Self-Assembled One-Dimensional Porphyrin Nanostructures with

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Self-Assembled One-Dimensional Porphyrin Nanostructures with Enhanced Photocatalytic Hydrogen Generation Na Zhang, Liang Wang, Haimiao Wang, Ronghui Cao, Jiefei Wang, Feng Bai, and Hongyou Fan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04701 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Self-Assembled One-Dimensional Porphyrin Nanostructures with Enhanced Photocatalytic Hydrogen Generation Na Zhang,1,2,‡ Liang Wang,1,2,‡ Haimiao Wang,1,2 Ronghui Cao,1,2 Jiefei Wang,1,2 Feng Bai,1,2,* and Hongyou Fan3, 4,* 1

Key Laboratory for Special Functional Materials of the Ministry of Education, Henan

University, Kaifeng 475004, P. R. China; 2

Collaborative Innovation Center of Nano Functional Materials and Applications, Henan

University, Kaifeng 475004, China; 3

Department of Chemical and Biological Engineering, Albuquerque, University of New Mexico,

Albuquerque, New Mexico 87106, United States; 4

Sandia National Laboratories, Albuquerque, New Mexico 87185, United States.

Corresponding author emails, phone numbers, and fax numbers: F.B. ([email protected]), Tel: 86-15039024866, Fax: 86-0371-23883868 H.F. ([email protected]), Tel: (505) 272-7128, Fax: (505) 272-7336

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ABSTRACT

There has been a widespread interest in the preparation of self-assembled porphyrin nanostructures and their ordered arrays, aiming to emulate natural light harvesting processes and energy storage and to develop new nanostructured materials for photocatalytic process. Here we report controlled synthesis of one-dimensional porphyrin nanostructures such as nanorods and nanowires with well-defined self-assembled porphyrin networks that enable efficient energy transfer for enhanced photocatalytic activity in hydrogen generation. Preparation of these onedimensional nanostructures is conducted through noncovalent self-assembly of porphyrins confined within surfactant micelles. X-ray diffraction and transmission electron microscopy results reveal that these one-dimensional nanostructures contain stable single crystalline structures with controlled interplanar separation distance. Optical absorption characterizations show that the self-assembly enables effective optical coupling of porphyrins, resulting in much more enhanced optical absorption in comparison with the original porphyrin monomers and the absorption bands red shift to more extensive visible light spectrum. The self-assembled porphyrin network facilitates efficient energy transfer among porphyrin molecules and the delocalization of excited state electrons for enhanced photocatalytic hydrogen production under visible light.

KEYWORDS: Self-assembly, porphyrins, visible light photocatalytic, hydrogen evolution.


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There has been a widespread interest in the preparation of self-assembled nanoparticles using active nanoparticles as a building block.1-9 Porphyrins are essential photosensitizers in many biological photosynthesis systems for light harvesting and energy storage.10-12 They have highly π-π conjugated electron structure that allows for efficient electron and/or energy transfer. Because of the π-π conjugated electron structure, they have strong visible light absorption. In addition, the rigid electron structure provides high chemical stability. These chemical and physical characters are of great interests for applications such as photocatalytic synthesis, phototherapy, and water splitting.13-18 Recently, porphyrins, as photosensitizers, have been extensively investigated from the viewpoint of photo induced electron transfer processes for water splitting and hydrogen generation, both at the molecular and supramolecular level, which was recently summarized in a review article.19 However, the porphyrin monomers have a narrow absorption range in visible light, which limits their practical applications where strong optical absorptions in broad visible spectrum are needed for photocatalytic synthesis, phototherapy, and water splitting applications. In natural photosynthesis systems, porphyrins self-assemble into ordered nanostructures as high performance photosensitizer for light harvesting and energy storage or transfer.10-12 Inspired by their utility and function, recently, there have been widespread efforts in fabrication of synthetic porphyrin nanostructures through self-assembly using a variety of porphyrin monomers as the building blocks, to emulate natural light harvesting processes and energy storage and to develop new nanostructured materials. It has been shown that efficient electron or energy transport relies on not only non-covalent interactions (e.g., hydrogen bonding, π-π stacking, ligand coordination, etc.) that initiates the self-assembly, but


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also the spatial arrangements of individual porphyrins, such as long range ordering of 14, 18, 20-22

intermolecular arrangements, porphyrin separation distance, etc.

