TiO2 hollow nanofibers with through-holes as high

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage 2

Coral-like Au/TiO hollow nanofibers with through-holes as high efficient catalyst through mass transfer enhancement Guichu Yue, Shuai Li, Dianming Li, Jing Liu, Yaqiong Wang, Yuchao Zhao, Nü Wang, Zhimin Cui, and Yong Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00004 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Coral-like Au/TiO2 hollow nanofibers with through-holes as high efficient catalyst through mass transfer enhancement Guichu Yue,a Shuai Li,a Dianming Li,a Jing Liu,a Yaqiong Wang,a Yuchao Zhao,b Nü Wang,*a Zhimin Cui,*a Yong Zhaoa a

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of

Ministry of Education, Beijing Key Laboratory of Bioinspired Energy Materials and Devices, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191, P. R. China. b

Shandong Collaborative Innovation Center of Light hydrocarbon

transformation and utilization, College of Chemistry & Chemical Engineering, Yantai University, Yantai, 264005, P. R. China. KEYWORDS. hollow nanofibers, through-holes, Au nanoparticles, mass transfer, coaxial electrospinning

ABSTRACT. One-dimensional hollow nanomaterials were widely used in catalysis field. However, the inner surfaces of one-dimensional hollow nanostructures could not be effectively utilized in liquid reaction due to diffusional limitation caused by the large ratio of length to diameter. In this work, 1 ACS Paragon Plus Environment

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a template assisted coaxial electrospinning method was developed to prepare TiO2 hollow nanofibers with through-holes which was further employed as carrier for Au nanoparticles. The Au/TiO2 hollow nanofibers with through-holes showed significantly catalytic activity enhancement to the reduction of 4nitrophenol in aqueous solution compared with solid and hollow nanofibers counterparts. The through-holes which provided unrestricted macropores for mass transfer in liquid solution were studied to be accounted for the catalytic activity enhancement. The through-hole structures can enlarge the application ranges and efficiencies of 0D or 1D hollow nanomaterials.

INTRODUCTION Hierarchical porous materials have been used in various applications such as catalysis, energy storages, sensors and life science due to their high specific surface areas, high pore volume ratios, high accessibilities, readily mass transfer properties.1-6 For catalytic application, the hierarchical porous structures are especially attractive since they not only possess micropores, mesopores which provide sufficient positions to support catalysts, but also afford macropores to minimize the diffusion resistances and thus enhance mass transfer.7-9 The rational design of heterogeneous catalysts with hierarchical porous structure has the possibility to endow next-generation industrial catalysts with enhanced performance.

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The heterogeneous catalysts commonly include zero-dimensional (0D) nanoparticles or nanospheres, one-dimensional (1D) nanowires, nanotubes or nanofibers,

two-dimensional

nanosheets.10-12

Although

many

0D

heterogeneous catalysts show large specific area, high catalytic activities in many reactions, the tedious costly separation processes and aggregation problems largely limit their practical industrial application. Comparatively, 1D nanocatalysts provided a good balance between catalytic activities and easy separation and recovery capability.13 Among 1D nanocatalysts family, 1D hollow nanocatalysts exhibit intrinsic advantages compared to solid structures. Firstly, the thin walls 1D hollow nanomaterials possess nearly the same inner surface area to outer surface.14 Secondly, hollow structure could effectively reduce materials consuming that are very important for expensive catalysts such as gold, platinum, palladium et al. Thirdly, 1D hollow nanocatalyst provides a confined inner spaces that significantly increase the contact probability between reactant and catalyst. Therefore, various methods were proposed to synthesize 1D hollow nanocatalysts, such as liquid phase method15, gas phase approach16 and electrospinning17, et al. For example, researchers presented a strategy to fabricating 1D hollow mesoporous ZrO2 nanofibers embedded with FeOx on the outer surfaces for the Fenton reaction with

good

performance.

