Highly Crystalline Mesoporous TiO2(B) Nanofibers - The Journal of

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Highly Crystalline Mesoporous TiO2(B) Nanofibers Wei Li,†,‡ Yang Bai,†,‡ Wei Zhuang,† Kwong-Yu Chan,*,‡ Chang Liu,† Zhuhong Yang,† Xin Feng,† and Xiaohua Lu*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China

‡

S Supporting Information *

ABSTRACT: Efforts are being made to improve crystalline properties of the skeletons of mesoporous TiO2. On the basis of the unique crystal structures and versatile phase evolution of K2Ti2O5, we obtain mesoporous TiO2(B) nanofibers (MNFs) with highly crystalline and high-energy facets exposed skeletons via a cheap and scalable route. Verified by X-ray diffraction, scanning electron microscopy, highresolution transmission electron microscopy, and N2 adsorption, these special structures come from layered structure of K2Ti2O5 and subsequent interlayer splitting and exfoliating and intralayer topotactic transformation. Because of their well-organized frameworks and high surface area, M-NFs exhibit efficient photogenerated charge transportation and hydrogen production, showing better charge mobility along one-dimensional and highly crystalline skeletons than irregularly shaped polycrystalline counterpart and more accessible reactive sites due to larger surface area than single crystal nonporous counterpart.

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efforts are required to develop easier and more economic routes. Diverse TiO2 nanostructures have been synthesized from titanates (A2TinO2n+1, A = Na, K, Cs; 2 ≤ n ≤ 8) upon different synthetic approaches.24−35 Interestingly, a relatively new TiO2 crystal phase TiO2(B) was first observed in one of these work by Marchand et al.35 TiO2(B) shows different crystal structures with anatase, rutile, and brookite and thus opens more opportunities for promising applications as electrochemical and photocatalytic materials.36−50 By using titanates as precursors, one can not only achieve tunable TiO2 structures (including crystalline phases, shapes, sizes, exposed facets, and hierarchical structures) but also be benefited from low-cost and scalable synthetic routes (involving hydration, ion exchange, and thermal annealing). In our previous work, single crystal anatase TiO2 nanofibers, mesoporous anatase nanofibers, fibrous anatase/TiO2(B) composites and thermally stable TiO2(B) nanofibers were synthesized from K2Ti2O5.7,50−54 In this work, we prepare highly crystalline mesoporous TiO2(B) nanofibers (M-NFs) through the tunable synthesis route from K2Ti2O5. The key point is controlling the hydration process, which is employed to adjust the crystal phase and to create separated microphase for subsequent mesopores. To the best of our knowledge, it is the first time that mesopores, onedimensional (1D) orientation, reactive facets, and highly crystalline structures were integrated into one TiO2 material employing a surfactant- and template-free route. Moreover, M-

iO2 nanomaterials are of increasing importance to broad applications in photocatalysis, photovoltaics, and energy storage. 1−5 In these applications, performance of TiO 2 nanomaterials strongly depends on their geometry and crystallographic structures.3−10 For instance, in photoelectrochemical (PEC) water-splitting photoelectrodes, early efforts included the study of nanoparticle (NP) and mesoporous (MP) thin films because of their large surface area.3,11 However, they suffer from high charge recombination loss because the electron mobility is about 2 orders of magnitude lower than that of a bulk single crystal due to the electron trapping/scattering at grain boundaries.12−14 Recent work has focused on onedimensional (1D) nanotubes (NTs) and nanorods (NRs) because of enhancements in charge mobility and light absorption.15−18 However, most of these structures, compared to NP and MP films, have smaller surface areas, which can negatively impact charge transfer process and kinetics of water oxidation. In addition, most of the 1D nanostructures are poorly crystalline as well, which still limit charge carrier mobility due to trap states at grain boundaries.19,20 Therefore, it is highly desirable to synthesize highly crystalline 1D nanostructure with enhanced surface area. In synthesis of high surface area mesoporous metal oxides, surfactants or templates are generally used to introduce porous structures.3,5,21 However, simpler systems, avoiding the use of additional surfactants or templates, are still needed for low impurities in the final products, easy collection, mass production, and low cost.3,4 In synthesis of 1D and highly crystalline TiO2 nanomaterials, there are a few of methods like hydro- or solvo-thermal, hot-injection, and vapor deposition.3,22,23 By the criterion of low cost and scalable, more © 2014 American Chemical Society

Received: August 13, 2013 Revised: January 8, 2014 Published: January 13, 2014 3049

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desorption isotherms, shown in Figure 2. The corresponding curves show type IV-like isotherms, indicating the presence of

NFs are characterized as model nanostructure for efficient charge mobility and hydrogen production, showing larger surface area for more accessible reactive sites than single crystal nonporous counterpart and presenting better charge transport than irregularly shaped polycrystalline counterpart. More importantly, the clearly demonstrated “structure−property” relationships could be valuable hints for rational design of other semiconducting nanomaterials for photocatalysis and photovoltaics.