Therefore, ability to

tune spatial arrangements during non-covalent self-assembly is highly desirable for engineering porphyrin nanostructures to improve efficient transport for enhanced photocatalytic activity. Herein, we developed synthesis of porphyrin nanostructures with self-assembled onedimensional porphyrin nanostructures with controlled internal spatial separation through oncovalent interactions. This spatial arrangement effectively improved electron transport to enhance the photocatalytic hydrogen production. These nanostructures were synthesized using 5,10,15,20-tetrakis (4-(hydroxyl)phenyl) porphyrin (THPP) (Figure 1A) as the building block. The synthesis process was conducted through an acid-base neutralization reaction to initiate micelle confined nucleation and growth of self-assembled porphyrin nanowires (Figure 1). Owing to the spatial ordered J-aggregate arrangement of porphyrin molecules within the assemblies, the resultant nanostructures possess strong absorption in broad visible spectrum region. Photocatalytic hydrogen evolution indicated that these nanostructures showed enhanced photocatalytic hydrogen evolution property than the original porphyrin powders under visible light irradiation. THPP has four hydroxyl groups at the outer core of porphyrin that can self-assemble to form ordered networks via hydrogen bonding and π-π stacking.23 THPP powders are not soluble in water. In order to initiate the self-assembly in aqueous phase, it is first alkalized so that its hydroxyl groups are deprotonated to form tetrahydroxy TPP4- anions that are soluble in water solutions (Figure 1A). An acidic surfactant aqueous solution was prepared with the surfactant concentration above critical micelle concentration (cmc), so that surfactant micelles are formed. The self-assembly process was then initiated by injection of the TPP4- basic solution into the


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acidic surfactant solution under vigorous stirring (see experimental details in Supporting Information). With the mixing of the solutions, acid-base neutralization reaction occurs (Figure 1A). While the four hydroxyl groups were recovered from corresponding anion state, the two


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Figure 1. Schematic illustration of formation of the self-assembled one-dimensional porphyrin nanostructures. (A) Deprotonation-protonation reactions. (B) Surfactant micelle confined nucleation and growth of one-dimensional porphyrin nanostructures.

nitrogen groups at the inner core of the THPP were protonated forming (H2THPP)2+ (Figure 1A) that is encapsulated within the hydrophobic interior of the surfactant micelles (Figure 1B).23 Although the existence of the mono-protonated cation has been reported, the deprotonated porphyrin form was practically the only stable product found in acidic solutions.24 The protonated THPP monomers (H2THPP)2+ can be confirmed by the green reaction product (Figure S1) and the UV-vis spectroscopy vide infra. It has been shown that the newly formed deprotonated porphyrin displays higher symmetry in electronic density distribution with respect to the initial porphyrin, favoring for the non-covalent self-assembly.24 Consequently, the combined non-covalent interactions such as hydrophobic-hydrophobic, hydrogen bonding, aromatic π-π stacking induce nucleation and growth of self-assembled one-dimensional THPP nanostructures within the micelles (Figure 1B). The final nanostructures were collected by centrifuging to remove unassembled porphyrins and extra free surfactants. The precipitated nanostructures were re-dispersed into DI water for structural characterizations and photocatalytic hydrogen evolution. The representative scanning electron microscopy (SEM) (Figure 2A) shows that the nanowires are uniform and monodispersed, the average diameter is about 110 nm and the average length of 4.25 µm (Figure 2E and 2F). Transmission electron microscopy (TEM) reveals a uniform electron contrast in these one-dimensional nanostructures without defects (Figure 2B). High-resolution transmission electron microscopy (HR-TEM) image shows well-resolved lattice


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fringes with an interplanar distance of 1.39 nm (Figure 2C). X-ray diffraction (XRD) patterns of the nanowires (Figure S2) and FFT of the lattice fringes (Figure 2C inset) confirmed the singlecrystalline nature of nanowires. In addition, the XRD patterns can be indexed as cubic space group P23 (number 195) with unit cell dimensions a = b = c = 1.40 nm, α = β = γ =90, and the


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Figure 2. Structure characterizations of the self-assembled THPP nanowires. (A). SEM image of the nanowires that were prepared using 0.5 mM THPP and 2.5 mM CTAB at pH 2.70 stirring for 48 hours. (B) Corresponding TEM image of the nanowires. (C) HR-TEM image of the nanowires, inset is the FFT from the image in (C). (D) The crystal structure simulated from the XRD data and the TEM image (chloride: green; nitrogen: blue; oxygen: red; carbon: gray). (E) Statistic analysis of the length of the nanowires. (F) Statistic analysis of the diameter of the nanowires.

