Ultra-Long Nanostructured Carbon Nitride Wires and Self-Standing C

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Ultra-Long Nanostructured Carbon Nitride Wires and SelfStanding C-rich Filters from Supramolecular Microspheres Jesus Barrio, and Menny Shalom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13873 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Applied Materials & Interfaces

Ultra-Long Nanostructured Carbon Nitride Wires and Self-Standing C-rich Filters from Supramolecular Microspheres Jesús Barrioa, Menny Shaloma*.

a.

Department of Chemistry and Ilse Katz Institute for Nanoscale Science and

Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel. Email: [email protected]

Keywords: Photocatalysis, sustainable hydrogen production, carbon nitride, ultra-long wires, water treatment.

ABSTRACT

The rational design of ultra-long carbon nitride nanostructures is highly attractive due to their high aspect ratio alongside their high surface-to-bulk ratio, which make them suitable candidates for various applications such as photocatalysts, water treatment, and sensors.

ACS Paragon Plus Environment

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However, the synthesis of ultra-long, continuous carbon nitride wires is highly challenging. Here we report the synthesis of 4-centimeter-long and large lateral size carbon nitride wires by utilizing unique supramolecular spheres, comprised of graphitic carbon nitride (g-CN) monomers as the reactants. In situ scanning electron microscopy studies reveal that upon calcination, the g-CN wires spontaneously start to grow from the spheres, while the remaining assembly which acts as a substrate creates self-standing carbon-rich g-CN porous films. The different morphology, chemical composition, and electronic properties of the wires and carbon-rich g-CN allow their utilization as both photocatalyst and water cleaning materials.

The g-CN wires exhibit excellent photoactivity for hydrogen

production whereas the porous carbon-rich g-CN porous film can be efficiently used for water cleaning applications. The reported work opens opportunities for tailored design of g-CN nanostructures and their use as multifunctional materials for photocatalysis, sensing, and other energy-related applications.

INTRODUCTION


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Graphitic carbon nitride (g-CN) attracts attention in material science due to its remarkable electronic properties, stability, and low price.1,

2

Its application is focused mainly in

environment and energy related devices, for example as photo- and electro-catalyst3, 4 for water splitting,5, 6 pollutant degradation,7, 8 CO2 reduction,9 solar cells,10 or light emitting diodes (LED).11 The main drawback of carbon nitride chemistry relies on the difficulty to fine-tune its chemical and electronic properties by the traditional synthetic methods. A significant advance in g-CN synthesis and performance as a photocatalyst has been obtained by several approaches like nanostructuring,12-14 copolymerization,15 the use of soft/hard templating approaches,16 doping with different heteroatoms (e.g., P, B, and S),17-20 formation of heterojunctions and composites with other semiconductors and metals21-24 or by post functionalization of g-CN with other molecules.25-27 These techniques provide enhanced surface-areas,28 better light harvesting properties, and improved charge transfer and separation processes.29,30 Nevertheless, the capability of designing g-CN materials for a given application has not been fully achieved. Our group and others showed that by manipulating a supramolecular assembly based on hydrogen bonding,31-35 hydrogen-halogen,36-38 or monomer-solvent interactions,39 novel electronic


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properties40-42 and morphologies43-45 can be achieved and therefore g-CN can be tailormade for specific applications, like fluorescent probes for cell imaging,46 1D materials for photocatalysis and semiconductor devices,47 or porous films for photoelectrochemical cells.48 However, the design of structural, chemical, and catalytic properties of g-CN, as well as the final application from the reactant level still remains as a current standing challenge. Here we report on the synthesis of ultra-long carbon nitride wires with a continuous 2D nanostructure and large lateral dimension, together with self-standing porous C-rich films using a unique supramolecular assembly as the starting reactant. The as-prepared carbon nitride wires display excellent photocatalytic activity for hydrogen production, while the remaining porous C-rich g-CN film was successfully utilized as selfstanding filters for organic pollutants and oil removal from water. The photocatalytic activity

was

evaluated

by

performing

hydrogen

evolution

reaction

in

a

water/triethanolamine mixture. The adsorption capacity, measured in weight percentage (%), was assessed using seven different solvents and oils with varying densities (ethanol, toluene, water, ethylene glycol, silicone oil, pump oil, and used engine oil).


