Self-Standing Carbon Nitride-Based Hydrogels ... - ACS Publications

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Self-standing Carbon Nitride-based Hydrogels with High Photocatalytic Activity Jingwen Sun, Bernhard V. K. J. Schmidt, Xin Wang, and Menny Shalom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14879 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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ACS Applied Materials & Interfaces

Self-standing Carbon Nitride-based Hydrogels with High Photocatalytic Activity Jingwen Sun,†,‡ Bernhard V.K.J. Schmidt,*,‡ Xin Wang,*,† and Menny Shalom*,‡,§ †

Key Laboratory of Soft Chemistry and Functional Materials, Nanjing University of Science and

Technology, Ministry of Education, Nanjing 210094, China. ‡

Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research

Campus Golm, Potsdam 14424, Germany. §

Chemistry Department, Ben Gurion University of the Negev, Beersheba 009728, Israel.

KEYWORDS: carbon nitride, hydrogel, selective adsorption, photodegradation, hydrogen production

ABSTRACT: Here we report a facile synthesis of carbon nitride-based hydrogels with adjustable shapes, ranging from cylinder to tube and thin sheet, by photo-polymerization process in confined templates. The fabricated hydrogel shows enhanced mechanical properties compared to the reference gel without carbon nitride incorporation, good adsorption capacity and promising photocatalytic activity toward hydrogen production. Meanwhile, the hydrogel also exhibits selective pollutants adsorption properties which could be attributed to the negative-charged carbon nitride as well as relatively high stability alongside enhanced light harvesting. The novel

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carbon nitride-based hydrogels offer a facile approach as photocatalyst and open-up many opportunities for smart-catalyst design with adjustable reaction sites.

Soft materials, including liquids, polymers, foams, gels, colloids, granular materials, as well as most soft biological materials,1-3 are materials that can be easily utilized in the formation of shape-persistent, free-standing objects as their exceptionally great mechanical strength and rapid self-healing behavior.4-6 One of the most familiar soft materials is plant chloroplasts, which can co-localize molecules involved in light absorption, charge transport and catalysis to create chemical bonds with solar energy.2,3 Inspired by this, photosynthetic or photocatalytic soft materials are widely investigated to generate storable fuels,7 reducing the world’s dependency on fossil-fuel. Among all the soft materials, hydrogels are a major class of materials, which are wet and soft, defined as a crosslinked-polymer network involving large volumes of water.1,8,9 Such systems could experience substantial swelling and collapsing as the most remarkable properties of these materials, in contrast to most industrial materials such as metals and plastics. Thus, hydrogels show a lot of advantages as potential hydrophilic adsorbents,10 active sensors11 and modulators for delivery of drugs.12 By virtue their different functional groups, hydrogels have found extensive applications in water and wastewater treatment.13 Usually, hydrogels are formed via free radical polymerization in water with crosslinker addition,14 physical crosslinking15 or mechanical crosslinking.9,16 The formation mechanism has a significant impact on the final properties of the hydrogels, e.g. high mechanical strength for double network hydrogels,17 high stretchability for slide-ring gels16 or self-healing properties in the case of supramolecular gels.18

Recently, carbon nitride (C3N4) has widely attracted attention due to its unique chemical, electronic and (photo) catalytic properties alongside its low price and the easy synthesis, which


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makes it a proper alternative for metal based catalysts for energy related applications19-22 or as initiator for polymerizations.17 Up to date, carbon nitride was mainly used as a powder for photocatalytic reactions as photodegradation of pollutants and for solar fuel production (i.e. reduction of water and carbon dioxide) thanks to the acquirement of high surface area and good dispersion in different solvents.23,24 Very recently, the concept of photoactive hydrogels was introduced.6 The hydrogel structure as a substrate for photo-induced reactions holds the opportunities to (1) control the reaction sites (2) improve loading of adsorbate, (3) facilitate the catalyst recycling and (4) enhance its mechanical properties. In addition, the photoactive material can act as an initiator for the polymerization process in the gel formation, thus allowing good photocatalyst distribution within the hydrogel in one step.4,5 Furthermore, hydrogels can be formed in various shapes in perfect adjustment to the intended application, e.g. tubes, spheres or thin films. In this work, we report the facile synthesis of a carbon nitride-based hydrogel with adjustable shape, i.e. cylindrical and tube-like, based on N,N-dimethylacrylamide (DMA) through photopolymerization in the presence of well-dispersed CNB (carbon nitride calcined from CMB, cyanuric acid-melamine-barbituric acids) as initiator in aqueous solution.23,25-27 Figure S1 and Figure S2 provides the basic morphology and structure of CMB and CNB, separately. The selfstanding carbon nitride-based hydrogel (CNB-G) presents higher mechanical strength, enhanced deformation restorability and better stability compared to the reference hydrogel without carbon nitride incorporation. Furthermore, the CNB-G shows good catalytic activity toward the H2 production as well as strong adsorption capacity and high catalytic activity for the degradation of various cationic dyes.


