TiO2 Heterojunctions Modified by Pt. J. Phys. Chem. C 2014, 118, 23117−23125. (8) El-Maghraby, E. M. Effect of Sn Ratio on the Photocatalytic Degradation of Methylene Blue and Soot of Ink by TiO2−SnO2 Nanostructured Thin Films. Phys. B 2010, 405, 2385−2389. (9) Hirano, M.; Dozono, H.; Kono, T. Hydrothermal Synthesis and Properties of Solid Solutions and Composite Nanoparticles in the TiO2−SnO2 System. Mater. Res. Bull. 2011, 46, 1384−1390. (10) Kaleji, B. K.; Sarraf-Mamoory, R. Nanocrystalline Sol-Gel TiO2SnO2 coatings: Preparation, Characterization and Photo-catalytic Performance. Mater. Res. Bull. 2012, 47, 362−369. (11) Beltrán, A.; Andrés, J.; Sambrano, J. R.; Longo, E. Density Functional Theory Study on the Structural and Electronic Properties of Low Index Rutile Surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/ SnO2 Composite Systems. J. Phys. Chem. A 2008, 112, 8943−8952. (12) Hou, L.; Yuan, C.; Peng, Y. Synthesis and Photocatalytic Property of SnO2/TiO2 Nanotubes Composites. J. Hazard. Mater. 2007, 139, 310−315. (13) Huang, M.; Yu, S.; Li, B.; Lihui, D.; Zhang, F.; Fan, M.; Wang, L.; Yu, J.; Deng, C. Influence of Preparation Methods on the Structure and Catalytic Performance of SnO2-Doped TiO2 Photocatalysts. Ceram. Int. 2014, 40, 13305−13312. (14) Wang, X.; Sang, Y.; Wang, D.; Ji, S.; Liu, H. Enhanced Gas Sensing Property of SnO2 Nanoparticles by Constructing the SnO2− TiO2 Nanobelt Heterostructure. J. Alloys Compd. 2015, 639, 571−576. (15) Wu, H.; Hon, M.; Kuan, C.; Leu, I. Synthesis of TiO2 (B)/SnO2 Composite Materials as an Anode for Lithium-Ion Batteries. Ceram. Int. 2015, 41, 9527−9533. (16) Wang, C.; Shao, C.; Zhang, X.; Liu, Y. SnO2 NanostructuresTiO2 Nanofibers Heterostructures: Controlled Fabrication and High Photocatalytic Properties. Inorg. Chem. 2009, 48, 7261−7268. (17) Huang, M.; Yu, J.; Li, B.; Deng, C.; Wang, L.; Wu, W.; Dong, L.; Zhang, F.; Fan, M. Intergrowth and Coexistence Effects of TiO2− SnO2 Nanocomposite with Excellent Photocatalytic Activity. J. Alloys Compd. 2015, 629, 55−61. (18) Mourão, H.; Junior, W.; Ribeiro, C. Hydrothermal Synthesis of Ti Oxide Nanostructures and TiO2:SnO2 Heterostructures Applied to the Photodegradation of Rhodamine B. Mater. Chem. Phys. 2012, 135, 524−532. (19) Tsai, C.; Chen, L.; Yeh, T.; Teng, H. In Situ Sn2+-Incorporation Synthesis of Titanate Nanotubes for Photocatalytic Dye Degradation under Visible Light Illumination. J. Alloys Compd. 2013, 546, 95−101. (20) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Titania Nanotubes Prepared by Chemical Processing. Adv. Mater. 1999, 11, 1307−1311. (21) Ohno, T.; Sarukawa, K.; Tokieda, K.; Matsumura, M. Morphology of a TiO2 Photocatalyst (Degussa, P-25) Consisting of Anatase and Rutile Crystalline Phases. J. Catal. 2001, 203, 82−86.

particles. Concurrently, smaller particles are generated by the division of polygonal SnO2 particles during the hydrothermal process. Nanosheets are then exfoliated and subsequently scrolled surrounding some small SnO2 particles. Additionally, some small SnO2 particles rotate and collide with TNTs in the hydrothermal environment until achieving a favorable geometry, such as orientation along the (101) plane, and coalescence occurs. Moreover, e−/h+ pair separation efficiency is increased as a result of the heterojunction favoring electron transfer from the TNTs to SnO 2. Furthermore, the crystallographic orientations promote faster electron transport, and consequently, photocatalytic performance is improved.

4. CONCLUSIONS We propose using cassiterite SnO2 and P-25 as starting materials to synthesize anatase−rutile mixed-phase SnO2/TNT nanocomposites by employing the hydrothermal method in a single step without additional calcination treatment. In our study, the BET surface areas for all resulting samples were over 300 m2/g. TNTs without SnO2 addition had the largest BET surface area of 392 m2/g. The BET surface area of Sn−TNT samples decreased as SnO2 was added. Anatase appeared to be the major crystalline phase for the TNT samples. After SnO2 addition, an anatase−rutile mixed phase was observed. The rutile intensity increased with increasing SnO2. Moreover, anatase (004), (211), and (204) planes of Sn−TNT samples were induced by addition of SnO2. FTIR and PL examinations suggest the presence of OH groups and oxygen vacancies on the surface of all samples. A minor redshift of the band gap occurred during SnO2 addition. XPS results showed that Ti4+ comprised the major species in the Sn5−TNT sample. The synthesized SnO2/TNT nanocomposite had a diameter of approximately 5 nm and a length within 100−200 nm. HRTEM provided direct evidence that a heterojunction was formed between SnO2 and TNTs within the Sn5−TNT sample through the oriented attachment mechanism. This explains why Sn5−TNTs possessed the highest removal efficiency for MB dye in the study. This study provides new insights into the fabrication of a SnO2/TNT nanocomposite, which can be applied in photocatalysis for environmental contamination removal.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cheng-Yen Tsai: 0000-0002-5611-407X Chen-Wuing Liu: 0000-0003-1198-2639 Chihhao Fan: 0000-0002-9840-1835 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology, Taiwan, under Grant No. 103-2313-B-002018-MY3.

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REFERENCES

(1) Roy, P.; Berger, S.; Schmuki, P. TiO2 Nanotubes: Synthesis and Applications. Angew. Chem., Int. Ed. 2011, 50, 2904−2939. 6058


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TNT Heterojunction Nanocomposite as a High

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