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EXPERIMENTAL SECTION

Synthesis of catalysts. The synthesis of the supramolecular assemblies was conducted in a similar way to reported procedures,41 but with important changes. Herein, the spherelike supramolecular assemblies were formed by using ethanol as the solvent, for optimal formation of g-CN wires. The sphere-like supramolecular assemblies were prepared using a precise 1:1:0.1 molar ratio of cyanuric acid:melamine:barbituric acid (CMB0.1) in 50 mL ethanol. The complex solutions were shaken for 4 h using an automatic shaker, followed by 5 min 6000 rpm centrifugation. The product powder was then dried at 60 °C in a vacuum oven, overnight. Finally, 4 h calcination under constant N2 flow at 500 °C (heating rate: 2.3 °C min-1) yielded the catalyst powder. Reference carbon nitride materials were prepared by calcination, under identical conditions, of (a) melamine in a ceramic crucible (MCN), and (b) 1:1 cyanuric acid:melamine complex (CM) in 50 mL of ethanol following the same preparation route as CMB.

CHARACTERIZATION


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The full characterization of the material was performed according to methods already described in previous articles.36,41 An EMPYREAN instrument was used to measure Xray diffraction (XRD) with Cu-Kα radiation. Nitrogen sorption measurements were conducted with a Quantachrome Novatouch porosimeter at 77 K. Before the adsorption measurements, 20 h degassing in vacuum at 150 °C took place. The apparent surfacearea of the final g-CN was calculated using a Brunauer-Emmett-Teller (BET) model. Atomic force microscopy (AFM) was conducted on a Dimension 3100 SPM equipped with a Nanoscope 4 controller. To quantify the carbon/nitrogen/hydrogen and oxygen in the materials, a Thermo-Scientific Flash elemental analyzer OEA 2000 was used. JEOL JSM7400F equipped with a FEG source operated at 3 kV was used for scanning electron microscopy (SEM). Transmission electron microscopy (TEM) images were obtained using an FEI Tecnai T-12 G2 TWIN transmission electron microscope operated at 120 kV. We used an ESEM system Quanta 250 FEG for in situ heating SEM measurements. A Nicolet 6700 spectrometer was used to record FTIR spectra. An integrating sphere-equipped Cary-100 spectrophotometer was used to measure optical absorbance spectra and an Edinburgh Instruments FLS920P spectrofluorimeter was used to measure fluorescence


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spectra. An X-ray photoelectron spectrometer ESCALAB 250 equipped with an ultrahigh vacuum (1×10–9 bar) apparatus, an Al-Kα X-ray source, and a monochromator was used to record XPS spectra. Details of measurement conditions: beam size = 500 μm, pass energy (PE) of 150 eV for survey spectra, PE of 20 eV for high-energy resolution spectra; calibration of all spectra relative to a carbon C 1s peak, positioned at 284.8 eV for charging effects correction. Results processing was performed using the AVANTGE software. The photocatalytic performance was assessed by measuring hydrogen evolution from aqueous solutions, which contained a platinum cocatalyst (3%) and a holescavenger (triethanolamine, TEOA). To maintain an inert atmosphere, a Schlenk flask, filled with argon and connected to a cooling system (constant 25 °C) was used. An array of white LEDs was the illumination source (Bridgelux BXRA-50C5300, 50 W, λ > 410 nm). A typical H2 evolution experiment was performed as follows: to a 50 mL Schlenk flask containing 15 mg of the sample and 19 mL of the 9:1 (v/v) ratio water:TEOA solvent mixture, 19.6 µL of an 8% aqueous H2PtCl6 solution was added. This corresponds to a calculated 3% Pt loading onto the catalyst, i.e., 207 µM total concentration). The reaction mixture was stirred for 30 min in the dark under constant Ar flow before illumination. The