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RESULTS AND DISCUSSION Scanning electron microscopy (SEM) analysis (Figure 1) is performed to examine the morphology evolution of the whole

Figure 2. N2 adsorption−desorption isotherms and the pore-size distribution (inset) of ion-exchanged titanate and M-NFs.

well-developed mesoporosity. The insert of Figure 2 shows the pore-size distribution plots calculated by the BJH (Barrett− Joyner− Halenda) equation from the desorption branch of the isotherm. They present very narrow pore-size distributions well agreed with the statistical pore size distributions from the SEM images. The typical values for the specific surface area, the average pore diameter, and the pore volume of the porous samples are summarized in Table 1. Table 1. Textual Properties of the Mesoporous Fibers particle (nanocrystallites in fibers) size/nm

Figure 1. SEM images of (A) K2Ti2O5; (B) hydrated intermediate; (C,E) Ion-exchanged titanate; and (D,F) M-NFs.

samples

SBET/m2· g−1

VP/cm3· g−1

pore size (DBJH)/nm

FESEM

XRD

H2Ti5O11 M-NFs

178 79

0.26 0.24

5.2 13.2

8 16

5.6 (200) 10.3 (110)

X-ray diffraction (XRD) is employed to understand the crystal phase evolution in the synthesis process and to show crystallinity of the final product. The XRD pattern of K2Ti2O5 is presented in Figure 3A, all diffraction peaks match well with the crystal structure of K2Ti2O5 (JCPDS 51−1890). The intensity of the (001) peak is much stronger than that of the (111) peak, which is opposite with the calculated diffraction

process from K2Ti2O5 particles to M-NFs. Figure 1A shows images of the K2Ti2O5 particles with some parallel clefts inside, which is a usual observation due to its particular layered structure. According to its layered crystal structure, the split planes are K2Ti2O5 {001} facets. After hydration, in Figure 1B the clefts develop much more compactly and uniformly attributed to interlayer splitting caused by introduced water molecules. Simultaneously, some elusive splitting generates in the direction nearly vertical to the interlayer {001} facets, that is, {100} planes of K2Ti2O5. Consequently, the ion-exchanged products (Figure 1C) show bundles of loosely aggregated fibers exfoliated from hydrated samples since the total removal of K+. Finally, cuboid-like fibers with an equivalent diameter between 100 to 200 nm and length from 1 to 10 μm are obtained (Figure 1D) by dehydration at 500 °C. In a much smaller size scale, we find porous structures are introduced after the removal of K+, shown in Figure 1E. The framework of a porous nanofiber is built up by loosely connected cuboid nanocrystals (average size around 8 nm) of titanate with numerous 3−8 nm gaps presented in between. In M-NFs, similar framework and porous structure are shown in Figure 1F. The size of nanocrystals (around 16 nm in average) and pores grow larger because of the crystallization and rearrangement of nanocrystals during the thermal treatment. The porosity is also investigated using N2 adsorption−

Figure 3. XRD patterns of (A) K2Ti2O5; (B) hydrated intermediate; (C) ion-exchanged titanate; (D) M-NFs. (Samples B and C are wet intermediates, it is unlikely to get better patterns.) 3050

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Figure 4. TEM images of (A) K2Ti2O5; (B) hydrated intermediate; (C) ion-exchanged titanate; (D) M-NFs. A1, B1, C1, and D1 are the TEM images of morphology. A2, B2, C2, and D2 are the corresponding high-resolution TEM images of the same samples. Insets of A1, B1, C1, and D1 are the corresponding SAED images of a single fiber.