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Figure 1. (a) Proposed mechanism for hydrogelation. (b,c) Cryo-SEM images of Ref.-G (hydrogels formed via conventional photoinitiator without carbon nitride incorporation), inset is the digital photo of untreated Ref.-G. (d-e) Cryo-SEM images of CNB-G with 0.03 wt% CNB, inset is the digital photo of untreated CNB-G. Cylindrical CNB-G was designed in inerratic injectors under light irradiation, as illustrated in Figure 1a and Figure S3. An aqueous colloidal dispersion of the semiconducting CNB produces radicals on its surface which induce in situ radical polymerization of added mono functional acrylamide

derivatives,

namely

DMA

and

bifunctional

crosslinker,

namely

N,N’-

methylenebis(acrylamide), to afford crosslinked hydrogel network in one step that is reinforced via the CNB structures.28 The stability of the CNB dispersion was confirmed via photoluminescence (PL) measurements and TEM images as shown in Figure S4. The transparency of the CMB sheets suggested that CNB sheets are well-dispersed in aqueous solution. In addition, The PL spectra don’t change during time, promising that the hydrogel contains well-dispersed CNB sheets. We note that photopolymerization without crosslinker also led to the formation of hydrogels, albeit with a significant lower mechanical strength (Figure S5).


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This behavior is an indication that the CNB is actually incorporated into the gel structure, as shown in Figure 1a. After fabrication, the fresh CNB-G and Ref.-G were taken out of the injectors and put into deionized water for one week to get the swelling equilibrium state. Water content, equilibrium swelling degree (ESD) and gel phase percentage were calculated in Table S1. The high water content and good swelling capacity encourage us to use the hydrogel for waste water treatment, e.g. in dye adsorption and as photocatalyst. Cryogenic scanning electron microscopy (Cryo-SEM) of the Ref.-G (hydrogels without carbon nitride incorporation, formed via photopolymerization with a standard photoinitiator, see SI for details) and CNB-G with differing CNB contents are displayed in Figure 1b-e and Figure S6. The transparent Ref.-G gradually turned to yellow with increasing CNB amounts. Besides, the pore size of CNB-G is much more narrow and uniform than the Ref.-G, especially the one with 0.03 wt% CNB. Such three-dimensional porous structure will efficiently anchor the CNB particles, substantially inhibit their aggregation or stacking. After introduction of increased amounts of CNB, the pores of CNB-Gs further grew due to the carbon nitride’s limited solubility in water. Thus, 0.03 wt% CNB-G was selected as the basic research object in the following investigations. Alongside the CNB-G and Ref.-G, Bulk-G (carbon nitride-based hydrogel formed with carbon nitride calcined form melamine only) was synthesized via the same method for comparison. SEM images and surface areas of the corresponding freeze-dried hydrogels are shown in Figure S7. The good dispersion of the CNB-G leads to more homogeneous structure compared to the Bulk-G. Figure S8 also provides the XRD and FTIR spectra of the freeze-dried hydrogels.


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However, only the features of the monomer and crosslinker could be obtained and no distinct differences can be discovered.

