TiO2 Nanofiber Composites as Efficient Visible

In addition, we also discuss the photoabsorption performance and the reaction .... three-dimensional TiO 2 photocatalysts with enhanced performance...
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One-Dimensional CdS/TiO2 Nanofiber Composites as Efficient VisibleLight-Driven Photocatalysts for Selective Organic Transformation: Synthesis, Characterization, and Performance Na Qin, Yuhao Liu, Weiming Wu, Lijuan Shen, Xun Chen, Zhaohui Li, and Ling Wu* State Key Laboratory of Photocatalysis on Energy and Environment, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China S Supporting Information *

ABSTRACT: CdS/TiO2 heterojunction nanofibers have been successfully synthesized through the photodeposition of CdS on 1D TiO2 nanofibers that were prepared via a facile electrospinning method. The as-synthesized samples showed high photocatalytic activities upon selectively oxidizing a series of alcohols into corresponding aldehydes under visible light irradiation. TEM observations revealed that CdS was closely grown on the TiO2 nanofibers. Moreover, it was found that the CdS/TiO2 nanofibers that were photodeposited for 4 h exhibited the highest catalytic activity, with a conversion of 22% and a selectivity of 99%, which were much higher than those of commercial CdS. In addition, we also discuss the photoabsorption performance and the reaction mechanism of the photocatalytic oxidation of alcohols. narrow-gap semiconductors, (e.g., CdS).21,22 The semiconducting metal sulfides usually have light-absorbing ability in the visible and short-wavelength near-infrared regions, which enable them to work as a promising class of visible-light-driven photocatalysts.23−27 Among them, CdS is currently the focus of significant attention23−25 because it can compensate for the disadvantages of the individual components to induce some synergistic effects, such as efficient charge separation and migration, and an expanded visible light response.28 Great progress has been achieved in developing methods for the rational synthesis of various nanocomposites. Currently, many nanocomposites such as TiO2/CdS,29 ZnO/CdS30,31 have been investigated as promising photocatalysts.32−34 Various protocols have been exploited such as precipitation,35 chemical bath deposition (CBD),36,37 electrochemical deposition,38 chemical vapor deposition,39 and so on. However, most of the current procedures suffer from complexity, heating, and use of organic solvents, which may waste resources and cause serious pollution. Therefore, there is still a certain space for fabricating the CdS/TiO2 nanocomposite. Recently, it was reported that TiO2-based heterojunctions such as graphene or carbon/TiO2, metal oxides or sulfides/TiO2, and so on may increase the photocatalytic activity of TiO2 photocatalysts by increasing the charge separation and extending the photoresponding range.40,41 Therefore, considerable efforts have been devoted

1. INTRODUCTION One-dimensional (1D) nanostructures such as nanofibers, nanorods, and nanotubes have become a hot topic in the area of photocatalysis owing to their unique physicochemical properties and anisotropic structures.1−3 Among the large number of fabrication methods designed to obtain 1D nanostructures, electrospinning is a convenient and scalable technique.4,5 Nanofibers prepared by the electrospinning method have numerous remarkable characteristics, for example, high porosity, a large surface area-to-volume ratio, and excellent substrates for the assembly of secondary nanostructures.6,7 Unique properties and the multifunctional nature of these electrospun nanofibers make them applicable to biotechnology, reinforced composites, catalyst supports, and sensors.8−12 Nowadays, TiO2 nanofibers (NFs) have attracted much attention because of their attractive characteristics of 1D nanostructure for applications in photocatalysis.13−18 It has also been reported that electrospun TiO2 NFs were beneficial to the vectorial transport of photogenerated charge carriers through grain boundaries, resulting in an enhanced separation of electron−hole charge pairs compared to TiO2 NPs.19 Thus, the design and architecture of TiO2-based nanostructure photocatalysts with a visible-light response are still a research focus. The large band gap (3.2 eV) of anatase TiO2 restricts its use to the narrow light-response range of UV, which accounts for only 3−5% of the spectrum of solar energy.20 In this regard, various strategies have been efficiently developed to tune the photoresponse of TiO2 to visible light and increase the photocatalytic efficiency, for example, combining TiO2 with © 2014 American Chemical Society