Figure 5. (A) Schematic illustrations of a TiO2(B) (100) facet in the M-NFs. (B,C) Schematic illustrations of crystal structure viewed along the [100] and [001] directions, respectively. (D,E) HRTEM analyses of exposed TiO2(B) (100) facets viewed from [100] and [001] directions, respectively. Insets in (D,E) are the corresponding SAED images (upper left), TEM images (upper right), and simulated HRTEM images (lower left).

calculated crystallite size indicate that the prepared M-NFs are highly crystallized. More explicit structural evolution is shown in transmission electron micrscopy (TEM) images, Figure 4. A scrap of K2Ti2O5 particle is shown in Figure 4A1; the crystallography is investigated using selected area electron diffraction (SAED) (inset of Figure 4A1) and HRTEM imaging (Figure 4A2). The SAED pattern is indexed to the K2Ti2O5 (C2/m) crystal structure with zone axis of [100], which reveals the growth direction of the fiber is [010]. The HRTEM image shows a clear image of a split (001) plane from the side view. Thus the splitting happens exactly at the interlayers of the layered K2Ti2O5, confirmed by the explanations in the SEM and XRD results. TEM images of hydrated intermediate, ion-exchanged H2Ti5O11·H2O and final M-NTs (Figure 4B1,B2,C1,C2,,D1,and D2) demonstrate consecutive interlayer splitting and exfoliating from large K2Ti2O5 particles to smaller TiO2(B) fibers. SAED and HRTEM results disclose that the directions of crystal axes in the fibers are preserved throughout the entire process, while TiO6 units are rearranged by intralayer topotactic transformation. There are two more interesting results worth further discussion. The first one is the clear single-crystal-like patterns of all the SAED analyses, including the porous ones in Figure

pattern of bulk K2Ti2O5. In K2Ti2O5, planes of (001) correspond to the adjacent layers. The strong diffraction peak of (001) indicates the preferred orientation along (001), well agreed with the SEM observation. The XRD pattern of hydrated titanate is shown in Figure 3B. Compared to the 13.7° main diffraction peak of K2Ti2O5 (001), the main peak of hydrated titanate (200) planes shifts to a smaller angle of 8.5°. It means that the lattice spacing of adjacent layers is broadened from 6.51 to 10.4 Å by the water molecules entered into the interlayers during the hydration. In the broadened interlayers, K+ and water molecules exist in the form of KOH·nH2O, confirmed by diffraction peaks of KOH·H2O (JCPDS 36-0791) near 30°. Meanwhile, layers of [Ti2O5]2− composed trigonal bipyramid TiO5 units in K2Ti2O5 are rearranged to [Ti5O11]2− comprised of TiO6 octahedra. The diffraction pattern associated with the ion-exchanged products (Figure 3C) matches well with the pattern of H2Ti5O11·H2O (JCPDS 44-0131), which confirms the [Ti5O11]2− structure of the hydrated intermediate. Finally, crystal phase of TiO2(B) (JCPDS 35-0088) of M-NFs is shown in Figure 3D. Crystallite size of the porous samples is calculated from the most intense peaks ((200) of H2Ti5O11 and (110) of TiO2(B), respectively) using Scherrer equation, summarized in Table 1. Both intensity of the peaks and 3051

