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Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol
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CERAMICS INTERNATIONAL
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Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol Jian Tiana,b,n, Jian Lia, Na Weia, Xiaohong Xuc, Hongzhi Cuia,n, Hong Liub,n a
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China c School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Received 13 August 2015; received in revised form 3 September 2015; accepted 19 September 2015
Abstract One-dimensional (1D) TiO2 nanostructures can be used as the support of metal particles to form the heterostructures, which can prevent the metal agglomeration and enhance the activity of metal in various applications. Here, well-dispersed Ru nanoparticles (NPs) supported on 1D TiO2 nanobelts (NBs) are firstly synthesized by a facile photo-reduction method. The as-synthesized Ru/TiO2 NB heterostructures exhibit an improved photocatalytic performance compared with P25 and TiO2 under both UV and visible light irradiation. Our mechanistic investigation reveals that the enhanced photocatalytic activity can be attributed to the Schottky barrier effect of Ru NPs, which efficiently harvest the electrons of TiO2 and enhance the separation of photogenerated electron–hole pairs. Moreover, the Ru/TiO2 NB heterostructures are assembled into the porous nanopaper via a modified paper-making process, which can be applied in heterogeneous catalysis. The formed Ru/TiO2 NB heterostructure nanopapers exhibit enhanced activity and selectivity for catalyzing aerobic oxidation of benzyl alcohol. Ru NPs are well dispersed with the average size of about 1.5 nm on the TiO2 NBs support, which could contribute to the high catalytic activity of Ru/TiO2 catalyst. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Schottky barrier effect; Heterostructures; Photocatalytic; Nanopaper; Gas phase oxidation catalysis
1. Introduction As a large electron gap semiconductor, TiO2 has aroused great attention due to its good chemical and thermal stability, and excellent electronic and optical properties, which has been used in numerous potential applications, such as photovoltaic devices, photodegradation of pollutants, photocatalytic watersplitting for hydrogen production, and heterogeneous catalysis [1–3]. Among various synthesized TiO2 nanostructures, onedimensional (1D) TiO2 nanostructures, for example, TiO2 nanobelts (NBs), are also extensively explored in the field of solar energy or catalysis due to their stable 1D nanostructure n
Corresponding authors. E-mail addresses: [email protected] (J. Tian), [email protected] (H. Cui), [email protected] (H. Liu).
and fast electron transfer [4–5]. Besides, this 1D nanostructure can easily assemble metal or metal oxide to form the heterostructure based on the band structure matching, P–N junction, surface plasmon resonance, or Schottky barrier effect [6], which gets some new properties because of the existence of the second phase, ensures a fast and efficient charge transfer between the two phases, and broadens the light absorption band to the visible region [7]. Photocatalysis provides a cost effective method for both renewable energy synthesis and environmental purification [8]. A large amount of materials, such as TiO2, ZnO and CdS, has been developed by researchers, which are considered as promising light-absorbing materials in photocatalysis, because of their versatile optical and electrical characteristics [9]. However, their poor visible light utilization, few surface active sites, as well as the high recombination of photogenerated
http://dx.doi.org/10.1016/j.ceramint.2015.09.112 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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electron–hole pairs greatly limit their practical applications [10]. Combining the semiconductor with highly dispersed noble metal nanoparticles (NPs), such as Au, Pd, and Ru, that can form a new Fermi level and promote the effective charge carrier transfers, which is a feasible approach for enhancing the photocatalytic performance [11]. Ruthenium (Ru), as one of the noble metals, has been investigated widely due to their unique properties, such as stability, ease of preparation, and remarkable electronic and optical properties, which are of great interest in the technological applications [12]. Ru NPs enable size dependent storage of electrons, which can then be released to the suitable acceptors [13]. Recently, it has been reported that electron transfer from TiO2 to Ru NPs occurred under irradiation of light due to the Schottky barrier effect at the metal-oxide interface [14]. However, in most studies on Ru/TiO2 nanocomposites, the preparation processes are very complicated and often lead to the lack of control over the size distribution [15]. Their practical applications are still hindered by the high price of precious metals and the agglomeration problem of metal NPs [16]. Thus minimizing the metal NPs loading while simultaneously improving the photocatalytic activity and stability of these materials are of vital importance in photocatalysis or heterogeneous catalysis [17]. Moreover, Ru is a very powerful oxidation catalyst and wellknown for its application in heterogeneous catalysis [18]. The catalytic properties of Ru NPs are strongly dependent on their size. Smaller metal particles have larger specific surface area and larger quantity of highly active sites, thus present higher catalytic activity [19]. However, the small size of Ru NPs tends to sinter during the catalytic process at elevated temperature and cause deactivation [20]. So Ru NPs need to be well dispersed on the support to achieve high mass activity and resistance to aggregation [21]. 1D TiO2 nanostructures can be used as the support materials for noble metal or metal oxide catalysts because of their large surface area, high stability in basic or acidic media, and ability to assemble above catalysts [22]. In the present work, we detail a facile photo-reduction method to construct Ru/TiO2 NB heterostructures with strongly electrically coupled interfaces. The well dispersed and ultra-small Ru NPs are uniformly distributed on the surface of TiO2 NBs. Ru is the metal chosen due to its potential for Schottky barrier enhancement of photocatalysis and as an excellent catalyst on the TiO2 support for heterogeneous catalysis. Ru/TiO2 NB heterostructures are tested for photodegradation of pollutants in solution using methyl orange (MO) as the model system under UV and visible light irradiation, and for gas-phase selective oxidation of benzyl alcohol.
(H2SO4), and ruthenium chloride trihydrate (RuCl3 3H2O) were purchased from Sinopharm and used without further treatment.