Received: September 18, 2014 Revised: December 7, 2014 Published: December 12, 2014 1203

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Langmuir to finding a new way to fabricate TiO2-based heterostructures as promising visible-light-driven photocatalysts. Photocatalytic selective organic transformations have attracted increasing interest because of their environmental friendliness and promising potential for utilizing solar energy.42−45 Among them, the selective oxidation of alcohols to carbonyls is a fundamental but significant transformation for the synthesis of fine chemicals because carbonyl compounds such as ketone and aldehyde derivatives are widely utilized in the fragrance, confectionary, and pharmaceutical industries.46−48 It is therefore highly desirable to exploit visiblelight-driven photocatalysts for photocatalytic selective organic transformations. In this study, we reported on the synthesis of CdS/TiO2 heterojunction nanofibers by coating CdS onto the 1D TiO2 nanofibers that were prepared via a facile electrospinning method. The photocatalytic performance of CdS/TiO2 heterostructure nanofibers on the selective oxidation of alcohols was investigated in detail under visible-light irradiation (λ ≥ 420 nm). The photocatalytic activity results revealed that the photocorrosion of CdS can be inhibited. The CdS/TiO2 heterostructure was confirmed by high-resolution transmission electron microscopy (HRTEM). It was found that the asprepared CdS/TiO2 heterojunction nanofibers displayed a tight contact and increasing surface catalytic sites.

oxygen.50−53 The mixture was then transferred to a 10 mL Pyrex glass bottle filled with molecular oxygen at a pressure of 0.1 MPa and stirred for half an hour to make the catalyst blend evenly in the solution. The suspensions were irradiated with a 300 W Xe arc lamp (Beijing Perfectlight, PLS-SXE300c) through a UV cut-off filter for 4 h. After the reaction, the mixture was centrifuged at 12 000 rpm for 20 min. The remaining solution was analyzed with an Agilent gas chromatograph (GC-6890). The conversion of alcohol, the yield of aldehyde, and the selectivity for aldehyde were defined as follows

⎛C − C ⎞ alcohol conversion(%) = ⎜⎜ 0 ⎟ × 100 C0 ⎠ ⎝ ⎛ Caldehyde ⎞ selectivity(%) = ⎜ ⎟ × 100 ⎝ C0 − Calcohol ⎠ where C0 is the initial concentration of alcohol and Calcohol and Caldehyde are the concentrations of the substrate alcohol and the corresponding aldehyde at a certain time after the photocatalytic reaction, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 shows XRD patterns of the 1D CdS/TiO2 samples with different photodeposition times.

2. EXPERIMENTAL SECTION All materials were used as received without further purification. Deionized water was obtained from local sources. 2.1. Preparation of Catalysts. 2.1.a. Fabrication of TiO2 Nanofibers. TiO2 nanofibers were fabricated through an electrospinning method with postcalcination. In a typical procedure,49 1.5 g of titanium tetraisopropoxide (TIIP, Aldrich) was mixed with 3 mL of acetic acid and 3 mL of ethanol. After 10 min, this solution was added to 7.5 mL of ethanol that contained of 0.45 g of PVP (Aldrich, Mw ≈ 1 300 000), followed by magnetic stirring for some time. The prepared precursor solution was then electrospun at 21 kV, and the above composite nanofibers were calcined at 500 °C for 3 h to obtain the anatase TiO2 nanofibers. 2.1.b. Synthesis of One-Dimensional CdS/TiO2 Heterojunction Nanofibers. The composite nanofibers were synthesized by a photodeposition method. First, a certain amount of the TiO 2 nanofibers were evenly dispersed in 25 mL of absolute ethanol. Then, we added 10 mg of S8 and 78 mg of CdCl2·2.5H2O and bubbled with nitrogen for 30 min in the dark. Last, the suspensions were irradiated with a 300 W Xe arc lamp with incident light at λ ≥ 365 nm for 2, 4, or 6 h. The samples were labeled C/T-2, C/T-4, or C/T-6, respectively. 2.2. Characteristics of Catalysts. X-ray diffraction (XRD) patterns of the products were measured on a Bruker D8 Advance Xray diffractometer using Ni-filtered Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. The morphology and microstructure of the obtained samples were recorded by field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010 EX). ICP measurements were examined on the Ultima2 inductively coupled plasma OES spectrometer. The optical properties of the samples were analyzed by UV−vis diffuse reflectance spectra (UV−vis DRS) using a UV−vis spectrophotometer (Varian Cary 500). The photoluminescence spectra (PL) were surveyed with an Edinburgh FL/FS 900 spectrometer. 2.3. Photoactivity Test. The photocatalytic selective oxidation of various alcohols was performed as follows. Alcohol (0.1 mmol) and catalyst (8 mg) were dissolved in a solvent of oxygen-saturated benzotrifluoride (BTF, 1.5 mL). Solvent BTF was chosen because of its inertness to oxidation and high solubility with respect to molecular