During

the

preparation

process,

magnetic

microspheres were decorated onto the inner surfaces which leading to the magnetic catalysts with good recycling and reusing properties.18 The 1D hollow 3 ACS Paragon Plus Environment

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nanocatalysts have showed improved catalytic performance in gas phase reactions such as highly selective electrchemical reduction of CO2 to C1 products19, high effective NO reduction reaction20. As well as, a Pd loaded WO3 nanotubes with macropores were prepared by template method and had superior hydrogen sensing performance21. However, the diffusion resistance of liquid is much larger than gas which severely restrict the roles played by the inner surfaces in liquid phase catalytic process. Therefore, how to break through the diffusion limitation of liquid phase reaction systems become the obstacle ahead the high efficiency utilization of 1D hollow nanocatalysts. In this respect, coral reef with myriad holes gives us a beneficial inspiration. Corals take calcium ions from sea water and turn to aragonite coral reef with numerous uniform holes. The coral reef framework with holes not only protect tender corals (structural stability), but also ensure efficient metabolism (mass transfer), which exactly cater requirements for ideal catalysts with both high efficiency and good structural stability. Therefore, introducing macropores into 1D hollow nanocatalysts offers an alternative strategy to minimize diffusion resistance, enhance mass transfer, and also potentially enhance the distribution of active sites during catalyst preparation.13 Herein, we designed a coral-like TiO2 hollow through-hole nanofibers (HTHNFs) via a template assisted coaxial electrospinning method. Au nanoparticles were loaded on the outer and inner surfaces of TiO2 HTHNFs to form a supported catalyst. The Au/TiO2 HTHNFs showed greatly improved 4 ACS Paragon Plus Environment

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activity in 4-nitrophenol (4-NP) reduction reaction than Au/TiO2 solid nanofibers or Au/TiO2 hollow nanofibers. This through-holes strategy would play the key role in improvement of 1D hollow catalytic activity because they provided mass transfer for the catalyst loading and catalytic reaction proceeding. EXPERIMENTAL METHODS Synthesize of SiO2 Nanoparticles The SiO2 nanoparticles (NPs) were synthesized by Stöber method.22, 23 2.0 mL of TEOS was added into 50 mL of absolute ethanol and stirred for 3 min. 1.4 mL of ammonia solution was mixed uniformly with 10 mL of deionized water. Then, two above solutions were transferred into an undefiled round-bottom flask to launch the synthetic process. The reaction was conducted under continuous stirring at 40°C for 16 h. At last, the SiO2 NPs were separated and washed three times by absolute ethanol and deionized water to remove the redundant reagents. Preparation of TiO2 Hollow Through-hole Nanofibers In a typical process, a certain amount of SiO2 NPs, 1.5 g PVP and 6.0 g of Ti(OBu)4 were add into a mixed solution of absolute ethanol (10.0 g) and acetic acid (2.0 g) in a conical flask with cover. After continuous stirring for 12 h in icebath, the shell fluid of coaxial electrospinning was obtained. The paraffin liquid was selected as the core fluid of coaxial electrospinning. The spinneret was assembled by two coaxial stainless steel capillaries. The fluids were pumped 5 ACS Paragon Plus Environment

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to the spinneret through pump tubes with individual micro-injection pumps (LSP01-2A, Longer Pump) respectively. The shell fluid was pumped into the outer capillary at flow rate of 3.0 mL/h and the flow rate of core fluid was 1.0 mL/h. The spinneret was connected to the positive of high-voltage generator, and a piece of aluminum foil was grounded to work as a collector. The work distance between the spinneret and collector was 15 cm, and the work voltage was 16 kV. Then, as-prepared nanofibers were calcined at 700°C for 1 h at a heating rate of 5°C/min. After calcination, the SiO2 NPs in the walls of TiO2 hollow nanofibers can be removed by hydrofluoric acid etching to produce through-holes. TiO2 solid nanofibers and hollow nanofibers as the control groups were prepared by uniaxial electrospinning or coaxial electrospinning without SiO2 NPs, respectively. Preparation of Au/TiO2 Hollow Through-hole Nanofibers 100 mg of TiO2 HTHNFs was dispersed in 25 mL of distilled water and ultrasonicated for 30 min. 10 mL of fresh SnCl2 aqueous solution (5 mg/mL, dissolved in 0.02 M HCl) was added into the dispersion liquid and stirred for 10 min. Then the suspension was centrifuged and washed with distilled water for five times to remove the redundant Sn2+. The precipitate was redispersed in 25 mL of distilled water and then 100 μL of 40 mg/mL HAuCl4 aqueous solution was added under stirring. After 10 min, 5 mL of 10.2 mg/mL HCOONa aqueous solution was added into the above reaction system followed by 3 h stirring to