2. Experimental section
X-ray powder diffraction (XRD) pattern of catalysts were recorded on a Bruke D8 Advance powder X-ray diffractometer with Cu Kα (λ¼ 0.15406 nm). HITACHI S-4800 field emission scanning electron microscope (FE-SEM) was used to characterize the morphologies and size of the synthesized samples. High resolution transmission electron microscopy (HRTEM) images were carried out with a JOEL JEM 2100 microscope. Fourier transform infrared (FTIR) spectra were collected on a Nicolet
2.1. Materials The chemicals used in this work were of analytical reagent grade. Solutions were freshly prepared with deionized water. Titania P25 (TiO2; ca. 80% anatase, and 20% rutile), sodium hydroxide (NaOH), hydrochloric acid (HCl), sulfuric acid
2.2. Preparation of TiO2 nanobelts TiO2 nanobelts were synthesized by a hydrothermal procedure. Typically, P25 powder (0.1 g) was mixed with an aqueous solution of NaOH (20 mL, 10 M), followed by a hydrothermal treatment at 180 1C in a 25 mL Teflon-lined autoclave for 48 h. The treated powder was washed thoroughly with deionized water followed by filtration and drying processes. The obtained Na2Ti3O7 nanobelts were then immersed in an aqueous solution of 0.1 M HCl for 48 h and then washed thoroughly with water to produce H2Ti3O7 nanobelts. The H2Ti3O7 nanobelts were added into a 25 mL Teflon vessel, which was filled with an aqueous solution of H2SO4 (0.02 M) up to 80% of the total volume and maintained at 100 1C for 12 h. Finally, the products were isolated from the solution by centrifugation and sequentially washed with deionized water several times, and dried at 70 1C for 10 h. Thermal annealing of the H2Ti3O7 nanobelts by acid corrosion at 600 1C for 2 h led to the production of TiO2 nanobelts with roughened surfaces. 2.3. Preparation of Ru/TiO2 NB heterostructures Ru/TiO2 NB heterostructures (2 wt %) were prepared by a photo-reduction method. TiO2 NBs (0.2 g) were added to ethanol solution (10 mL) under magnetic stirring. Then RuCl3 (10 mL, 3.09 mmol L 1) ethanol solution was added to the suspension, and the mixture was stirred for 30 min. The obtained mixing solution was illuminated with a 300 W ultraviolet lamp for 1 h under magnetic agitation. The product was washed with deionized water and ethanol to remove any ionic residual, and then dried in a vacuum oven at 70 1C for 12 h. 2.4. Ru/TiO2 NB heterostructure nanopapers Ru/TiO2 NB heterostructure nanopapers were fabricated by a modified paper-making process. Briefly, 0.2 g Ru/TiO2 NB heterostructures without any surfactants were dispersed in 200 mL distilled water under magnetic stirring, followed by filtering the resultant pulp on a sub-microporous filter paper (pore size of 0.22 μm) surface through a vacuum-filter (SHBIII, Gongyi, the vacuum is 0.1 MPa) with ceramic filter (pore size of 0.45 μm). 2.5. Characterizations
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Avatar 370 infrared spectrometer in the range 400–4000 cm 1 using pressed KBr pellets. The KBr pellets technique was used where 10 mg of each sample is mixed with 1000 mg of KBr in an agate mortar. From this stock, 200 mg were then pressed into pellets of 13 mm diameter. UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV–vis spectrophotometer (UV-2550, Shimadzu) with an integrating sphere attachment within the range of 200–700 nm and with BaSO4 as the reflectance standard. 2.6. Photocatalytic activity test The photocatalytic activity of the Ru/TiO2 NB heterostructures was investigated by means of the photodegradation of methyl orange (MO, 20 mg/L). In a typical measurement, 20 mL of an aqueous suspension of MO and 20 mg of the heterostructure powders were placed in a 50 mL beaker and were conducted in an XPA-photochemical reactor (Xujiang Electromech-anical Plant, Nanjing, China). Prior to photoirradiation, the suspensions were magnetically stirred in the dark for 30 min to establish adsorption–desorption equilibrium between the dye and the surface of the catalysts under ambient conditions. A 350 W mercury lamp with a maximum emission at 356 nm was used as the UV source, a 300 W Xe arc lamp as the visible light source where the UV components were filtered out during visible light photocatalysis. At varied irradiation time intervals, an aliquot of the mixed solution was collected and centrifuged, and the residual MO concentration in the supernatant was analyzed by UV–vis spectroscopic measurements (Hitachi UV-3100). 2.7. Catalytic reaction tests The gas-phase selective oxidation of benzyl alcohol was carried out with a continuous-flow fixed-bed microreactor (8 mm i.d.). In the catalytic test, four pieces of Ru/TiO2 NB heterostructure nanopapers (8 mm in diameter, total weight 20 mg) were stacked vertically on a Teflon ring fixed to the inner wall of the reactor, and the process was carried out at atmospheric pressure and a lower reaction temperature of 240 1C. The gas streams (44 mL/min, O2/N2 ¼ 21:79) were supplied by mass flow controllers, and benzyl alcohol (20 μL/ min, WHSV of 63 h 1) was fed continuously through a syringe pump. Liquid vaporization occurred in the preheater prior to the catalytic reaction bed. The condensable reaction products and the unreacted benzyl alcohol were cooled and collected using a cold trap (0 1C). Then the mixture was analyzed with a Shimadzu Type GC-14C equipped with a flame ionization detector, using a SGE-30QC2/AC5 capillary column and N2 as carrier gas. 1-Butanol was employed as an internal standard. GC-MS (Thermo Trace GC Ultra DSQ) was also employed to determine the reaction products.