Figure 2. (a,b) UV-Vis and PL spectra of Ref.-G (1), Bulk-G (2) and CNB-G (3) with 0.03 wt% carbon nitrides inside, respectively. (c) The digital photos of Ref.-G (1), Bulk-G (2) and CNB-G (3) without and with excitation at 254 nm. (d) CNB-G (3) supporting a 500 g counterweight without structural collapse, and the digital photos of Ref.-G (1) and CNB-G before and after supporting the counterweight. CNB-G, Ref.-G and Bulk-G were further investigated via UV-Vis spectra in Figure 2a. Alike the powder form, the absorption of CNB-G is much stronger and red-shifted compared to the other samples.25 Another evidence for the good distribution and the lack of materials aggregation is given by the enhanced emission, accompanying with the gradual color changing thanks to the


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genuine carbon nitride, of the hydrogel compared to the powder form (Figure 2b,c). The latter also emphasizes the possible utilization of g-CN hydrogel for other opto-based devices, e.g. sensors. The mechanical property of CNB-G was valued through a simple compression test with an applied pressure of 500 g counterweight. As shown in Figure 2d and Figure S9, CNB-G shows attractive deformation restorability integrity and better mechanical strength compared to the Ref.-G, which can be attributed to the strong repulsion between the negatively-charged carbon nitride layers (zeta potential of -25.5 mV) under compression as was shown for TiO2 sheets as well as the reduced mobility of the polymer chains due to the interpenetrating carbon nitride sheets.28-30 Moreover, rheological measurements of as synthesized CNB-G showed a storage modulus (G’) of 840 Pa at 0.1% strain as well as a loss modulus (G’’) of 274 Pa at 0.1% strain, which underpins the improved mechanical properties of the hydrogel. However, in the present study we mainly focused on the photocatalytic activity and adsorption properties while the elucidation of the detailed mechanical properties is still under investigation. The superior swelling capacity and good stability of the three-dimensional CNB-G positions it as a perfect candidate for pollutant removal. Therefore, the original cylindrical CNB-G was sliced in several pieces with the same volume of 1 mL (Figure S3) and the adsorption of different dyes was checked. Figure 3a describes the adsorption capacity of CNB-Gs for methylene blue (MB), rhodamine B (RhB) and methyl orange (MO), which can be visually discovered through the changes in gels’ colors. CNB-G shows the highest adsorption capacity for MB, almost two fold to RhB or MO, indicating a selective adsorption of cationic dyes in the gel, speaking for the preservation of the negative CNB charge. The latter indicates that the dye adsorption is not only diffusion driven (due to the concentration difference) but strongly bound due to chemical


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interaction between the adsorbate and the CNB. Figure S10 also provides the adsorption capacity of Ref.-G on mixed dyes for reference.

Figure 3. (a) Adsorption capacity of CNB-G on MB, RhB or MO, inset is the digital photos of CNB-Gs before and after adsorption. (b) Adsorption capacity of CNB-G on mixed dyes. (c) PL spectra of CNB-Gs before and after adsorption equilibrium, inset is the digital photos of these


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four gels without (left) or with (right) 254 nm excitation. (d) Photodegradation of CNB-G on MB, RhB or MO. (e) Photodegradation of CNB-G on various dyes (congo red, crystal violet, methyl orange, methylene blue and rhodamine B, from left to right). Taking the advantage of the strong ionic interaction, qualitative testing of its adsorption selectivity was evaluated through an intelligent separation of multifarious dyes’ mixture in Figure 3b and Figure S10. With CNB-Gs immersed in RhB/MB mixture and MB/MO mixture for 5 h, the color of the solutions turned from purple to pink and green to yellow, respectively. And there was no discernible color changing in the RhB/MO system. The Ref-G shows similar dyes adsorption, but without any selectivity. The latter further confirms the better adsorption affinity of the hydrogel with carbon nitride. Both of the above efficiently prove our previous hypothesis, the CNB-G shows selective adsorption of cationic dyes like MB, especially in the mixture with different kinds of ionic dyes (Figure 3b). To further support the above-mentioned assumption, cationic red x-GRL, acid black and neutral dark yellow were further chosen as differing dyes to prove the assumptions above (Figure S11). PL spectra of original CNB-G and CNB-Gs after adsorption equilibrium with various dyes (MB, RhB, MO) are shown in Figure 3c. In comparison with the original CNB-G, CNB-G/RhB or CNB-G/MO, only CNB-G/MB suffered a strong quenching, which might be contributed to the strong interaction between hydrogels and specific cationic dyes, which allows the direct charge transfer between the CNB and the dye.8 Such selective adsorption and separation via CNB-G also gives rise to more opportunities in the photocatalytic fields. Figure 3d,e displays the photocatalytic activity of CNB-G under white LED irradiation. Analogous to the adsorption results, CNB-G also exhibited superior photocatalytic abilities in the degradation of cationic dyes, which is usually driven by direct


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electron injection from the photoexcited CNB to MB, further accentuating the direct contact between the CNB to the cationic dyes. The degradation intermediates were investigated by LCMS and TOC as shown in Figure S12. The unique properties of the hydrogel opens up the opportunity to first load the pollutants (for example overnight) and to remove it during day light. The good stability and the easy recycling of the hydrogel allows repeating the process many times. Moreover, taking into account its chemical interactions the gels can be used for smart materials release design based on its ionic interactions.