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achieve the complete reduction of AuCl4- to Au NPs. After centrifugation and washing with distilled water, the precipitate was dried at 60°C for 12 h. Au NPs were loaded on the TiO2 solid nanofibers and hollow nanofibers as control groups using the same experiment parameters. Material characterization Scanning electron microscope (SEM) images were obtained on field emission scanning electron microscope (JEOL, JSM-7500F). Transmission electron microscope (TEM) images were obtained on transmission electron microscope (JEOL, JEM-2100F). High angel annular dark field transmission electron microscope (HAADF-TEM) images were obtained on spherical aberrationcorrected transmission electron microscope (JEOL, JEM-ARM200F) equipped with energy dispersive spectroscopy detector. The TG was analyzed by high temperature synchronous thermal analyzer (Netzsch, STA-449F3). The crystalline structure of samples was analyzed by X-ray diffractometer (Shimadzu, XRD-6000) with Cu Kα radiation (λ = 1.54184Å). Catalytic characterization To investigate the catalytic activity of the as-prepared Au/TiO2 HTHNFs, the reduction of 4-NP was tested in a quartz cuvette. The catalytic process was monitored using UV–Vis spectrophotometer (Shimadzu, UV-2600) from 250 nm to 500 nm at constant time intervals. In brief, 1.0 mL of 0.0139 mg/mL 4-NP aqueous solution and 1.0 mL of 0.1 mg/mL Au/TiO2 HTHNFs dispersion liquid 7 ACS Paragon Plus Environment

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were added to the cuvette. The obtained 4-NP solution contained Au/TiO2 HTHNFs has noticeable light yellow color. After addition of freshly prepared 1.0 mg/mL of NaBH4 aqueous solution (1.0 mL), the color change from light yellow to bright yellow rapidly which can be observed obviously. The color of the solution changed gradually from bright yellow to colorless. The catalytic activity of Au/TiO2 HTHNFs, Au/TiO2 hollow nanofibers, Au/TiO2 nanofibers are compared at same experiment condition.


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RESULTS AND DISCUSSION

Scheme 1. Schematic diagram for the preparation of Au/TiO2 hollow throughhole

nanofibers

(HTHNFs).

A

template

method

assistant

coaxial

electrospinning followed by high temperature calcination and template removing steps were used to obtain TiO2 HTHNFs. In this process, liquid paraffin was selected as core fluid of coaxial electrospinning and SiO2 nanoparticles were selected as templates in out fluid. After the calcination and chemical etching, the Au nanoparticles were loaded onto TiO2 HTHNFs by reduction of AuCl4- to form Au/TiO2 HTHNFs catalyst. The specific procedures were illustrated in Scheme 1. The template assistant coaxial electrospinning method, calcination and chemical etching were used sequentially to prepare the TiO2 hollow through-hole nanofibers (HTHNFs). SiO2 nanoparticles (NPs) were used as templates in the coaxial electrospinning process to form hollow nanofibers with through-holes. The SiO2 NPs 9 ACS Paragon Plus Environment

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synthesized by Stöber method22,

23

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with uniform sphere morphology can be

dispersed in ethanol to form a milky white suspension homogeneously without any sediment (Figure S1). In order to remove the organic components and ensure most of anatase TiO2 were transformed to rutile TiO2 which is hydrofluoric acid resistant24, the calcination temperature of as-prepared liquid paraffin@PVP/SiO2 NPs/Ti(OBu)4 nanofibers was set at 700°C according to the TGA and XRD analyse (Figure S2, S3). After the calcination, SiO2 NPs@TiO2 hollow nanofibers were prepared. SEM observations showed that the nanofibers were hollow structures with bumpy surface morphology due to the embedding of SiO2 NPs in the walls of TiO2 hollow nanofibers (Figure 1a). The spherical bump swells on the surface of TiO2 hollow nanofibers (inset image of Figure 1a, S4) represent the independent existence of SiO2 NPs. TEM image (Figure 1b, S5) further proved that SiO2 NPs penetrated the walls of the hollow nanofibers. SEM and TEM images (Figure 1c, S6) showed apparent holes on the intact hollow nanofibers after the chemical etching to remove the SiO2 NPs. And the through-holes structure can be observed directly (inset image of Figure 1c, Figure 1d). Moreover, the XRD results (Figure 1e) showed that the diffraction peaks of amorphous SiO2 and anatase TiO2 in SiO2 NPs@TiO2 hollow nanofibers were both disappeared after the hydrofluoric acid etching and rutile TiO2 is the sole phase in the TiO2 HTHNFs. The sizes of SiO2 NPs and TiO2 HTHNFs were conducted statistics and a series of analysis (Figure S7S10). The density of through-holes can be easily controlled by adjusting the 10 ACS Paragon Plus Environment

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mass fraction of SiO2 NPs among TiO2, the SEM images showed the changes of density of through-holes clearly when the mass fraction of SiO2 NPs among TiO2 was added to 15% from 7.5% (Figure S11). These results means that coral-like TiO2 HTHNFs (Figure 1f) were prepared.