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nanoparticles (NPs) were assembled onto the surface of TiO2 NBs using a simple photo-reduction method to obtain the Ru/ TiO2 NB heterostructures. The powder X-ray diffraction (XRD) patterns of TiO2 NBs and Ru/TiO2 NB heterostructures are shown in Fig. 1. In curve a, all of the diffraction peaks at 2θ ¼ 25.281, 37.801, 48.051, 53.891, 55.061, and 62.691 can be readily indexed to anatase phase of TiO2 (PDF-21-1272) [17]. For the XRD pattern of Ru/TiO2 NB heterostructures (curve b), in addition to the diffraction peaks of TiO2, no characteristic peak for Ru can be observed. The reasonable interpretation should be that the Ru components are uniformly dispersed in a way with rather a small particle size that the X-rays are insensitive to detect them. The morphology and crystalline property of the as-prepared samples are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which are shown in Figs. 2 and 3. A panoramic SEM image shows that the TiO2 consists of uniform belts with width of 50– 200 nm, and lengths of up to several of micrometers (Fig. 2a). The magnified SEM images (Fig. 2b) further reveal that these NBs possess rough surface, which exhibit a large specific surface area and provide numerous nucleation sites for the growth of Ru NPs. The SEM image of Ru/TiO2 NB heterostructures is shown in Fig. 2c. Ru NPs are not clearly observed on the surface of TiO2 NBs. Regarding the TEM images, described in detail below, there are highly crystalline Ru NPs with a diameter of o 5 nm. The very small size of these particles beyond current SEM resolution limits. As shown in the low magnification TEM image (Fig. 3a), the Ru/TiO2 NB heterostructures still keep the onedimensional (1D) structure. The successful formation of Ru/ TiO2 NB heterostructures is confirmed by the magnified TEM image (Fig. 3b). The TiO2 NBs are homogeneously covered by numerous Ru NPs with several nanometers in size. The particle sizes of Ru (Fig. 4) are about 1.5 7 0.3 nm. It is the 1D structure of TiO2 NBs that prevents Ru particles form aggregation in the photo-reduction procedure. Then the welldistributed Ru NPs with a small size are obtained [23,24], suggesting the formation of the Ru/TiO2 NB heterostructure, which facilitate the charge transfer in the heterostructure
3. Results and discussion In the preparation of the catalyst, TiO2 nanobelts (NBs) were first synthesized by a hydrothermal method. Then the Ru
Fig. 1. XRD patterns of (a) TiO2 NBs and (b) Ru/TiO2 NB heterostructures.
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Fig. 2. SEM images of (a, b) TiO2 NBs and (c) Ru/TiO2 NB heterostructures.
Fig. 3. TEM images of Ru/TiO2 NB heterostructures.
photocatalyst. Consequently, the photocatalytic performance is positively influenced. The optical properties of pristine TiO2 NBs and Ru/TiO2 NB heterostructures were investigated by UV–vis absorption spectra. As shown in Fig. 5, TiO2 NBs have been already shown to have high absorption in the UV part of the spectrum (3.2 eV for anatase TiO2), which can be confirmed from the measured absorption edge at 380 nm (black line). Interestingly, Ru/TiO2 NB heterostructures display surprisingly strong absorption in the UV and visible light region due to the metallic electronic structure of Ru with partially filled band which can absorb light in the broad UV–vis range (red line). This indicates that the photocatalysis might cover the UV and visible light range. To examine the photocatalytic behavior of the Ru/TiO2 NB heterostructures, the degradation under UV and visible light irradiation of aqueous methyl orange (MO) was tested as a model system. For comparison, the respective performance of P25 and TiO2 NBs was also measured. The solution was stirred in the dark for 30 min to achieve the equilibrium adsorption. Furthermore, blank experiments in the absence of irradiation with the photocatalyst (Fig. 6) or in the presence of UV and visible light irradiation without the photocatalyst (Fig. S1 in Supporting information) were carried out to rationalize the photocatalytic activity of the Ru/TiO2 NB heterostructures. Fig. S1 shows that MO can only be slightly degraded under UV and visible light irradiation without catalysts, indicating that MO is a stable molecule and that the photolysis mechanism can be ignored. At the same time, our experimental data in the MO decolorization with photocatalyst in the dark show that the dye photosensitization
Fig. 4. Particle size distribution of Ru NPs in Ru/TiO2 NB heterostructures.
Fig. 5. UV–vis absorption spectra of TiO2 NBs and Ru/TiO2 NB heterostructures. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Fig. 6. Photocatalytic degradation of MO for P25, TiO2 NBs and Ru/TiO2 NB heterostructures under (a) UV and (b) visible light irradiation.
Fig. 7. Stability examination of photocatalytic degradation of MO for Ru/TiO2 NB heterostructures under (a) UV and (b) visible light irradiation.
process is not obvious. Therefore, the dye photosensitization process can be ignored in our experiments. The above blank experiments demonstrate that the degradation reaction was truly driven by a photocatalytsis process. FTIR spectra also suggest that the MO decolorization is due to the photocatalytic reaction of Ru/TiO2 NB heterostructures instead of the adsorption (Fig. S2 in Supporting information). P25 and TiO2 NBs are good UV light active photocatalysts, which can be attributed to the large bandgap. Extraordinarily, under UV light irradiation for 25 min, Ru/TiO2 NB heterostructures exhibit enhanced UV photocatalytic activity (Fig. 6a) with 84% of MO degraded, whereas it takes 25 min to decompose 66% MO for P25 and 63% MO for pure TiO2 NBs. This result can be attributed to the Ru loading as well as the positioning of the Ru NPs on the surface the TiO2 NBs, which enable Ru NPs to act as efficient electron storage system, promote the migration of photogenerated electrons from TiO2 to Ru and suppress the recombination of separated electrons and holes. Under visible light irradiation for 25 min, neither P25 nor TiO2 NBs shows good visible photocatalytic activity, with 17% and 26% of MO degraded, respectively (Fig. 6b). In contrast, the Ru/TiO2 heterostructures exhibit improved visible photocatalytic performance, and the degree of decomposition increases to 45% after visible light irradiation for 25 min. The excellent photocatalytic performance of Ru/TiO2 NB
heterostructures can be attributed to the improved visible light absorption of Ru NPs, resulting in an increase in the activity of Ru/TiO2 NB heterostructures. The photostability of the Ru/TiO2 NB heterostructures was tested by measuring the degradation of MO by four consecutive operations. After each run, the photocatalyst was recovered by centrifugation and re-dispersed in a new MO water solution. As shown in Fig. 7, the photocatalytic degradation rate of MO increases steadily with irradiation time, without an apparent decrease after four runs under UV and visible light irradiation. This result suggests the high stability of Ru/TiO2 NB heterostructures during the photocatalytic process. Firstly, four kinds of TiO2 nanopapers were prepared with P25, TiO2 NBs, Ru/bulk TiO2 (P25) heterostructures and Ru/TiO2 NB heterostructures by a modified papermaking process [25]. The diameter of TiO2 nanopapers is about 8 cm. TiO2 nanopapers can be used as the monolithic catalyst in the selective gas-phase oxidation of benzyl alcohol, which is shown in Fig. 8. For the P25, TiO2 NB, and Ru/P25 heterostructure nanopapers, the benzyl alcohol conversion is 11%, 10%, and 58.3% after 5 h, respectively (Fig. 8a). In the case of Ru/TiO2 NB heterostructure nanopapers, however, the maximum conversion of 72% is obtained at the same reaction time, which is significantly better than that of P25, TiO2 NB and Ru/P25 heterostructure. The ultra-small particle size and high density of the supported Ru NPs on the surface of
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Fig. 8. The (a) yield and (b) selectivity of benzaldehyde for benzyl alcohol oxidation over nanopaper catalysts composed of P25, TiO2 NBs, Ru/bulk TiO2 (P25) heterostructures and Ru/TiO2 NB heterostructures, respectively.