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reactor headspace was analyzed using an Agilent 7820 gas chromatography (GC) system, confirming the production of hydrogen. The quantum yield was calculated from measuring under a flow of argon (via an Ar line) in a sealed GC-connected vessel. In order to remove the existing gases (e.g., H2, N2, and O2), a purging argon flow was maintained in the dark, while samples were taken automatically every 11 min to monitor the purging process.49 A 405 nm LED (Thorlabs M405L2) was then lit to record the H2 production and we used the integrated area to calculate an average quantum yield (AQY%).

(1) AQY% = Ne/Np × 100% = 2M/Np × 100%

where Ne is the number of reaction electrons, Np are the incident photons, and M represents the number of evolved hydrogen molecules.

RESULTS AND DISCUSSION

The initial supramolecular assembly was prepared by mixing cyanuric acid-melamine complex (1:1 molar ratio) with barbituric acid (0.1 molar ratio), referred here as CMB0.1,


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in ethanol as a solvent. The selection of ethanol as solvent and the precise monomers ratio resulted in the creation of supramolecular assemblies that are composed from nanoparticles that aggregate into microspheres (Figure S1). We have previously observed that these monomers self-assemble in water, formatting rod-like structure.31,41 However, here we found that the CMB0.1 in ethanol leads to the formation of spheres. To our surprise, after pyrolysis, long wires had grown from the surface of these spheres as shown later in this manuscript. Intrigued by this behavior, we decided to explore it further and try to understand how these wires were formed and study their properties.

TEM images (Figure S2) show the typical hexagonal structure of the CMB0.1 supramolecular units and the round shape (~400 nm) of the aggregate layer. The g-CN materials were acquired by the calcination of CMB0.1 at different temperatures under N2 atmosphere (2.3 °C min–1 heating ramp). We observed that after calcination at 500 °C, the sample consisted of two different materials. A hard, self-standing brown powder filmlike structure located at the bottom of the ceramic crucible (CMBT(°C)). From the brown substrate, many ultra-long yellow wires grew until they reached the lid (WCN, Figure 1a,


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Figure S3). The wire length reached up to 4 cm (Figure 1a, S4), depending on the distance between the bottom and the lid of the ceramic crucible. It could be clearly observed that the wires grew directly from the g-CN substrate and after their removal, an empty shell can be detected (Figure S5). In order to reveal the complicated growth mechanism of the ultra-long wires we followed the condensation of CMB0.1 into g-CN by

in situ heating SEM (heating ramp 2 °C min–1. Figure 1b, Figure S6).


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Figure 1. a) (From left to right) Images of a 4-centimeter free-standing g-CN wire, selfstanding CMB500, and a crucible with CMB500, and g-CN wires growing from it. b) In situ heating SEM images at different temperatures. The in situ heating experiments reveal that the surface of the supramolecular microspheres becomes smoother at 150 °C and the growth of wires from the surface of spheres begins spontaneously and continues until 220 °C. Afterward, the growth is completed and the condensation to g-CN is finished at higher temperatures. The population of carbon nitride wires (which reach more than 100 µm length) is well spread over the whole sample (Figure S6).

FTIR and XRD measurements of the obtained CMB CN powders show that the condensation is completed at 500 °C (Figure S7). FTIR spectra indicate the stretching vibrations of the heptazine motifs at 1200–1600 cm–1, and vibrations related to the triazine units’ breathing mode ca. 810 cm–1. XRD patterns of the material synthetized at 500 and 600 °C display two peaks at 13.1 and 27.2 degrees, that belong to g-CN’s (100) and (200) crystal planes, respectively. We observed that the production of wires (WCN) was optimal


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at a calcination temperature of 500 °C. FTIR and XRD measurements confirm the typical characteristic features of carbon nitride (Figure S8). SEM images (Figure 2a) indicate that the wires exhibit a layered porous structure, and that their width reach up to 150 µm (Figure S9). TEM images (Figure 2b) reveal that the WCN is composed by thin foldable sheets reaching several micrometer lengths. The thickness of the WCN layer ranges from 2 to 2.5 nm as was determined by AFM (Figure 3). The sheet-like structure together with the high porosity lead to poor mechanical strength of the wires.