Figure 1. Characterization of SiO2 NPs@TiO2 hollow nanofibers and TiO2 HTHNFs. SEM (a) and TEM (b) images of SiO2 NPs@TiO2 hollow nanofibers showed that the SiO2 NPs were incorporated on TiO2 hollow nanofibers and tracked down possibility to prepared through-holes on the TiO2 hollow nanofibers. SEM (c) and TEM (d) images of intact TiO2 HTHNFs after chemical etching showed that the existence of through-holes on the walls of TiO2 hollow 11 ACS Paragon Plus Environment

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nanofibers. The XRD pattern (e) showed the change of crystal phases after chemical etching. (f) The through-holes structure achieves a coral-like TiO2 HTHNFs.

Figure 2. Characterization of Au/TiO2 HTHNFs. (a) XRD pattern shows rutile TiO2 and small size Au NPs with diffuse peak. (b) HAADF-TEM image shows the Au NPs as bright spots which were dispersed on TiO2 HTHNFs uniformly. (c) The average diameters of HTHNFs, through-holes and Au NPs were 880 nm, 250 nm and 8.14 nm, respectively, which demonstrated the hierarchical structure of Au/TiO2 HTHNFs. (e-f) EDS mapping of Au, O, Ti on selected area (d).


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After preparation of TiO2 HTHNFs, Au nanocatalyst were deposited onto the surface of HTHNFs. Au is demonstrated to be an active catalyst for many heterogeneous catalytic reaction such as hydrogenation of olefins, alkynes and carboxyl/aldehyde, low temperature CO oxidation, and reduction of nitrophenol et al.25-28 The Au NPs were loaded on TiO2 HTHNFs with the assistance of Sn2+ ions, which acts as reductant to in situ reduce AuCl4- and also as a bridge to link the TiO2 HTHNFs and Au NPs.29, 30 The XRD pattern of Au/TiO2 HTHNFs (Figure 2a) showed several strong diffraction peaks of rutile TiO2 (P42/mnm, JCPDS No. 21-1276). The diffuse peak of Au (111) at 38.2° (Fm3m, JCPDS No. 04-0784) also suggest the successful loading of Au NPs on the TiO2 HTHNFs. The HAADF-TEM image of Au/TiO2 HTHNFs showed the Au NPs directly which were distributed uniforml on the TiO2 HTHNFs (Figure 2b). The average diameters of HTHNFs, through-holes and Au NPs, were 880 nm, 250 nm and 8.14 nm, respectively (Figure 2c, S7, S8b, S12), which indicated the hierarchical structure of Au/TiO2 HTHNFs. The EDS element mapping of selected area (Figure 2d) showed that the distributions of Au and O, Ti (Figure 2e, f, S13). These results demonstrated that Au/TiO2 HTHNFs hierarchical 1D catalyst was fabricated. Further, the catalytic performance of Au/TiO2 HTHNFs was investigated in liquid phase reaction.


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Figure 3. The optical photograph for the color of 4-NP solution by NaBH4 in the presence of the Au/TiO2 HTHNFs (a) and the UV/Vis absorption spectrum of the reaction solution with different reaction time (b). The reduction of 4-NP could be completed in 12 min catalyzed by Au/TiO2 HTHNFs. The catalytic reaction kinetic plots of the reduction of 4-NP by different catalyst (c). The calculated ‘k’ was 0, 0.045 min-1, 0.046 min-1, 0.121 min-1 and 0.169 min-1 for TiO2 nanofibers, Au/TiO2 nanofibers, Au/TiO2 hollow nanofibers, Au/TiO2 HTHNFs-L and Au/TiO2 HTHNFs, respectively. The catalytic activity of Au/TiO2 HTHNFs in reduction of 4-NP showed negligible decrease in five cycles (d), and the morphology (inset of (d)) remains intact after five cycles. In this work, the reduction of 4-NP was carried out as a model reaction to evaluate the catalytic activity of the Au/TiO2 HTHNFs, and further understanding the structural advantage of the through-holes on the hollow nanofibers in mass transfer. The reaction was carried out at room temperature (25°C) and atmospheric conditions. When the freshly prepared NaBH4 aqueous 14 ACS Paragon Plus Environment