Scheme 1. Schematic illustration for the charge separation of Ru/TiO2 NB heterostructures under UV and visible light irradiation.
one-dimensional TiO2 NBs prevent agglomeration of metal catalysts, accounting for the high activity of Ru/TiO2 NB heterostructures. Moreover, it should be noted that all of nanopapers have very satisfactory selectivity towards benzyl alcohols oxidation. Among them, the Ru/TiO2 NB heterostructure nanopaper presents the higher selectivity (81%) than that of P25 (69.2%), TiO2 NBs (70%), and Ru/P25 heterostructure (75.3%) (Fig. 8b). The results conclusively demonstrate that the 1D TiO2 NBs are a desirable support for the nanoscale metal catalysts. On the basis of the above experimental results and discussion, the mechanism of enhanced photocatalytic activity over the Ru/TiO2 NB heterostructures is proposed (Scheme 1). The enhanced photocatalytic activity for Ru/TiO2 NB heterostructures can be ascribed to the Schottky barrier effect of Ru NPs. The Schottky barrier produced at a metal-semiconductor interface serves as an efficient electron trap. Ru=TiO2 þ hv-Ru=TiO2 ðeCB þ hVB Þ-RuðeCB Þ=TiO2 ðhVB Þ ð1Þ RuðeCB Þþ O2 -Ru þ O2 ∙
ð2Þ
TiO2 ðhVB Þ þ H2 O-TiO2 ðhVB Þ þ H þ þ OH -TiO2 þ H þ þ ∙OH
ð3Þ
Under UV light irradiation, the electron and hole pairs are produced in TiO2. Electrons are excited to the conduction band (CB), leaving holes in the valence band (VB). The work function of Ru metals ( 4.71 eV) is more negative that the
CB edge of TiO2 ( 4.21 eV) [26,27]. The electron in the CB may recombine with the holes or transfer from TiO2 to the Ru metals (Eq. 1). The enriched electrons on the surface of Ru could be trapped by molecular oxygen in solution to form O2 (Eq. 2). At the same time, the holes on the surface of TiO2 can react with H2O to generate OH (Eq. 3). Both O2 and OH radicals can degrade MO dye. As a result, the formation of the Schottky barrier at the Ru–TiO2 heterojunction helps to separate the photoexcited electrons and holes, and the photocatalytic efficiency of Ru/TiO2 NB heterostructures is greatly enhanced. Under visible-light irradiation, the plasmonic and quantumsize effect of Ru NPs make the Ru/TiO2 NB heterostructures present enhanced visible absorption, which is determined to be the main reason for the enhanced visible photocatalytic activity. 4. Conclusions In summary, a novel and stable Ru/TiO2 NB heterostructured catalyst has been successfully synthesized by a simple photoreduction method. Ru clusters of ca. 1.570.3 nm are spontaneously and uniformly deposited on the surface of TiO2 NB, thus the Ru/TiO2 NB heterostructures are formed. Due to the Schottky barrier effect of Ru clusters, the Ru/TiO2 NB heterostructures manifest an enhanced absorption in the UV–vis region and an efficient charge transfer of the photogenerated carriers from the semiconductor to the metal. This has led to a significant increase in photocatalytic activity compared to P25 and TiO2 NBs under UV and visible light irradiation. In addition, the nanopapers based on Ru/TiO2 NB heterostructures are fabricated via a modified paper-making process. The nanopaper catalysts based on the Ru/TiO2 NB heterostructures exhibit superior yield and selectivity for gas-phase selective oxidation of benzyl alcohol. The TiO2 NB support can ensure high catalytic activity and selectivity by providing the platform for the good dispersion of nanosize Ru NPs and preventing the Ru NPs from aggregating or sintering. This work not only provides a facile method for the preparation of highly dispersed ultra-small Ru NPs on 1D TiO2 NBs that serving as an excellent photocatalyst, but also demonstrates that the suitable choice of a metal oxide support is very important for the
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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improvement of the heterogeneous catalytic activity and selectivity of metal NPs. Acknowledgments The authors are thankful for fundings from National Natural Science Foundation of China (Nos. 51502160, 51372142 and 51272141), Natural Science Foundation of Shandong Province (No. ZR2015EQ001), Taishan Scholars Project of Shandong Province (No. TS20110828), National High Technology Research and Development Program of China (863 Program, No. 2015AA034404), Innovation Research Group (IRG: 51321091), and the Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint. 2015.09.112.