crystal structure was simulated as shown in Figure 2D. The d-spacing of 1.40 nm from the XRD data that corresponds to the diffraction peak of (100) face, is in agreement well with the lattice spacing of 1.39 nm observed in the HRTEM image (Figure 2C), indicating that the nanowires preferred to grow along direction perpendicular to the (100) crystal face. In addition, the strong diffraction peaks at 30˚and 32˚ with d-spacing of 0.30 nm and 0.28 nm indicated the possible formation of multiple hydrogen bonding, such as (pyrrol) N+H···Cl and OH···H, among THPP molecules within the nanowires.25,

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Noted that the protonation of the porphyrin molecule

(H2THPP)2+ induces strong repulsion between porphyrin molecule, which makes the π-π stacking interactions weak. These overall non-covalent interactions are balanced favoring formation of Jaggregates (Figure 2D and Figure S3). We found that the diameter and the length of the nanowires can be systematically tuned by self-assembly conditions such as the surfactant concentrations and the pH conditions of the reaction solutions (Figures S4 and S5). The surfactant concentration is critical on the formation and morphology control of the nanowires. When the surfactant concentration is lower than cmc, irregular shape will be obtained (Figure S4A). This is probably due to the lack of micelles to


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assist the confined assembly. pH is another important factor to control the morphology of the final nanowires. When the acidic surfactant aqueous solution was mixed with the TPP4- basic solution, acid-base neutralization reaction occurred and the four hydroxyl groups were recovered from corresponding anion state to neutral neat THPP monomers. Recovery of THPP monomers provides an important opportunity for THPP monomers to self-assemble through hydrogen bonding.25, 26 At even lower pH, the pyrrole nitrogen groups from the inner porphyrin core can also be protonated25, 26 so that positive charges are added to the porphyrin core, which adds into the overall non-covalent self-assembly interactions. Additionally, the protonated porphyrin species may serve as convenient models to explore the strength and photochemistry of the chromophore in structurally different porphyrin units.24 Finally, three different one-dimensional nanostructures with different lengths and widths were obtained by just tuning pH and used for photocatalytic studies, including long nanowires with length of 10 µm, nanowires of 4.25 µm and nanorods of 0.50 µm (Figures S5B, S5C, and S5E). To reveal the formation and growth mechanism of the one-dimensional nanostructures, the self-assembly process of the nanowires with length of 4.25 µm was investigated to track the structure evolution in-situ using TEM and UV-vis spectroscopy (Figure 3). From the TEM images (Figure 3A), large amount of random small THPP aggregate seeds and some short nanowires were formed right after the TPP4- basic aqueous solution was injected to the acidic surfactant solution. UV-vis absorption spectra (Figure 3B) showed that the four weak Q absorption bands at 519 nm, 558 nm, 595 nm, and 653 nm from porphyrin monomers (Figure S6) disappeared; for the self-assembled nanowries, two Q bands in the similar areas but with much broader coverage of the visible spectrum appeared, which indicated the formation of protonated (H2THPP)2+.23 After 30 min, more well organized nanowires were formed, indicating


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that the porphyrin monomers are rearranged by multiple supramolecular interactions. Meanwhile, the amount of random aggregates is reduced as confirmed by the TEM image. In addition, a shoulder absorption peak located at 468 nm starts to appear, indicating initial


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Figure 3. Time progression of THPP monomer aggregation to form nanowires, monitored by (A) TEM imaging and (B) UV-vis absorption spectroscopy. In this reaction, 5 mL of stock THPP solution in water (0.01 M, 0.2 M NaOH) was added into 95 mL of CTAB (2.5 mM) and HCl


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12mM) while stirring continuously at room temperature for 48 h. The 0.5 mL samples of the reaction mixture were withdrawn at different times and separated for TEM imaging and UV-vis characterizations by centrifugation at 8000 rpm.

formation of J-aggregates of THPP. From 1 h to 48 h, random THPP aggregates are gradually disappeared and monodisperae nanowires are fully formed. Fourier transform infrared spectroscopy (FTIR, Figure S7) was further performed to characterize the final nanostructures. The results showed that free porphyrin THPP had the characteristic stretching vibration peak of C-N in pyrrole at 1470 cm-1, as well as the bending vibration peaks of C-N at 972 cm-1. When the non-covalent self-assembly occurs to form the nanowires, the characteristic peak at 1470 cm-1 shifts to 1478 cm-1, and the peak at 972 cm-1 splits into two peaks at 964 cm-1and 984 cm-1. This is probably due to the formation of N-H and N-H+ that are resulted from protonation of the inner core N groups, forming (H2THPP)2+ cations within nanowires, which is consistent with results from UV-vis spectroscopy analysis. In addition, the characteristic peaks at 3400 cm-1 and 3500 cm-1 corresponding to N-H and O-H stretching vibration of THPP molecule shift to a broad peak at 3400 cm-1 when the selfassembled nanowires are formed. Formation of the broad multiple peaks around 3400 cm-1 suggests the formation of multiple hydrogen bonding from N-H groups and O-H groups during self-assembly, which are in agreement with the corresponding diffraction peaks derived from multiple hydrogen bondings in XRD. In order to further illustrate the formation of (H2THPP)2+ cations within the THPP nanowires, we compared the FTIR spectra of the self-assembled THPP nanowires with protonated THPP powders. Protonated THPP powders were prepared by adding