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solution was added into 4-NP aqueous solution, the color of solution changed to bright yellow rapidly and the absorption peak was red-shifted from 317 nm to 400 nm (absorption of 4-NP), confirming the formation of 4-nitrophenolate anion.31, 32, 33 After the addition of Au/TiO2 HTHNFs aqueous dispersion, the color of solution changed gradually from bright yellow to colorless during the reaction (Figure 3a). The UV-Vis spectrum of the reaction solution (Figure 3b) showed that the intensity of peak centered at 400 nm decreased gradually and a new peak at 300 nm emerged and increased concurrently, indicating the successive catalytic hydrogenation of 4-NP to produce 4-AP and the reaction was completed in 12 min. To demonstrate the advantages of through-holes, two control examples were also prepared including Au/TiO2 solid nanofibers and hollow nanofibers (Figure S14, S15). To compare the catalytic activity of different catalysts, reaction kinetics of the reduction of 4-NP catalyzed by TiO2 nanofibers and Au NPs supported on three kinds of nanofibers were monitored. The linear curve of the plot ln(c/c0) vs time indicate that the reduction of 4-NP obeys pseudo first order reaction mechanism (Figure 3c, S16), which coincide with previous report32, 34. The rate constant ‘k’ of reduction 4-NP catalyzed by TiO2 nanofibers is nearly zero, indicating that solo TiO2 is not active for the reaction. For the Au/TiO2 solid nanofibers, hollow nanofibers, HTHNFs catalyst, the rate constant ‘k’ were calculated to be 0.045 min-1, 0.046 min-1, and 0.169 min-1, respectively. The Au/TiO2 HTHNFs catalyst showed much more enhanced catalytic activity than other two kinds of catalysts, although they were 15 ACS Paragon Plus Environment

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loaded Au NPs in the same condition. To further explore the structural advantage of through-holes, Au NPs was loaded onto TiO2 HTHNFs with lower through-holes density (the mass fraction of SiO2 NPs among TiO2 was reduced to 7.5% from 15%, denoted as Au/TiO2 HTHNFs-L) by the same experiment parameters. The rate constant ‘k’ of the reduction of 4-NP with Au/TiO2 HTHNFs-L catalyst was calculated to be 0.121 min-1. The lower catalytic activity of Au/TiO2 HTHNFs-L than Au/TiO2 HTHNFs suggested that higher density of through-holes possessed better catalytic performance. The turnover frequency (TOF) of four kinds of catalysts was calculated and the results showed that the TOF of Au/TiO2 nanofibers, Au/TiO2 hollow nanofibers, Au/TiO2 HTHNFs-L and Au/TiO2 HTHNFs were 0.47 min-1, 0.51 min-1, 1.1 min-1, 1.9 min-1 respectively. Besides high activity, the cycling stability of the catalysts are essential factors for the practical applications.35 The cycling stability of Au/TiO2 HTHNFs were estimated by recovered the catalyst and reused in the reduction of 4-NP. Figure 3d showed that the catalytic activity of Au/TiO2 HTHNFs showed negligible decrease after five consecutive reaction runs. The inset HAADF-TEM images showed that Au NPs kept almost same as the fresh Au/TiO2 HTHNFs (Figure 3d (I)) and no apparent smash could be observed after five cycles (Figure 3d (V)).


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Figure 4. (a) The reaction mechanism of hydrogenation of 4-NP under the exist of NaBH4 in aqueous solution catalyzed by Au NPs. (b) Mass transfer models of Au/TiO2 solid nanofibers during the reaction processes. (c-d) Random cross section SEM images of Au/TiO2 hollow nanofibers and Au/TiO2 HTHNFs to prove the existence of Au NPs on the inner and outer surfaces. (e-f) Mass transfer models of Au/TiO2 hollow nanofibers and Au/TiO2 HTHNFs during the reaction processes which demonstrate that Au/TiO2 HTHNFs have more aisles for reactants access into and for resultants get out from the inner spaces during the reaction process. The effect of the through-holes on the TiO2 hollow nanofibers to the catalytic activity of the supported Au NPs was further studied and discussed. As illustrated in Figure 4a and Figure S17, the reduction of 4-NP was proceeded on the surface of Au NPs, where BH4- was firstly diffused and adsorbed to release electrons and then -NO2 of 4-NP 17 ACS Paragon Plus Environment