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Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
CERAMICS INTERNATIONAL
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Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol Jian Tiana,b,n, Jian Lia, Na Weia, Xiaohong Xuc, Hongzhi Cuia,n, Hong Liub,n a
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China c School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Received 13 August 2015; received in revised form 3 September 2015; accepted 19 September 2015
Abstract One-dimensional (1D) TiO2 nanostructures can be used as the support of metal particles to form the heterostructures, which can prevent the metal agglomeration and enhance the activity of metal in various applications. Here, well-dispersed Ru nanoparticles (NPs) supported on 1D TiO2 nanobelts (NBs) are firstly synthesized by a facile photo-reduction method. The as-synthesized Ru/TiO2 NB heterostructures exhibit an improved photocatalytic performance compared with P25 and TiO2 under both UV and visible light irradiation. Our mechanistic investigation reveals that the enhanced photocatalytic activity can be attributed to the Schottky barrier effect of Ru NPs, which efficiently harvest the electrons of TiO2 and enhance the separation of photogenerated electron–hole pairs. Moreover, the Ru/TiO2 NB heterostructures are assembled into the porous nanopaper via a modified paper-making process, which can be applied in heterogeneous catalysis. The formed Ru/TiO2 NB heterostructure nanopapers exhibit enhanced activity and selectivity for catalyzing aerobic oxidation of benzyl alcohol. Ru NPs are well dispersed with the average size of about 1.5 nm on the TiO2 NBs support, which could contribute to the high catalytic activity of Ru/TiO2 catalyst. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Schottky barrier effect; Heterostructures; Photocatalytic; Nanopaper; Gas phase oxidation catalysis
1. Introduction As a large electron gap semiconductor, TiO2 has aroused great attention due to its good chemical and thermal stability, and excellent electronic and optical properties, which has been used in numerous potential applications, such as photovoltaic devices, photodegradation of pollutants, photocatalytic watersplitting for hydrogen production, and heterogeneous catalysis [1–3]. Among various synthesized TiO2 nanostructures, onedimensional (1D) TiO2 nanostructures, for example, TiO2 nanobelts (NBs), are also extensively explored in the field of solar energy or catalysis due to their stable 1D nanostructure n
Corresponding authors. E-mail addresses: [email protected] (J. Tian), [email protected] (H. Cui), [email protected] (H. Liu).
and fast electron transfer [4–5]. Besides, this 1D nanostructure can easily assemble metal or metal oxide to form the heterostructure based on the band structure matching, P–N junction, surface plasmon resonance, or Schottky barrier effect [6], which gets some new properties because of the existence of the second phase, ensures a fast and efficient charge transfer between the two phases, and broadens the light absorption band to the visible region [7]. Photocatalysis provides a cost effective method for both renewable energy synthesis and environmental purification [8]. A large amount of materials, such as TiO2, ZnO and CdS, has been developed by researchers, which are considered as promising light-absorbing materials in photocatalysis, because of their versatile optical and electrical characteristics [9]. However, their poor visible light utilization, few surface active sites, as well as the high recombination of photogenerated
http://dx.doi.org/10.1016/j.ceramint.2015.09.112 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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electron–hole pairs greatly limit their practical applications [10]. Combining the semiconductor with highly dispersed noble metal nanoparticles (NPs), such as Au, Pd, and Ru, that can form a new Fermi level and promote the effective charge carrier transfers, which is a feasible approach for enhancing the photocatalytic performance [11]. Ruthenium (Ru), as one of the noble metals, has been investigated widely due to their unique properties, such as stability, ease of preparation, and remarkable electronic and optical properties, which are of great interest in the technological applications [12]. Ru NPs enable size dependent storage of electrons, which can then be released to the suitable acceptors [13]. Recently, it has been reported that electron transfer from TiO2 to Ru NPs occurred under irradiation of light due to the Schottky barrier effect at the metal-oxide interface [14]. However, in most studies on Ru/TiO2 nanocomposites, the preparation processes are very complicated and often lead to the lack of control over the size distribution [15]. Their practical applications are still hindered by the high price of precious metals and the agglomeration problem of metal NPs [16]. Thus minimizing the metal NPs loading while simultaneously improving the photocatalytic activity and stability of these materials are of vital importance in photocatalysis or heterogeneous catalysis [17]. Moreover, Ru is a very powerful oxidation catalyst and wellknown for its application in heterogeneous catalysis [18]. The catalytic properties of Ru NPs are strongly dependent on their size. Smaller metal particles have larger specific surface area and larger quantity of highly active sites, thus present higher catalytic activity [19]. However, the small size of Ru NPs tends to sinter during the catalytic process at elevated temperature and cause deactivation [20]. So Ru NPs need to be well dispersed on the support to achieve high mass activity and resistance to aggregation [21]. 1D TiO2 nanostructures can be used as the support materials for noble metal or metal oxide catalysts because of their large surface area, high stability in basic or acidic media, and ability to assemble above catalysts [22]. In the present work, we detail a facile photo-reduction method to construct Ru/TiO2 NB heterostructures with strongly electrically coupled interfaces. The well dispersed and ultra-small Ru NPs are uniformly distributed on the surface of TiO2 NBs. Ru is the metal chosen due to its potential for Schottky barrier enhancement of photocatalysis and as an excellent catalyst on the TiO2 support for heterogeneous catalysis. Ru/TiO2 NB heterostructures are tested for photodegradation of pollutants in solution using methyl orange (MO) as the model system under UV and visible light irradiation, and for gas-phase selective oxidation of benzyl alcohol.
(H2SO4), and ruthenium chloride trihydrate (RuCl3 3H2O) were purchased from Sinopharm and used without further treatment.