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2 mg THPP powders into 10 mL 0.1N HCl (without using surfactants) to ensure that all molecules are protonated.23 We found that both specimens had identifcal FTIR peak positions

Figure 4. Photocatalytic hydrogen production and characterizations of the THPP powders and different self-assembled THPP nanostructures. (A) Hydrogen evolution photocatalyzed by THPP powder and different self-assembled nanostructures. (B) UV-vis spectra of different selfassembled THPP nanostructures. (C) XRD of different nanostructures. (D) Schematic illustration of the self-assembled J-aggregates and photo induced charge process for hydrogen generation. Reaction conditions: 4 mg THPP nanowires loaded with 5 wt% of Pt co-catalyst; 0.2 M of AA aqueous solution (50 mL); 1 N NaOH was used to adjust the solution pH to 4; 300 W Xe lamp with a 420 nm cut off filter for 5 h.


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(Figure S7), which confirms formation of (H2THPP)2+ cations within the THPP nanowires. Overall, the combined results from electron microscopy, UV-vis, and FTIR suggest that the selfassembly process is a confined nucleation and growth process driven by balanced non-covalent interactions. The photocatalytic hydrogen generation experiments of different THPP nanostructures were performed under visible light (λ > 420 nm) irradiation with ascorbic acid (AA) as the sacrificial agent and Pt as the co-catalyst. After 5 h light irradiation, nanorods with length of 0.50 µm exhibited a hydrogen evolution rate of 19.5 mmol/g/h (Figure 4A). When the length of nanowires was increased to 4.5 µm and 10 µm, respectively, the photocatalytic activity reduced to 17.6 mmol/g/h and 1.67 mmol/g/h accordingly. In comparison, the THPP powders only showed a negligible photocatalytic activity with a hydrogen evolution rate of 0.7 mmol/g/h. These results show two interesting obvious catalytic features. The self-assembled nanostructured exhibit enhanced hydrogen production rate; and the overall the self-assembled nanostructured exhibit structure-dependent photocatalytic hydrogen evolution activity. To be noted that the hydrogen production rates of the self-assembled porphyrin nanostructures are much higher than the most reported porphyrin and other organic material based photocatalysts (Table S1) under visible light irradiation on the basis of our best knowledge. Although some metallic photocatalysts have higher hydrogen production rates,27,

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organic porphyrin materials exhibit advantages such as

light weight and absorption of broad visible light. Based on above results, the structure-dependent photocatalytic hydrogen evolution was understood as follows. Firstly, UV-vis absorption spectrum indicates that all peaks red-shifted in comparison with the THPP monomer (Figure 3B and Figure S6), indicating the formation of Jaggregate, which results in a stronger exciton coupling between THPP. Such coupling is


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supposed to activate the porphyrin core by increasing the electronic density within to a definitely larger extent,24 which helps to stabilize the photo excited electrons-holes and leads to the increase of hydrogen evolution rate of all nanowires to the monomer. Such arrangement is similar to the role of light-harvesting complex in natural photosynthesis systems that contain chlorophyll aggregates, in which chromophores form J-aggregates along the head-to-tail arrangement via a stronger transition-dipole moment and harvest solar energy then turn into chemical resource. In addition, as shown in Figure 4B, the UV-vis absorption spectrum of nanorods with length of 0.50 µm covers broad range from 400 to 750 nm. Although the UV-vis absorption spectrum of the nanowires with the length of 4.5 µm also covers broad range from 400 to 750 nm, the intensity of the spectrum shows a lower absorption than that of the nanorods. In comparison, the UV-vis absorption spectrum of the long nanowires with the length of 10 µm only shows main absorption in 400 nm - 500 nm, and further the spectrum intensity is much lower than those of the nanorods and nanowires in the range of 450 nm – 600 nm. The absorption spectroscopy results unambiguously establish that self-assembled nanostructure induce different coverage of the light range of spectra and their intensity that result in the different hydrogen evolution rates. Secondly, from XRD patterns (Figure 4C), we discover that the internal spatial arrangement of the THPP within the self-assembled nanostructure plays a critical role on the photocatalytic activity. The result shows that all three nanostructures have the well-defined diffraction peaks that are similar except the diffraction peaks in the low angle range from 5° - 7°, which attribute to the inter-planar distance of lattice fringes as shown in HR-TEM (Figure 2C). As shown in Figure S8, the inter-planar distance of lattice fringes decreases from 1.42 nm to 1.37 nm when the lengths of the nanowires decrease from 10 µm to 0.50 µm. Note that in the direction along