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captured the electros and the -H gradually replace the -O of -NO2 to form -NH2 with the existence of H2O.36 Besides the intrinsic properties of Au NPs, the diffusion rate of the reactants to the surfaces of Au NPs would significantly affect the reaction rate. As shown in Figure 4b, Au/TiO2 solid nanofibers exhibited the lowest activity since Au NPs were supported merely on the outer surfaces. Moreover, the random broken cross section SEM of catalysts showed that Au NPs were dispersed uniformly on both inner and outer surfaces of Au/TiO2 hollow nanofibers and Au/TiO2 HTHNFs (Figure 4c, d). In addition, the loading amount of Au NPs were detected by ICP-AES. The 1.17 wt%,1.09 wt%, 1.12 wt%, 1.13 wt% loading amount of Au NPs respectively for Au/TiO2 nanofibers, Au/TiO2 hollow nanofibers, Au/TiO2 HTHNFs-L and Au/TiO2 HTHNFs show negligible difference. The through-holes which is the sole difference between Au/TiO2 hollow nanofibers and Au/TiO2 HTHNFs most probably accounts for the enhancement of the catalytic activity by facilitating mass transfer. For the Au/TiO2 hollow nanofibers, Au NPs were supported on both the inner and outer surfaces of the TiO2 hollow nanofibers. However, the Au NPs on the inner surfaces were difficult to be accessed for the reagents in short reaction time due to the diffusional limitation caused by large ratio of length to diameter of TiO2 hollow nanofibers (Figure 4e). Through- holes on the Au/TiO2 HTHNFs provided abundant mass transfer aisles for reactants to access inner space, which could take full advantage of Au NPs on the inner surfaces and for resultants to get out from inside (Figure 4f). Therefore, the reaction rate was 18 ACS Paragon Plus Environment

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improved significantly via the mass transfer enhancement. The influence factors of mass transfer in Au/TiO2 HTHNFs can be further explained as follows: (a) the existence of through-holes made more catalytic active sites of Au NPs to be accessible for reagents. (b) the through-holes are much larger than the molecular average free path so that the reactants and resultants can free diffused with low diffusion resistance. In other words, the active molecules can be refreshed quickly so that increased the reaction rate. CONCLUSIONS In summary, TiO2 hollow nanofibers with through-holes were fabricated by template assisted coaxial electrospinning method. The Au/TiO2 HTHNFs possesses high catalytic activity in liquid phase 4-NP reduction reaction through facilitating mass transfer efficiency. This mass transfer enhancement strategy can be easily extended to many other 0D and 1D hollow nanomaterials that would play promising roles in catalyst and energy fields. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details on the morphology characterizations of SiO2 NPs, SiO2 NPs@TiO2 hollow nanofibers, TiO2 HTHNFs, TGA and XRD patterns of liquid 19 ACS Paragon Plus Environment

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paraffin@PVP/SiO2

NPs/Ti(OBu)4

nanofibers,

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SiO2

NPs@TiO2

hollow

nanofibers, TiO2 hollow through-hole nanofibers et al. (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Note The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (NSFC) (Grant Nos. 51772010, 21774005, 21433012, and 21374001), National Natural Science Foundation for Outstanding Youth Foundation, the Fundamental Research Funds for the Central Universities, the National Program for Support of Top-notch Young Professionals, and the 111 project (Grant No. B14009). ABBREVIATIONS HTHNFs, hollow through-hole nanofibers REFERENCES


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1. Zhong, W.; Liu, H.; Bai, C.; Liao, S.; Li, Y., Base-Free Oxidation of Alcohols to Esters at Room Temperature and Atmospheric Conditions using Nanoscale Co-Based Catalysts. ACS Catal. 2015, 5, 1850-1856. 2. Wei, Y.; Parmentier, T. E.; de Jong, K. P.; Zecevic, J., Tailoring and visualizing the pore architecture of hierarchical zeolites. Chem. Soc. Rev. 2015, 44, 7234-7261. 3. Wei, J.; Zhou, D.; Sun, Z.; Deng, Y.; Xia, Y.; Zhao, D., A Controllable Synthesis of Rich Nitrogen-Doped Ordered Mesoporous Carbon for CO2 Capture and Supercapacitors. Adv. Funct. Mater. 2013, 23, 2322-2328. 4. Zhao, Q.; Yin, M.; Zhang, A. P.; Prescher, S.; Antonietti, M.; Yuan, J., Hierarchically structured nanoporous poly(ionic liquid) membranes: facile preparation and application in fiber-optic pH sensing. J. Am. Chem. Soc. 2013, 135, 5549-5552. 5. Liang, H. W.; Zhuang, X.; Bruller, S.; Feng, X.; Mullen, K., Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction. Nat. Commun. 2014, 5, 4973. 6. Tsujimura, S.; Murata, K.; Akatsuka, W., Exceptionally high glucose current on a hierarchically structured porous carbon electrode with "wired" flavin adenine dinucleotide-dependent glucose dehydrogenase. J. Am. Chem.