2. Experimental section
X-ray powder diffraction (XRD) pattern of catalysts were recorded on a Bruke D8 Advance powder X-ray diffractometer with Cu Kα (λ¼ 0.15406 nm). HITACHI S-4800 field emission scanning electron microscope (FE-SEM) was used to characterize the morphologies and size of the synthesized samples. High resolution transmission electron microscopy (HRTEM) images were carried out with a JOEL JEM 2100 microscope. Fourier transform infrared (FTIR) spectra were collected on a Nicolet
2.1. Materials The chemicals used in this work were of analytical reagent grade. Solutions were freshly prepared with deionized water. Titania P25 (TiO2; ca. 80% anatase, and 20% rutile), sodium hydroxide (NaOH), hydrochloric acid (HCl), sulfuric acid
2.2. Preparation of TiO2 nanobelts TiO2 nanobelts were synthesized by a hydrothermal procedure. Typically, P25 powder (0.1 g) was mixed with an aqueous solution of NaOH (20 mL, 10 M), followed by a hydrothermal treatment at 180 1C in a 25 mL Teflon-lined autoclave for 48 h. The treated powder was washed thoroughly with deionized water followed by filtration and drying processes. The obtained Na2Ti3O7 nanobelts were then immersed in an aqueous solution of 0.1 M HCl for 48 h and then washed thoroughly with water to produce H2Ti3O7 nanobelts. The H2Ti3O7 nanobelts were added into a 25 mL Teflon vessel, which was filled with an aqueous solution of H2SO4 (0.02 M) up to 80% of the total volume and maintained at 100 1C for 12 h. Finally, the products were isolated from the solution by centrifugation and sequentially washed with deionized water several times, and dried at 70 1C for 10 h. Thermal annealing of the H2Ti3O7 nanobelts by acid corrosion at 600 1C for 2 h led to the production of TiO2 nanobelts with roughened surfaces. 2.3. Preparation of Ru/TiO2 NB heterostructures Ru/TiO2 NB heterostructures (2 wt %) were prepared by a photo-reduction method. TiO2 NBs (0.2 g) were added to ethanol solution (10 mL) under magnetic stirring. Then RuCl3 (10 mL, 3.09 mmol L 1) ethanol solution was added to the suspension, and the mixture was stirred for 30 min. The obtained mixing solution was illuminated with a 300 W ultraviolet lamp for 1 h under magnetic agitation. The product was washed with deionized water and ethanol to remove any ionic residual, and then dried in a vacuum oven at 70 1C for 12 h. 2.4. Ru/TiO2 NB heterostructure nanopapers Ru/TiO2 NB heterostructure nanopapers were fabricated by a modified paper-making process. Briefly, 0.2 g Ru/TiO2 NB heterostructures without any surfactants were dispersed in 200 mL distilled water under magnetic stirring, followed by filtering the resultant pulp on a sub-microporous filter paper (pore size of 0.22 μm) surface through a vacuum-filter (SHBIII, Gongyi, the vacuum is 0.1 MPa) with ceramic filter (pore size of 0.45 μm). 2.5. Characterizations
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Avatar 370 infrared spectrometer in the range 400–4000 cm 1 using pressed KBr pellets. The KBr pellets technique was used where 10 mg of each sample is mixed with 1000 mg of KBr in an agate mortar. From this stock, 200 mg were then pressed into pellets of 13 mm diameter. UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV–vis spectrophotometer (UV-2550, Shimadzu) with an integrating sphere attachment within the range of 200–700 nm and with BaSO4 as the reflectance standard. 2.6. Photocatalytic activity test The photocatalytic activity of the Ru/TiO2 NB heterostructures was investigated by means of the photodegradation of methyl orange (MO, 20 mg/L). In a typical measurement, 20 mL of an aqueous suspension of MO and 20 mg of the heterostructure powders were placed in a 50 mL beaker and were conducted in an XPA-photochemical reactor (Xujiang Electromech-anical Plant, Nanjing, China). Prior to photoirradiation, the suspensions were magnetically stirred in the dark for 30 min to establish adsorption–desorption equilibrium between the dye and the surface of the catalysts under ambient conditions. A 350 W mercury lamp with a maximum emission at 356 nm was used as the UV source, a 300 W Xe arc lamp as the visible light source where the UV components were filtered out during visible light photocatalysis. At varied irradiation time intervals, an aliquot of the mixed solution was collected and centrifuged, and the residual MO concentration in the supernatant was analyzed by UV–vis spectroscopic measurements (Hitachi UV-3100). 2.7. Catalytic reaction tests The gas-phase selective oxidation of benzyl alcohol was carried out with a continuous-flow fixed-bed microreactor (8 mm i.d.). In the catalytic test, four pieces of Ru/TiO2 NB heterostructure nanopapers (8 mm in diameter, total weight 20 mg) were stacked vertically on a Teflon ring fixed to the inner wall of the reactor, and the process was carried out at atmospheric pressure and a lower reaction temperature of 240 1C. The gas streams (44 mL/min, O2/N2 ¼ 21:79) were supplied by mass flow controllers, and benzyl alcohol (20 μL/ min, WHSV of 63 h 1) was fed continuously through a syringe pump. Liquid vaporization occurred in the preheater prior to the catalytic reaction bed. The condensable reaction products and the unreacted benzyl alcohol were cooled and collected using a cold trap (0 1C). Then the mixture was analyzed with a Shimadzu Type GC-14C equipped with a flame ionization detector, using a SGE-30QC2/AC5 capillary column and N2 as carrier gas. 1-Butanol was employed as an internal standard. GC-MS (Thermo Trace GC Ultra DSQ) was also employed to determine the reaction products.