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Figure 4. (a) Time courses of H2 production from water with FD-CNB-G, CNB-G, Bulk-G, Ref.-G and Ref.-G-CNB under white LED irradiation. (b) Cycling measurements of H2 generation of wet CNB-G, inset is the photo of CNB-G before and after H2 production. (c,d) Cryo-SEM images of CNB-G before and after H2 production. (e) Tube-like CNB-G design. The application of CNB-G can be further extended to the hydrogen production under illumination. The morphology of the hydrogel results in much more active sites, which can lead to better H2 production. Figure 4a shows the H2 production from water with freeze-dried CNB-G (FD-CNB-G), CNB-G, Bulk-G, Ref.-G or Ref.-G with adequately adsorbed CNB suspension (Ref.-G-CNB) under white light irradiation. Obviously, CNB-G displays the highest activity compared to the other wet gels. In the freeze-dried CNB-G system, water affinity is much more significant than that of untreated gels, thus water, triethanolamine (TEOA) and platinum ions may diffuse into the dried gel rapidly. Sufficient active sites, with available water in the surrounding, accelerate the speed of H2 production. While in the untreated CNB-G system, TEOA and platinum ions will slowly enter into the wet gel, retarding the reaction to some extent. As a result, the freeze-dried CNB-G system shows two fold increased activity compared to the untreated CNB-G. The cycle performance of CNB-G is displayed in Figure 4b, even after 3 cycles, the H2 production was still higher than 85%. Cryo-SEM images of CNB-G before and after reaction revealed no significant differences, further indicating its favorable stability (Figure 4c,d). Although the hydrogen production is not yet as high as the most active powder form mostly due to the diffusion limitation, it is possible to increase the efficiency by adjusting its configuration via hydrogel thickness and structure. As a proof of principle, we therefore also designed and fabricated a tube-like hydrogel, as exhibited in Figure 4e. In general, the great


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simplicity of the system encompasses the opportunity to shape the photoactive hydrogel according to the desired application. For example the tube-like structure opens possibilities for flow-chemistry and even sequential catalysis by employing multi-functional groups along the tube. The latter offers a new model for integrated chemical systems that can be applied in the environmentally interesting chemical transformations, production of solar fuels or artificial systems. In summary, three-dimensional carbon nitride-based hydrogel was generated through photopolymerization of acrylamide derivatives in inerratic injectors under white LED illumination in an easy, cheap and versatile fashion, leading to materials with strong mechanical strength and deformation restorability. Motivated by the advantages of CNB-G, like attractive swelling capacity, enhanced light absorption properties, remarkable stability and facile recycling, it was successfully applied in selective dyes’ adsorption and photodegradation, especially for specific cationic dyes. Furthermore, we showed as a proof of principle that CNB-G hydrogel can act as an efficient photocatalyst for hydrogen production. The carbon nitride-based hydrogel offers many new possibilities in the field of smart-catalysis, sensors and mechanical based devices.

ASSOCIATED CONTENT Supporting Information. Supplementary methods, theoretical calculation, characterization, and SEM, XRD, FTIR, BET, heating TEM, elemental analysis are shown. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected] (Bernhard V.K.J. Schmidt), [email protected] (Xin Wang), [email protected] (Menny Shalom).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources NNSF of China (No. 51572125). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We would like to thank the technical staff at MPIKG and our group members for performing service measurements. We thank Baris Kumru for rheology measurements. ABBREVIATIONS DMA, N,N-dimethylacrylamide; CNB, carbon nitride calcined from cyanuric acid-melaminebabituric acid; Bulk, carbon nitride calcined from melamine; CNB-G, hydrogel fabricated with CNB; Bulk-G, hydrogel fabricated with Bulk; Ref.-G, hydrogel fabricated without carbon nitride; FD-CNB-G, freeze-dried CNB-G. REFERENCES


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Self-Standing Carbon Nitride-Based Hydrogels ... - ACS Publications

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