Soc. 2014, 136, 14432-14437. 7. Parlett, C. M.; Wilson, K.; Lee, A. F., Hierarchical porous materials: catalytic applications. Chem. Soc. Rev. 2013, 42, 3876-3893. 21 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

8. Wang, J.; Tang, H.; Wang, H.; Yu, R.; Wang, D., Multi-shelled hollow micro/nanostructures: promising platforms for lithium-ion batteries. Mater. Chem.

Front. 2017, 1, 414-430. 9. Sun, L. B.; Liu, X. Q.; Zhou, H. C., Design and fabrication of mesoporous heterogeneous basic catalysts. Chem. Soc. Rev. 2015, 44, 5092-5147. 10. Kim, H.; Cho, M. K.; Kwon, J. A.; Jeong, Y. H.; Lee, K. J.; Kim, N. Y.; Kim, M. J.; Yoo, S. J.; Jang, J. H.; Kim, H. J.; Nam, S. W.; Lim, D. H.; Cho, E.; Lee, K. Y.; Kim, J. Y., Highly efficient and durable TiN nanofiber electrocatalyst supports. Nanoscale 2015, 7, 18429-18434. 11. Chen, Z.; Wang, W.; Zhang, Y.; Liang, Y.; Cui, Z.; Wang, X., Pd Nanoparticles Confined in the Porous Graphene-like Carbon Nanosheets for Olefin Hydrogenation. Langmuir 2018, 34, 12809-12814. 12. Kwok, K. M.; Ong, S. W. D.; Chen, L.; Zeng, H. C., Constrained Growth of MoS2 Nanosheets within a Mesoporous Silica Shell and Its Effects on Defect Sites and Catalyst Stability for H2S Decomposition. ACS Catal. 2018, 8, 714-724. 13. Perego, C.; Millini, R., Porous materials in catalysis: challenges for mesoporous materials. Chem. Soc. Rev. 2013, 42, 3956-3976. 14. Xiao, F. X.; Miao, J.; Tao, H. B.; Hung, S. F.; Wang, H. Y.; Yang, H. B.; Chen, J.; Chen, R.; Liu, B., One-dimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis. Small 2015, 11, 2115-2131. 22 ACS Paragon Plus Environment

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

15. Liang, H. W.; Liu, S.; Yu, S. H., Controlled synthesis of one-dimensional inorganic

nanostructures

using

pre-existing

one-dimensional

nanostructures as templates. Adv. Mater. 2010, 22, 3925-37. 16. Liang, H.; Zhang, B.; Ge, H.; Gu, X.; Zhang, S.; Qin, Y., Porous TiO2/Pt/TiO2 Sandwich Catalyst for Highly Selective Semihydrogenation of Alkyne to Olefin. ACS Catal. 2017, 7, 6567-6572. 17. Niu, C.; Meng, J.; Wang, X.; Han, C.; Yan, M.; Zhao, K.; Xu, X.; Ren, W.; Zhao, Y.; Xu, L.; Zhang, Q.; Zhao, D.; Mai, L., General synthesis of complex nanotubes by gradient electrospinning and controlled pyrolysis. Nat.

Commun. 2015, 6, 7402. 18. Zhou, Y.; Song, W.; Zhang, L.; Tao, S., Preparation of hollow magnetic porous zirconia fibers as effective catalyst carriers for Fenton reaction. J.

Mater. Chem. A 2018, 6, 12298-12307. 19. Fan, L.; Xia, Z.; Xu, M.; Lu, Y.; Li, Z., 1D SnO2 with Wire-in-Tube Architectures for Highly Selective Electrochemical Reduction of CO2 to C1 Products. Adv. Funct. Mater. 2018, 28, 1706289. 20. Ruiz-Rosas, R.; Rosas, J. M.; Loscertales, I. G.; Rodríguez-Mirasol, J.; Cordero, T., Electrospinning of silica sub-microtubes mats with platinum nanoparticles

for

NO

catalytic

reduction.