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4C,D. In the TEM image of Figure 4D2, one can see how the porous structure is organized. For such a thick TEM sample, the exact size of nanocrystallites is difficult to measure because they give overlapped dark domains without clear grain boundaries in the image. But quasi-quantitatively it is clear that numerous 5−15 nm sharp polyhedral-shaped bright domains corresponding to the pores of the TiO2 arrange along the longitudinal direction in which 10−25 nm wide nanocrystallites build up the brick-like and porous framework, well agreed with the morphology showed in the SEM image. Meanwhile, the SAED and HRTEM analyses reveal that the analyzed “bricks” are highly crystalline and crystallographically preferred oriented along the [010] crystal axis preserved from the precursor. The second interesting result is exposed facets of TiO2(B) (100) (shown in the inset of Figure 4D2 and sketched in the cartoon in Figure 5A), which are derived from (001) of K2Ti2O5 through interlayer splitting and exfoliation. Figure 5 shows more detailed structural information of exposed TiO2(B) (100) facets viewed from [100] and [001] directions. The corresponding crystal cell models are shown in Figure 5B,C, respectively. Figure 5D,E shows corresponding HRTEM and SAED images. Comparing the simulated HRTEM images (insets in lower left quarters of Figure 5D,E) with the crystal models, atomic arrangement observed from HRTEM agreed well with the theoretical models and our explicit observations in first-principle calculations.55 The calculation results revealed that TiO2(B) (100) is another new high-energy facet of TiO2.56−58 Dissociated water on TiO2(B) (100) are much more thermodynamically favorable than molecular adsorption, implying promising photocatalytic efficiency of such a novel TiO2(B) material for H2 evolution. Before moving to the results of photoelectrochemical (PEC) measurements, the entire structural evolution is summarized in the Supporting Information and a detailed mechanism is proposed in Supporting Information Figure S1 and S2. PEC cell is set up to compare the efficiency of charge mobility of M-NFs with other two reference samples. Transient photocurrent of photoanodes made of M-NFs, as well as reference samples of TiO2(B) fibers from K2Ti4O9 and Degussa-P25, are measured in Na2SO4 electrolyte without any sacrificial reagents and cocatalysts. Amperometric I−t curves are collected with light on/off cycles at 0.8 V versus RHE (Figure 6A). All three curves show evident photoresponse; the photocurrent increases quickly and then keeps a steady state in illumination following a fast decrease to dark current as soon as the UV is turned off. Photocurrent is produced by photostimulated charge carriers and bias voltage in the cell. In a certain bias voltage, the higher value of photocurrent density means more efficient charge separation and collection. Steady photocurrent densities of mesoporous TiO2(B) nanofibers, P25 and TiO2(B) fibers from K2Ti4O9 are 11, 6.1, and 0.32 μA cm−2, respectively. The highest photocurrent density of mesoporous TiO2(B) reveals its most efficient generation and migration of charge carriers in the three candidates. The exposed active facets and larger surface area in M-NFs could be beneficial for more efficient charge generation and more accessible reactive sites, meanwhile the 1D and highly crystalline frameworks could enhance the smooth migration of photogenerated charge carriers. In the same setup, we conduct open circuit photovoltage (OCP) measurements by monitoring the cell potential response during on−off cycles of UV light illumination,

Figure 6. (A) Amperometric I−t curves and (B) open-circuit potential of M-NFs in comparison with P25 and low surface area TiO2(B) from K2Ti4O9 in photoelectrochemical experiments. (C). H2 production rate of M-NFs, P25, and TiO2(B) from K2Ti4O9 (inset, curves of H2 production amount vs reaction time).

shown in Figure 6B. Under open circuit conditions, the photostimulated electrons are accumulated in the conduction band of TiO2 under illumination and then migrated to electrode substrate, leading to a negative shift of OCP. The shifts of OCP are 0.38, 0.31, and 0.032 V for M-NFs, P25, and TiO2(B) fibers from K2Ti4O9, respectively. Higher shift means more photostimulated electrons are accumulated and thus a more efficient generation of charge carriers. When UV is switched off, free electrons recombine with the holes, resulting in the decay of potential. Photovoltage decay of M-NFs electrode is a bit slower than that of the P25 electrode. The photovoltage decay directly reflects the longer lifetime of accumulated electrons. Because the unique 1D and highly crystalline structures in M-NFs, we expect charge recombination to be less favored. From the curves, M-NFs show the 3052

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Crystal phase of samples was determined by powder XRD (Bruker D8, Cu−Kα radiation). Sample morphology was evaluated by field emission SEM (Hitachi S4800). Textural properties were studied by N2 adsorption−desorption measurements (ASAP 2020 M) at liquid nitrogen temperature. Before analysis, the samples were degassed for 2 h at 423 K in vacuum. The microstructure was observed by TEM (Philips Tecnai G2 20 S-TWIN) at 200 kV. The PEC measurement of transient photocurrent and open circuit photovoltage was examined using the workstation (SI 1287, Solartron Analytical) with a three electrode system, which consists of a glassy carbon working electrode with the sample on it, a Pt foil counter electrode, and an Hg/HgSO4 reference electrode. The electrochemical cell provided a 60 mm diameter quartz window for light illumination. A 3 W UV lamp with a central wavelength of 254 nm was used as a light source. A 0.5 M Na2SO4 solution (pH = 6.3) was used as supporting electrolyte, which was deaerated with nitrogen prior to PEC experiments. Working electrodes were prepared as follows: first, 50 mg of TiO2 sample was dispersed in a mixture containing 0.45 mL of ethanol, 0.45 mL of water, and 100 μL of Nafion solution. This mixture was then sonicated for 20 min. Twenty microliters of the above mixture was measured, dropcast onto a polished glassy carbon electrode, and dried gently under air flow. The photocatalytic H2 production was carried out in a 500 mL Pyrex reactor with a 300 W high-pressure mercury lamp inside, instead of the setup used in PEC measurement. A 0.3 g sample of platinized catalyst (with 0.5 wt % Pt) was dispersed in to 500 mL methanol−water solution (1:9, vol %). Pt loading is performed in the same procedure described in our previous work (ref 50). This method ensured the Pt particle size and dispersion are similar in all samples, which had been confirmed by TEM analysis (ref 50). Evolved H2 gas was identified by gas chromatography (Shimadzu, GC-8A TCD, molecular sieves 5 Å, argon gas). Volume of H2 evolved was collected and measured with a sweeping bubble film through a buret.