Figure 1. XRD patterns of the as-prepared samples prepared at different photodeposition times (0, 2, 4, and 6h) under UV light (λ ≥ 365 nm): (a) TiO2, (b) C/T-2, (c) C/T-4, and (d) C/T-6.

For the pure TiO2 sample, the identified peaks match perfectly with those of anatase TiO 2 (JCPDS, no. 21-1272). Furthermore, the diffraction peaks at 2θ values of 24.8, 26.5, 28.2, 43.9, 48.2 and 52° can be attributed to the diffraction of the (100), (002), (101), (110), (103), and (112) planes of hexagonal CdS (JCPDS, no. 41-1049), respectively. It is also found that the intensity of the diffraction peaks for CdS is enhanced with the increase in the photodeposition time, indicating that the content and the average crystallite size of CdS in the as-prepared samples can be increased by prolonging the photodeposition time. As shown in Figure S1 (Supporting Information), the obtained PVP/TiO2 and TiO2 NFs samples align in random orientations with uniform diameters at around 150 nm along the entire length. TEM technology has also been introduced to characterize the morphologies of the as-prepared samples. Figure 2a gives the TEM image of the TiO2 nanofibers after they have been calcined in air at 500 °C. The image clearly indicates the difference in morphology between the TiO2 nanofibers and the CdS/TiO2 nanofibers. Figure 2b−d shows that CdS in the as-prepared samples obtained at different photodeposition times is well dispersed on the surface of TiO2 NFs. It is found that the sample with a few nanorod-like CdS 1204

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Figure 2. TEM images of samples (a) TiO2 NF, (b) C/T-2, (c) C/T4, and (d) C/T-6 and HRTEM images of sample (e) C/T-4.

can be obtained after 2 h of photodeposition time. With the photodeposition time prolonged to 4 h, more and more nanorod-like CdS are found to decorate the surface of TiO2 NFs. When the photodeposition time is increased to 6 h, more and larger CdS is deposited on the TiO2 NF surface. This might be caused by the local aggregation between CdS particles. A high-resolution TEM (HRTEM) image of sample C/T-4 is shown in Figure 2e. The lattice spacing of d = 0.336 nm matches that of the (002) crystallographic plane of CdS, and the fringes of d = 0.352 nm correspond to the interplanar spacing of the (101) plane for TiO2. The heterojuction structure of sample C/T-4 is also be observed. The heterojunction structure can helpful in reducing the recombination of photogenerated carriers and therefore can enhance the photocatalytic activity of the as-prepared sample. This will be discussed in section 3.2. Surface chemical compositions of a typical sample (sample C/T-4) have been studied by XPS. As shown in Figure 3a, characteristic binding energy values of 459.0 and 464.5 eV for Ti 2p3/2 and Ti 2p1/2 reveal a Ti4+ oxidation state.54,55 The O 1s spectrum of the sample (Figure 3b) is composed of two lines: the main peak at ∼530.2 eV is assigned to the lattice oxygen, and the binding-energy peak at ∼531.7 eV can be attributed to the oxygen in surface hydroxyl groups.55 The binding energies of 405.4 and 412.1 eV for Cd 3d5/2 and Cd 3d3/2 confirm a bivalent oxidation state for Cd.56 It is also found that the binding energy of S 2p is 161.5 eV,56 indicating that a S2− oxidation state exists on the sample surface. Furthermore, the amount of Cd deposited on a TiO2 nanofiber has been determined by ICP. It is found that the percentage of Cd in sample C/T-4 is 15.23%. UV−vis DRS spectra of the as-prepared samples are shown in Figure 4. It is found that the absorption of the samples in the visible light region can be enhanced by increasing the photodeposition time. This can be attributed to the increase in the amount of CdS in the as-prepared samples because CdS is a well-known visible-light-induced photocatalyst (band gap ∼2.4 eV). It is noted that sample C/T-6 shows relatively low absorption in the visible light region as compared to sample C/ T-4, which may be ascribed to the excessive CdS loading on the surface of TiO2 NFs.