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Figure 2. SEM (a) and TEM (b) images of WCN. The preparation of a transparent colloidal suspension for TEM analysis was achieved by exposing a suspension of WCN (0.1 mg mL–1) in ultrapure water to strong sonication for 1 h, followed by a centrifugation step, which removes residual aggregates.

The chemical composition of WCN was studied by XPS analysis (Figure S10). C1s spectrum shows two different binding energies corresponding to adventitious C-C bond at 284.8 eV and C=N-C from the heptazine units at 288.2 eV. N1s spectra displays the typical contributions of carbon nitride as well, at 398.7 eV the C=N-C specie, N-(C)3 at 400.0 eV and remaining amino groups (N-H) at 401.4 eV. The structure and morphology of CMB500 were studied by SEM showing a layered porous framework (Figure S11). Elemental analysis (EA) data (Table S1) indicates that the bottom part (CMB500) contains high carbon amounts due to the barbituric acid in the precursor. However, the atomic composition of the wires perfectly matches the C3N4 atomic ratio, suggesting that the wires formation involves only g-CN precursors. The differences between CMB and WCN are further analyzed using their photophysical properties. UV-vis absorbance results


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(Figure S12) indicate that the absorption of CMB500 is red-shifted compared to WCN, owing to the higher carbon content in the material (as confirmed by EA, Table S1). The fluorescence of CMB CN is strongly quenched compared to WCN due to higher carbon amount.50 Mott-Schottky analysis alongside the Tauc plot (Figure S13) show a CMB500 conduction band position at a more positive position relative to WCN, and its bandgap is reduced to 1.95 eV due to a higher C content.


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Figure 3. AFM images (a, b) and the corresponding thickness profile (c, d) of WCN layers. The preparation of a transparent colloidal suspension for AFM analysis was achieved by exposing a suspension of WCN (0.1 mg mL–1) to strong sonication for 1 h in ultrapure water, followed by a centrifugation step, which removes residual aggregates.

In situ heating SEM imaging, along with the structural and photophysical characterizations suggests that the supramolecular spheres created by self-assembly act as a micro reactor, where a gas released upon calcination pushes mainly melamine/cyanuric acid units upwards. During the calcination process, the monomers involved in the WCN formation emerge and eventually break the spheres, leading to the growth of long wires from their surface. The latter leads to pure graphitic carbon nitride wires that can reach up to 4 cm, while at the bottom of the crucible remains a material with higher carbon content. (Scheme 1)


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Scheme 1. Proposed growth mechanism of WCN.

H2 production from an illuminated aqueous solution containing a platinum cocatalyst and a triethanolamine (TEOA) hole scavenger was used to test the photocatalytic activity. For this measurement, typically 15 mg of catalyst is employed. In this case, due to the 2D characteristics (large lateral size alongside atomic thickness), good surfacearea (40 m2 g–1) and enhanced dispersibility (Figure S14) of the material, a lower amount of only 4 mg could be used in the same water/triethanolamine solution volume. WCN showed a performance more than 3 times higher than g-CN fabricated from the cyanuric acid-melamine complex, CM, (1725 vs 550 µmol H2 h–1 g–1) and more than 40-fold


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enhancement relative to melamine-derived g-CN (MCN) (42 µmol H2 h–1 g–1) and an average quantum yield (AQY%) at 405 nm of 3.1% (Figure 4a).