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nanoparticles (NPs) were assembled onto the surface of TiO2 NBs using a simple photo-reduction method to obtain the Ru/ TiO2 NB heterostructures. The powder X-ray diffraction (XRD) patterns of TiO2 NBs and Ru/TiO2 NB heterostructures are shown in Fig. 1. In curve a, all of the diffraction peaks at 2θ ¼ 25.281, 37.801, 48.051, 53.891, 55.061, and 62.691 can be readily indexed to anatase phase of TiO2 (PDF-21-1272) [17]. For the XRD pattern of Ru/TiO2 NB heterostructures (curve b), in addition to the diffraction peaks of TiO2, no characteristic peak for Ru can be observed. The reasonable interpretation should be that the Ru components are uniformly dispersed in a way with rather a small particle size that the X-rays are insensitive to detect them. The morphology and crystalline property of the as-prepared samples are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which are shown in Figs. 2 and 3. A panoramic SEM image shows that the TiO2 consists of uniform belts with width of 50– 200 nm, and lengths of up to several of micrometers (Fig. 2a). The magnified SEM images (Fig. 2b) further reveal that these NBs possess rough surface, which exhibit a large specific surface area and provide numerous nucleation sites for the growth of Ru NPs. The SEM image of Ru/TiO2 NB heterostructures is shown in Fig. 2c. Ru NPs are not clearly observed on the surface of TiO2 NBs. Regarding the TEM images, described in detail below, there are highly crystalline Ru NPs with a diameter of o 5 nm. The very small size of these particles beyond current SEM resolution limits. As shown in the low magnification TEM image (Fig. 3a), the Ru/TiO2 NB heterostructures still keep the onedimensional (1D) structure. The successful formation of Ru/ TiO2 NB heterostructures is confirmed by the magnified TEM image (Fig. 3b). The TiO2 NBs are homogeneously covered by numerous Ru NPs with several nanometers in size. The particle sizes of Ru (Fig. 4) are about 1.5 7 0.3 nm. It is the 1D structure of TiO2 NBs that prevents Ru particles form aggregation in the photo-reduction procedure. Then the welldistributed Ru NPs with a small size are obtained [23,24], suggesting the formation of the Ru/TiO2 NB heterostructure, which facilitate the charge transfer in the heterostructure
3. Results and discussion In the preparation of the catalyst, TiO2 nanobelts (NBs) were first synthesized by a hydrothermal method. Then the Ru
Fig. 1. XRD patterns of (a) TiO2 NBs and (b) Ru/TiO2 NB heterostructures.
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Fig. 2. SEM images of (a, b) TiO2 NBs and (c) Ru/TiO2 NB heterostructures.
Fig. 3. TEM images of Ru/TiO2 NB heterostructures.
photocatalyst. Consequently, the photocatalytic performance is positively influenced. The optical properties of pristine TiO2 NBs and Ru/TiO2 NB heterostructures were investigated by UV–vis absorption spectra. As shown in Fig. 5, TiO2 NBs have been already shown to have high absorption in the UV part of the spectrum (3.2 eV for anatase TiO2), which can be confirmed from the measured absorption edge at 380 nm (black line). Interestingly, Ru/TiO2 NB heterostructures display surprisingly strong absorption in the UV and visible light region due to the metallic electronic structure of Ru with partially filled band which can absorb light in the broad UV–vis range (red line). This indicates that the photocatalysis might cover the UV and visible light range. To examine the photocatalytic behavior of the Ru/TiO2 NB heterostructures, the degradation under UV and visible light irradiation of aqueous methyl orange (MO) was tested as a model system. For comparison, the respective performance of P25 and TiO2 NBs was also measured. The solution was stirred in the dark for 30 min to achieve the equilibrium adsorption. Furthermore, blank experiments in the absence of irradiation with the photocatalyst (Fig. 6) or in the presence of UV and visible light irradiation without the photocatalyst (Fig. S1 in Supporting information) were carried out to rationalize the photocatalytic activity of the Ru/TiO2 NB heterostructures. Fig. S1 shows that MO can only be slightly degraded under UV and visible light irradiation without catalysts, indicating that MO is a stable molecule and that the photolysis mechanism can be ignored. At the same time, our experimental data in the MO decolorization with photocatalyst in the dark show that the dye photosensitization
Fig. 4. Particle size distribution of Ru NPs in Ru/TiO2 NB heterostructures.
Fig. 5. UV–vis absorption spectra of TiO2 NBs and Ru/TiO2 NB heterostructures. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Fig. 6. Photocatalytic degradation of MO for P25, TiO2 NBs and Ru/TiO2 NB heterostructures under (a) UV and (b) visible light irradiation.
Fig. 7. Stability examination of photocatalytic degradation of MO for Ru/TiO2 NB heterostructures under (a) UV and (b) visible light irradiation.
process is not obvious. Therefore, the dye photosensitization process can be ignored in our experiments. The above blank experiments demonstrate that the degradation reaction was truly driven by a photocatalytsis process. FTIR spectra also suggest that the MO decolorization is due to the photocatalytic reaction of Ru/TiO2 NB heterostructures instead of the adsorption (Fig. S2 in Supporting information). P25 and TiO2 NBs are good UV light active photocatalysts, which can be attributed to the large bandgap. Extraordinarily, under UV light irradiation for 25 min, Ru/TiO2 NB heterostructures exhibit enhanced UV photocatalytic activity (Fig. 6a) with 84% of MO degraded, whereas it takes 25 min to decompose 66% MO for P25 and 63% MO for pure TiO2 NBs. This result can be attributed to the Ru loading as well as the positioning of the Ru NPs on the surface the TiO2 NBs, which enable Ru NPs to act as efficient electron storage system, promote the migration of photogenerated electrons from TiO2 to Ru and suppress the recombination of separated electrons and holes. Under visible light irradiation for 25 min, neither P25 nor TiO2 NBs shows good visible photocatalytic activity, with 17% and 26% of MO degraded, respectively (Fig. 6b). In contrast, the Ru/TiO2 heterostructures exhibit improved visible photocatalytic performance, and the degree of decomposition increases to 45% after visible light irradiation for 25 min. The excellent photocatalytic performance of Ru/TiO2 NB
heterostructures can be attributed to the improved visible light absorption of Ru NPs, resulting in an increase in the activity of Ru/TiO2 NB heterostructures. The photostability of the Ru/TiO2 NB heterostructures was tested by measuring the degradation of MO by four consecutive operations. After each run, the photocatalyst was recovered by centrifugation and re-dispersed in a new MO water solution. As shown in Fig. 7, the photocatalytic degradation rate of MO increases steadily with irradiation time, without an apparent decrease after four runs under UV and visible light irradiation. This result suggests the high stability of Ru/TiO2 NB heterostructures during the photocatalytic process. Firstly, four kinds of TiO2 nanopapers were prepared with P25, TiO2 NBs, Ru/bulk TiO2 (P25) heterostructures and Ru/TiO2 NB heterostructures by a modified papermaking process [25]. The diameter of TiO2 nanopapers is about 8 cm. TiO2 nanopapers can be used as the monolithic catalyst in the selective gas-phase oxidation of benzyl alcohol, which is shown in Fig. 8. For the P25, TiO2 NB, and Ru/P25 heterostructure nanopapers, the benzyl alcohol conversion is 11%, 10%, and 58.3% after 5 h, respectively (Fig. 8a). In the case of Ru/TiO2 NB heterostructure nanopapers, however, the maximum conversion of 72% is obtained at the same reaction time, which is significantly better than that of P25, TiO2 NB and Ru/P25 heterostructure. The ultra-small particle size and high density of the supported Ru NPs on the surface of
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
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Fig. 8. The (a) yield and (b) selectivity of benzaldehyde for benzyl alcohol oxidation over nanopaper catalysts composed of P25, TiO2 NBs, Ru/bulk TiO2 (P25) heterostructures and Ru/TiO2 NB heterostructures, respectively.