Applied

Catalysis

B:

Environmental 2014, 156-157, 15-24. 21. Choi, S. J.; Chattopadhyay, S.; Kim, J. J.; Kim, S. J.; Tuller, H. L.; Rutledge, G. C.; Kim, I. D., Coaxial electrospinning of WO3 nanotubes functionalized 23 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

with bio-inspired Pd catalysts and their superior hydrogen sensing performance. Nanoscale 2016, 8, 9159-9166. 22. Wu, S. H.; Mou, C. Y.; Lin, H. P., Synthesis of mesoporous silica nanoparticles. Chem. Soc. Rev. 2013, 42, 3862-3875. 23. Liu, J.; Wickramaratne, N. P.; Qiao, S. Z.; Jaroniec, M., Molecular-based design and emerging applications of nanoporous carbon spheres. Nat.

Mater. 2015, 14, 763-774. 24. Li, Z.; Guan, B. Y.; Zhang, J.; Lou, X. W., A Compact Nanoconfined Sulfur Cathode for High-Performance Lithium-Sulfur Batteries. Joule 2017, 1, 576-587. 25. Liu, X.; Hu, B.; Fujimoto, K.; Haruta, M.; Tokunaga, M., Hydroformylation of olefins by Au/Co3O4 catalysts. Applied Catalysis B: Environmental 2009,

92, 411-421. 26. Gao, Z.; Qin, Y., Design and Properties of Confined Nanocatalysts by Atomic Layer Deposition. Acc. Chem. Res. 2017, 50, 2309-2316. 27. Fu, Y.; Huang, T.; Jia, B.; Zhu, J.; Wang, X., Reduction of nitrophenols to aminophenols under concerted catalysis by Au/g-C3N4 contact system.

Applied Catalysis B: Environmental 2017, 202, 430-437. 28. Ha, H.; Yoon, S.; An, K.; Kim, H. Y., Catalytic CO Oxidation over Au Nanoparticles Supported on CeO2 Nanocrystals: Effect of the Au-CeO2 Interface. ACS Catal. 2018, 8, 11491-11501.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

29. Zhang, Z.; Shao, C.; Zou, P.; Zhang, P.; Zhang, M.; Mu, J.; Guo, Z.; Li, X.; Wang, C.; Liu, Y., In situ assembly of well-dispersed gold nanoparticles on electrospun silica nanotubes for catalytic reduction of 4-nitrophenol. Chem.

Commun. 2011, 47, 3906-3908. 30. Zhong, L. S.; Hu, J. S.; Cui, Z. M.; Wan, L. J.; Song, W. G., In-Situ Loading of Noble Metal Nanoparticles on Hydroxyl-Group-Rich Titania Precursor and Their Catalytic Applications. Chem. Mater. 2007, 19, 4557-4562. 31. Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T., Photochemical green synthesis of calcium-alginate-stabilized Ag and Au nanoparticles and their catalytic application to 4-nitrophenol reduction. Langmuir 2010, 26, 28852893. 32. Zhang, S.; Chang, C. R.; Huang, Z. Q.; Li, J.; Wu, Z.; Ma, Y.; Zhang, Z.; Wang, Y.; Qu, Y., High Catalytic Activity and Chemoselectivity of Subnanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes. J. Am. Chem. Soc. 2016, 138, 2629−2637. 33. Zhang, W.; Lu, G; Cui, C.; Liu, Y.; Li, S.; Yan, W.; Xing, C.; Chi, Y. R.; Yang, Y.; Huo, F., A Family of Metal-Organic Frameworks Exhibiting SizeSelective Catalysis with Encapsulated Noble-Metal Nanoparticles. Adv.

Mater. 2014, 26, 4056–4060. 34. Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.; Moller, M.; Breu, J., Single nanocrystals of platinum prepared by partial dissolution of Au-Pt nanoalloys. Science 2009, 323, 617-20. 25 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

35. Song, J.; Wang, X.; Yan, J.; Yu, J.; Sun, G.; Ding, B., Soft Zr-doped TiO2 Nanofibrous Membranes with Enhanced Photocatalytic Activity for Water Purification. Sci. Rep. 2017, 7, 1636. 36. Deng, Y.; Cai, Y.; Sun, Z.; Liu, J.; Liu, C.; Wei, J.; Li, W.; Liu, C.; Wang, Y.; Zhao, D., Multifunctional mesoporous composite microspheres with welldesigned nanostructure: a highly integrated catalyst system. J. Am. Chem.

Soc. 2010, 132, 8466-8473.


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Table of Contents/Abstract Graphics


TiO2 hollow nanofibers with through-holes as high

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