highest shift and lowest decay of potential. Thus, M-NFs present the most efficient generation and lowest recombination of charge carriers, which could be attributed to its unique structures and agree well with the results from photocurrent density. Finally, photocatalytic H2 evolution is tested in the assistance of Pt cocatalyst and methanol sacrificial reagent in a 500 mL slurry photoreactor. The highly active and stable H2 evolution of M-NFs is demonstrated in Figure 6C. In particular, obvious decrease in the H2 evolution rates over time is observed for both reference samples, which is the same as other platinumdecorated TiO2 materials.52,59,60 In contrast, M-NFs display good stability in H2 evolution. The average H2 evolution rate of M-NFs is about 18.9 mmol·g−1cat·h−1, compared with 15.8 mmol·g−1cat·h−1 for P25 and 4.52 mmol·g−1cat·h−1 TiO2(B) fibers from K2Ti4O9 in a 15 h period. Efficient H2 evolution of M-NFs agrees well with PEC results and clearly demonstrates the advantages of M-NFs’ unique structures. Porous and high active surface structures provide large reactive surface area and work as a stable support for Pt cocatalysts. The 1D and highly crystalline structures allow efficient charge mobility in the bulk. More importantly, these unique structures could also be promising for other applications like dye sensitized solar cells, Li-ion batteries, and so forth.

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CONCLUSION We have shown a surfactant- and template-free scalable synthetic route for novel highly crystalline mesoporous TiO2(B) nanofibers with high-energy facet from layered titanate. The explicit structural evolution of interlayer splitting and exfoliating and intralayer topotactic transformation is clearly demonstrated by experimental characterizations. The origin of pores and crystal structures are well explained by the whole structural evolution. In PEC water splitting and H2 evolution investigations, highly crystalline mesoporous TiO2(B) nanofibers presented efficient generation, migration and low recombination of charge carriers, and thus efficient H2 evolution due to the unique structures. Moreover, the clear “structure−property” relationships give hints for rational design of other semiconducting nanomaterials for applications in photocatalysis, photovoltaics, and energy storage.

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ASSOCIATED CONTENT

S Supporting Information *

Summarized structural evolution and mechanism, statistic size distribution of crystallites in ion-exchanged titanate and calcinated TiO2 from SEM images, XRD pattern and SEM image of TiO2(B) from K2Ti4O9. This material is available free of charge via the Internet at http://pubs.acs.org.

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EXPERIMENTAL SECTION M-NFs were prepared as follows: (I) Calcination: A TiO2/K2O mixture of 1.9 molar ratio was prepared by uniformly adding K2CO3 (reagent grade) to TiO2·nH2O and then sintered at 810 °C for 2 h. (II) Hydration: Ten grams of the product was held in a crucible and kept in a steam box for 12 h. (III) Ionexchange: The product was suspended in 100 mL of vigorously stirred 0.1 M HCl solution until K+ ion was completely exchanged. (IV) Collection: The product was filtered and washed with distilled water and dried in a desiccator at 60 °C under vacuum. (V) Heat treatment: dried titania sample were heated in a muffle oven at 500 °C in air for 2 h. Reference samples: (1) TiO2(B) fibers without any porosity (Supporting Information Figure S3, SBET, 8−12 m2g−1) were synthesized from K2Ti4O9 with the same ion-exchange and calcination steps. (2) Degussa P25 (TiO2 nanopowder with 80% anatase and 20% rutile), a wide accepted benchmark material in the field of photocatalytic reactions, was purchased from Evonik Degussa Inc.

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AUTHOR INFORMATION

Corresponding Authors

*(K.-Y.C.) E-mail: [email protected]. Tel: +852-28597917. *(X.L.) E-mail: [email protected]. Tel: +86-25-8358-8063. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The collaborative research between Nanjing University of Technology and University of Hong Kong was assisted by a NSFC-RGC Joint Research Award (No. 20731160614 and HKU 735/07). The authors also thank the support by Changjiang Scholars and Innovative Research Team in University (No. IRT0732). 3053

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