Scheme 1. Schematic illustration for the charge separation of Ru/TiO2 NB heterostructures under UV and visible light irradiation.
one-dimensional TiO2 NBs prevent agglomeration of metal catalysts, accounting for the high activity of Ru/TiO2 NB heterostructures. Moreover, it should be noted that all of nanopapers have very satisfactory selectivity towards benzyl alcohols oxidation. Among them, the Ru/TiO2 NB heterostructure nanopaper presents the higher selectivity (81%) than that of P25 (69.2%), TiO2 NBs (70%), and Ru/P25 heterostructure (75.3%) (Fig. 8b). The results conclusively demonstrate that the 1D TiO2 NBs are a desirable support for the nanoscale metal catalysts. On the basis of the above experimental results and discussion, the mechanism of enhanced photocatalytic activity over the Ru/TiO2 NB heterostructures is proposed (Scheme 1). The enhanced photocatalytic activity for Ru/TiO2 NB heterostructures can be ascribed to the Schottky barrier effect of Ru NPs. The Schottky barrier produced at a metal-semiconductor interface serves as an efficient electron trap. Ru=TiO2 þ hv-Ru=TiO2 ðeCB þ hVB Þ-RuðeCB Þ=TiO2 ðhVB Þ ð1Þ RuðeCB Þþ O2 -Ru þ O2 ∙
ð2Þ
TiO2 ðhVB Þ þ H2 O-TiO2 ðhVB Þ þ H þ þ OH -TiO2 þ H þ þ ∙OH
ð3Þ
Under UV light irradiation, the electron and hole pairs are produced in TiO2. Electrons are excited to the conduction band (CB), leaving holes in the valence band (VB). The work function of Ru metals ( 4.71 eV) is more negative that the
CB edge of TiO2 ( 4.21 eV) [26,27]. The electron in the CB may recombine with the holes or transfer from TiO2 to the Ru metals (Eq. 1). The enriched electrons on the surface of Ru could be trapped by molecular oxygen in solution to form O2 (Eq. 2). At the same time, the holes on the surface of TiO2 can react with H2O to generate OH (Eq. 3). Both O2 and OH radicals can degrade MO dye. As a result, the formation of the Schottky barrier at the Ru–TiO2 heterojunction helps to separate the photoexcited electrons and holes, and the photocatalytic efficiency of Ru/TiO2 NB heterostructures is greatly enhanced. Under visible-light irradiation, the plasmonic and quantumsize effect of Ru NPs make the Ru/TiO2 NB heterostructures present enhanced visible absorption, which is determined to be the main reason for the enhanced visible photocatalytic activity. 4. Conclusions In summary, a novel and stable Ru/TiO2 NB heterostructured catalyst has been successfully synthesized by a simple photoreduction method. Ru clusters of ca. 1.570.3 nm are spontaneously and uniformly deposited on the surface of TiO2 NB, thus the Ru/TiO2 NB heterostructures are formed. Due to the Schottky barrier effect of Ru clusters, the Ru/TiO2 NB heterostructures manifest an enhanced absorption in the UV–vis region and an efficient charge transfer of the photogenerated carriers from the semiconductor to the metal. This has led to a significant increase in photocatalytic activity compared to P25 and TiO2 NBs under UV and visible light irradiation. In addition, the nanopapers based on Ru/TiO2 NB heterostructures are fabricated via a modified paper-making process. The nanopaper catalysts based on the Ru/TiO2 NB heterostructures exhibit superior yield and selectivity for gas-phase selective oxidation of benzyl alcohol. The TiO2 NB support can ensure high catalytic activity and selectivity by providing the platform for the good dispersion of nanosize Ru NPs and preventing the Ru NPs from aggregating or sintering. This work not only provides a facile method for the preparation of highly dispersed ultra-small Ru NPs on 1D TiO2 NBs that serving as an excellent photocatalyst, but also demonstrates that the suitable choice of a metal oxide support is very important for the
Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112
J. Tian et al. / Ceramics International ] (]]]]) ]]]–]]]
improvement of the heterogeneous catalytic activity and selectivity of metal NPs. Acknowledgments The authors are thankful for fundings from National Natural Science Foundation of China (Nos. 51502160, 51372142 and 51272141), Natural Science Foundation of Shandong Province (No. ZR2015EQ001), Taishan Scholars Project of Shandong Province (No. TS20110828), National High Technology Research and Development Program of China (863 Program, No. 2015AA034404), Innovation Research Group (IRG: 51321091), and the Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint. 2015.09.112.
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Please cite this article as: J. Tian, et al., Ru nanoparticles decorated TiO2 nanobelts: A heterostructure towards enhanced photocatalytic activity and gas-phase selective oxidation of benzyl alcohol, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.09.112