Figure 3. XPS spectra of (a) Ti 2p, (b) O 1s, (c) Cd 3d, and (d) S 2p for sample C/T-4.

Figure 4. UV−vis DRS spectra of the as-prepared samples.

Figure S2 (Supporting Information) shows the N 2 adsorption−desorption isothermals and the corresponding pore size distribution curve of TiO2 NFs (a) and sample C/ T-4 (b). Both samples exhibit a type IV adsorption−desorption isotherms. The BET surface area of the TiO2 NFs is measured to be ca. 33.5 m2 g−1. As for sample C/T-4, it has a mesoporous structure and a relative high surface area. The average pore size was determined to be 9 nm, and the BET specific surface area is ca. 42.9 m2 g−1. 3.2. Selective Oxidation of Alcohols. The photocatalytic activities of the as-prepared samples for the selective oxidation of alcohols have been investigated under visible light irradiation (λ ≥ 420 nm). As shown in Figure 5, the samples exhibit good photocatalytic activities for the selective oxidation of benzyl alcohol to benzaldehyde. Sample C/T-4 has the highest conversion of benzyl alcohol (∼22%), and the selectivity of benzaldehyde is ∼99% after 4 h of visible light irradiation. It is found that the photodeposition time plays an important role in the photocatalytic activities of the as-prepared samples. Initially, the increase in photodeposition time from 2 to 4 h can improve the photocatalytic activity of the as-prepared sample because of 1205

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photoinducted charge carrier is greatly inhibited in the CdS/ TiO2 composite nanofiber. Generally, the stability of a catalyst is a very important factor for its practical applications. As shown in Figure 7, the

Figure 5. Photocatalytic selective oxidation of benzyl alcohol over the CdS/TiO2 heterojunction nanofiber under visible light irradiation (λ ≥ 420 nm) for 4 h.

the increase in the amount of CdS on the surface of TiO2 NFs. When the photodeposition time is greater than 4 h, excess CdS will load on the surface of the TiO2 nanofiber. Therefore, as mentioned above, sample C/T-6 has low absorption in the visible light region. As a result, sample C/T-4 shows the highest catalytic activity for the selective oxidation of benzyl alcohol to benzaldehyde. For comparison, the photocatalytic activities of the TiO2 nanofiber and commercial CdS and CdS-TiO2, which is prepared by simply mixing TiO2 nanofiber and commercial CdS for the selective oxidation of benzyl alcohol to benzaldehyde, have been investigated under similar conditions. As shown in Figure 6, the TiO2 nanofiber shows negligible

Figure 7. Recycled testing of the photocatalytic activity of sample C/ T-4 toward the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation (λ ≥ 420 nm) for 4 h.