The self-standing film and porous layered morphology of CMB500 allow its simple utilization as adsorbent of organic pollutants. Self-standing films of CMB500 were placed in a petri dish together with 20 mL of 20 mg L–1 rhodamine B (RhB) or (separately) methylene blue (MB) solutions (Figure 4b). The remaining concentration of the organic dye was monitored by the changes of their absorption maximum, 554 nm for RhB and 664 nm for MB. (Figure 4c, d). The CMB self-standing films display very good pollutant adsorption capability owing to its porous structure, considering that no stirring or exfoliation techniques were employed, and the materials adsorb after two days 90% and 80% of the MB and RhB, respectively (Figure 4e). Furthermore, the filters can be easily recycled by washing them with pure water, (Figure 4f) where they release the adsorbed dye, and be reutilized, maintaining most of their initial activity (Figure S15). The water cleaning capacity of the self-standing C-rich filters was further evaluated by analyzing the adsorption capacity using seven different solvents and oils having different densities


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(ethanol, toluene, water, ethylene glycol, silicone oil, pump oil, and used engine oil). The adsorption capacities expressed using weight percentage were obtained by weighing the dry g-CN self-standing filters before and after solvent/oil adsorption.51 The materials were left submerged in each solvent/oil overnight for acquiring complete saturation. The porous self-standing films exhibit good adsorption capacities for both oils and solvents (Figure S16) reaching up to more than 3 times their own weight for used oil or silicone oil. Although there are higher values reported,52-54 these uptake capacities are higher compared to commercial materials like activated carbon or BN particles.51 Furthermore, to the best of our knowledge we report for the first time the application of a self-standing graphitic carbon nitride film for this purpose.


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Figure 4. a) Produced hydrogen per hour and gram of CN materials. The inset shows a picture of 15 mg of MCN, 15 mg of CM, and 4 mg of WCN (from left to right) in order to emphasize the effect of the surface-area of WCN. b) Self-standing carbon nitride filters in RhB and MB solutions. UV-vis spectra measured during 48 h of adsorbing c) RhB, and d) MB. e) RhB and MB adsorption curves using the carbon nitride filters, and f) images of the recycling carbon nitride filters in pure water.


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CONCLUSIONS We demonstrated a simple and efficient synthesis of continuous, ultra-long carbon nitride wires, up to 4 cm in length, with a large lateral size by using unique supramolecular microspheres comprised of carbon nitride monomers. Upon heating, condensation occurs and g-CN wires spontaneously emerge and grow upward from the sphere surface. The resulting g-CN wires exhibit excellent photocatalytic activity for hydrogen production thanks to their good dispersibility in water, nanostructure, and large lateral dimension. In addition, during the condensation process the supramolecular assembly which acts as a substrate for the growth of the g-CN wires, a self-standing carbon-rich porous film is formed. The self-standing film acts as an efficient filter for pollutants and oil removal from water (Scheme 2). It is suggested that new possibilities for rational design of g-CN (and other related carbon nitride-based polymers) as multifunctional materials for sustainable applications such as photocatalysis, water treatment, and other energy-related applications are achievable via this approach.


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Scheme 2. Illustration of the obtained materials after calcination of the supramolecular microreactors. ASSOCIATED CONTENT Supporting Information Characterization of prepared supramolecular assemblies and of the final materials using TEM, SEM, FTIR, XRD, UV-vis, and AFM. Electrochemical characterization and photocatalysis performance assessment of carbon nitrides. AUTHOR INFORMATION

Corresponding Author


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Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel. Email: [email protected]

Author Contributions

The manuscript was written through contributions of all authors and they all have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors thank assistance from Dr. Merav Muallem and Dr. Yossi Talyosef from BarIlan University for the in situ heating SEM experiments. We thank Dr. Michael Volokh and Jonathan Tzadikov for fruitful discussion. The authors also thank the IKI technical staff at Ben-Gurion University of the Negev: Dr. Alexander Upcher and Dr. Einat Nativ-Roth for electron microscopy and Mr. Jürgen Jopp for AFM measurements. Funding was granted through the Israel Science Foundation (ISF), grant No. 1161/17.

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Ultra-Long Nanostructured Carbon Nitride Wires and Self-Standing C

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