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the width of the nanowires, the porphyrin molecules are arranged in J-aggregate (Figures S3 and S8), which facilitates the delocalization of excite state electrons instead of the charge recombination. In this case, with the decrease of the inter-separation distance of lattice fringes, the interactions between THPP molecules increase, which enhances the delocalization of excite state electrons, and extends the lifetime of electron-hole pairs. Taken together, the onedimensional nanostructures consist of different internal spatial arrangements of THPP molecules lead to distinct range and intensity of absorption spectra and excitations, which is responsible for such an interesting structure-dependent photocatalytic hydrogen evolution activity. The light harvesting performance and XRD results provide important evidence for the structure or morphology dependent photocatalytic hydrogen generation (Figure 4D). Direct evidence such as real charge lifetimes will require further studies to evaluate the photocatalytic performance. Finally, although the surface areas of the nanostructures could also play a certain role in the catalytic process, we do not have surface area data to support the correlation of the surface areas with performance. In general, organic photocatalysts are limited by their photo stability because they are easy to undergo photobleaching or solvolysis by the solvents. The self-assembled THPP nanowires have geometrical constraints with rigid aggregate framework, which would potentially prevent them from being attacked by reactive species and ensure better stability. The reusability experiments were performed to investigate the stability and robustness of the THPP nanowire catalysts. As shown in Figure 5, the hydrogen production activity of nanowires with length of 4.5 µm was carried out up to 40 h under visible light irradiation. The hydrogen evolution rate in 10 h under visible light irradiation is 14.6 mmol/h/g. The slight reduction in hydrogen evolution rate


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could be due to the consumption of the sacrificial agent during the reaction. The result indicates that these nanowires are fairly stable with very good recycling ability, further confirming the

Figure 5. Cycling runs for the photocatalyzed hydrogen evolution by self-assembled THPP nanowires under visible light irradiation. Reaction conditions. 4 mg THPP nanowires loaded with 5 wt% of Pt co-catalyst; 0.2 M of AA aqueous solution (50 ml) and1 N NaOH was used to adjust the solution pH to 4; 300 W Xe lamp with a 420 nm cutoff filter; T = 279 K; 10% of AA sacrificial agent was re-added after second and third cycles.

stability advantage of self-assembled nanostructures for photocatalytic hydrogen production application. Our initial results indicated that the photostability of nanorods was not as good as


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that of the nanowires. This is probably due to the photocatalytic degradation or dissolution of nanorods. In summary, we developed controlled synthesis of self-assembled THPP nanorods and nanowires through nucleation and growth that is confined within surfactant micelles. The deprotonation-protonation reactions effectively enables the confined self-assembly process. Optical absorption characterizations showed that the self-assembly induced effective optical coupling of THPP, resulting in much more enhanced optical absorption in comparison with the original porphyrin monomers and the absorption bands red shift to more extensive visible light spectrum. This enables efficient energy transfer for enhanced photocatalytic activity for hydrogen generation. The photocatalytic hydrogen generation experiments showed that the hydrogen evolution rate of the self-assembled nanostructures is almost 20 times better than that of the original THPP powders. To further understand the photocatalytic process, experiments such as measurements of real charge lifetime are underway.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author F.B. ([email protected]), Tel: 86-15039024866, Fax: 86-0371-23883868 H.F. ([email protected]), Tel: (505) 272-7128, Fax: (505) 272-7336

Author Contributions ‡

These authors contributed equally. H.F. conceived the idea. N.Z., L.W., H.W., R.C., J.W, and


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F.B. performed the experiments. All authors commented on the manuscript and contributed to the writing of the manuscript.

Note The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. F.B. acknowledges the support from the National Natural Science Foundation of China (21422102, 21771055, U1604139, 21601049, 21171049), Plan for Scientific Innovation Talent of Henan Province (No. 174200510019), and Program for Changjiang Scholars and Innovative Research Team in University (No.PCS IRT_15R18). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Preparation of self-assembled nanowires and their characterizations including TEM, SEM, XRD, FTIR, simulation, and UV− vis (PDF).

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For TOC only.


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Self-Assembled One-Dimensional Porphyrin Nanostructures with

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