photocatalytic activity of sample C/T-4 basically remains unchanged in the recycling experiment. Its catalytic activity remains at ∼100% in the fourth cycle of testing. The photogenerated holes trend to oxidize the benzyl alcohol, and thus the photocorrosion of CdS can be inhibited. In this way, the stability of the composite is guaranteed. Moreover, comparison experiments over commercial CdS have been performed, as shown in Figure S4 (Supporting Information). It is clear that the photocatalytic activity of commercial CdS obviously decreases in the recycling experiments. The result indicates that the as-prepared CdS/TiO2 composite nanofiber is a stable visible-light-driven photocatalyst for the selective oxidation of benzyl alcohol under visible light irradiation. To understand the photocatalytic activity of the CdS/TiO2 composite nanofiber better, the selective oxidation of various psubstituted benzyl alcohols over the typical sample (sample C/ T-4) has been investigated in this work.57,58 As shown in Table 1, various p-substituted benzyl alcohols can be selectively converted to the corresponding benzaldehydes under visible light irradiation. Furthermore, it is noted that the as-prepared sample exhibits a high conversion rate for the selective oxidation of benzyl alcohol with electron-donating groups (such as −OCH3). This may suggests the presence of the electron-donating groups in favor of the selective oxidation of benzyl alcohols over the TiO2/CdS composite nanofiber. Accordingly, a probable mechanism has been proposed, as illustrated in Figure 8. Under visible-light irradiation (λ ≥ 420 nm), photoinduced carriers are created and migrate to the CdS surface. The photogenerated electrons can more easily transfer to TiO2 nanofibers owing to their intimate interfacial contact and matchable energy band position.59 An oxygen molecule reacts with electrons to give superoxide radicals. Simultaneously, the holes in the valence band of CdS can directly oxidize the organic reactive substrates that are adsorbed on the surface of the photocatalyst to form corresponding carbocations. Then, the formed carbocations further react with the superoxide radicals to give the final products. This is why the photocorrosion of CdS can be inhibited.

Figure 6. Photocatalytic selective oxidation of benzyl alcohol over (a) TiO2 NFs, (b) CdS, (c) TiO2 + CdS, and (d) sample C/T-4 under visible light irradiation (λ ≥ 420 nm) for 4 h.

photocatalytic activity. The photocatalytic activity of sample C/ T-4 is about 3 times as high as those of the other samples. The high photocatalytic activity of sample C/T-4 can be attributed to the increasing surface catalytic sites and the close contact with the substrate, and a closely connected heterojunction is formed at the interface between nanorod-like CdS and TiO2 NFs. These tight heterojunction structures of the photocatalysts are beneficial for separating the photogenerated carriers in space and improving the photocatalytic activity. A PL measurement has been applied to determine the chargeseparation efficiency of the CdS/TiO2 composite nanofiber. Figure S3 (Supporting Information) shows that the PL intensity of sample C/T-4 is weaker than that of commercial CdS. The result confirms that the recombination of the 1206

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Table 1. Photocatalytic Selective Oxidation of Various Alcohols over Sample C/T-4 under Visible-Light (λ ≥ 420 nm) Irradiationa

a

Reaction conditions: 8 mg C/T-4, 0.1 mmol alcohol, 1.5 mL of BTF, O2 (1 bar), 4 h, λ ≥ 420 nm.

for the selective oxidation of benzyl alcohols over the CdS/ TiO2 heterojunction nanofibers.

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ASSOCIATED CONTENT

S Supporting Information *

SEM pattern of PVP/TiO2 NFs and TiO2 NFs, BET adsorption−desorption isotherms of the TiO2 NFs and sample C/T-4, PL spectra of sample C/T-4 and commercial CdS, and recycled testing of the photocatalytic activity of commercial CdS. 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]. Notes

The authors declare no competing financial interest.

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Figure 8. Possible mechanism of photocatalytic selective oxidation of alcohols to corresponding aldehydes over the 1D CdS/TiO 2 composite nanofiber under visible light irradiation.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21273036 and 21177024) and the National Key Basic Research Program of China (2014CB239303).

4. CONCLUSIONS CdS/TiO2 heterojunction nanofibers have been constructed by coating CdS onto 1D TiO2 nanofibers, by which TiO2 nanofibers were prepared via a facile electrospinning method. It could be seen that CdS with nanorod-like morphology was well incorporated with TiO2 nanofibers by changing the photodeposition time. This method was simple and capable of preparing 1D CdS/TiO2 heterojunction nanofibers with a closer interface and increasing surface catalytic sites. Furthermore, the photocatalytic activities of the as-prepared samples for the selective oxidation of alcohols had been investigated under visible light irradiation. Sample C/T-4 showed the highest catalytic activity because of the suitable amount of CdS. Its photocatalytic activity was also higher than those of commercial CdS and mixed TiO2−CdS because nanorod-like CdS closely contacted the TiO2 nanofibers to form a heterojunction structure, which could reduce the recombination of photogenerated carriers. Moreover, it was found that the presence of the electron-donating groups is good

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REFERENCES

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DOI: 10.1021/la503731y Langmuir 2015, 31, 1203−1209