Water Oxidation

Dispersing Molecular Cobalt in Graphitic Carbon Nitride Frameworks for Photocatalytic Water Oxidation Guigang Zhang, Caijin Huang,* and Xinchen Wang*

The

development of water oxidation catalysts (WOCs) to cooperate with lightenergy transducers for solar energy conversion by water splitting and CO2 fixation is a demanding challenge. The key measure is to develop efficient and sustainable WOCs that can support a sustainable photocatalyst to reduce over-potentials and thus to enhance reaction rate of water oxidation reaction. Cobalt has been indentified as active component of WOCs for photo/electrochemical water oxidation, and its performance relies strongly on the contact and adhesion of the cobalt species with photoactive substrates. Here, cobalt is homogeneously engineered into the framework of pristine graphitic carbon nitride (g-C3N4) via chemical interaction, establishing surface junctions on the polymeric photocatalyst for the water oxidation reaction. This modification promotes the surface kinetics of oxygen evolution reaction by the g-C3N4-based photocatalytic system made of inexpensive substances, and further optimizations in the optical and textural structure of Co-g-C3N4 is envisaged by considering ample choice of modification schemes for carbon nitride materials.

1. Introduction Photocatalytic water oxidation to liberate dioxygen gas is a key bottleneck for the conversion and storage of solar energy in the form of chemical bonds, because it is a multi-electron process coupled with a multiple-proton transference in an uphill energy transformation process.[1] Inspired by the plant Mn4CaOx cluster stabilized by proteins for water oxidation at soft biological interfaces in Photosystem II, significant efforts have been devoted to developing functional and structural mimics of the cubene Mn4CaOx cluster for water oxidation using artificial materials in combination with organic linkers to tune redox properties.[2] Up to now, noble ruthenium[3] and iridium[4] metals have been indentified as highly effective artificial catalysts that operate under homogeneous conditions, with ca. 10,000 turnover (TON). However, their limited availability, high toxicity, and unacceptable cost restrict

G. G. Zhang, Prof. C. J. Huang, Prof. X. C. Wang State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry Fuzhou University Fuzhou 350002, China E-mail: [email protected]; [email protected] DOI: 10.1002/smll.201402636 small 2014, DOI: 10.1002/smll.201402636

sustainable applications. Thus, the development of WOCs based on the first-row transition metals, which are regarded as abundant, environmentally benign and cost-effective catalyst alternatives, is of great admiration. Recently, some first-row transition metal based oxides such as λ-MnO2, α-Fe2O3, Co3O4 have shown promising ability towards water oxidation,[5] because they are available to be fabricated as anode materials for (photo) electrolytic water oxidation, while the energy need to drive this reaction can be provided either by sunlight or by electric current. Among the numerous WOCs, cobalt based materials (e.g.: Co-Pi, Co3O4) draw particular attentions because of their low cost, low toxicity, earth abundance, and most importantly, redox transformation between different chemical valence states (e.g. Co2+, Co3+) is convenient, thus lead to a high water oxidation performance with tunable properties, even with self-healing functions.[6] However, for a classical heterogeneous catalytic reaction, the photocatalytic performance of the water oxidation catalysts is strongly depended on the size, morphology and dispersion of catalysts.[7] A convincing fact to reveal this issue is that the commercial CoO (72 nm) with a low surface area (25.5 m2 g−1) is inert, but CoO nanoparticle (15 nm) with a higher surface area (75.9 m2 g−1) become reactive for oxygen oxidation. Moreover, significantly enhanced activity can be

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(3, 5, 10, 20, and 100 mg) were dissolved in 10 mL deionized water together. After being mixed completely, it was evaporated under vigorously stirring to remove water. The resultant solids were grinded and then heated at 600 °C in tubular furnace for 4 h with N2 flow. The obtained solids were denoted as 0.3, 0.5, 1, 2 10 wt-Co-g-C3N4 for simplicity.

2. Results and Discussion In the first place, to investigate the local structure of Co2+ modified g-C3N4 material, XRD, solid-state 13C NMR characterizations were performed. Powder XRD patterns of the samples are shown in Figure 1a. The most intense XRD peak located at 27.4o is a characteristic peak (002) of graphitic stacking, while the other one at 13.0o (d = 0.675 nm) can be attributed to the in-plane structural repeating units of tri-s-triazine.[17] It is interesting that few difference can be viewed, except for a gradually decrease in the diffraction peak intensity as the cobalt content increased. This result illustrates an inhibition of polymeric condensation by excessive cobalt species. Besides, there is not any peak belong to

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obtained when molecular Co2+ is incorporated into substrate material (e.g. ZnO) with an appropriate lattice substitution.[8] This is not a unique instance, but has its counterpart in many catalysts. For instance, by placing Pt monolayer on a bulk substrate of tungsten monocarbide (WC), comparable hydrogen evolution reaction (HER) activity as a bulk Pt foil electrode can be obtained due to reduced kinetic barriers.[9] It is therefore desirable to engineer cobalt based materials into ultrafine nanoparticles even molecular scale so as to enhance the contact and adhesion of the guest with electro/ photo active substrates for water oxidation. Very recently, intense attention has been attracted by a π-conjugated g-C3N4 polymer, which is a promising candidate for photocatalytic water splitting due to the appropriate band structures (CB = −1.42 V, VB = 1.28 V, versus Ag/AgCl).[10] The photocatalytic activity of pristine g-C3N4 may be promoted via various chemical modifications, such as doping,[11] sensitizing,[12] nanostructure engineering,[13] copolymerization,[14] and hybridization.[15] At present, however, the photocatalytic activity for O2 evolution is an order of magnitude lower than that for H2 evolution, which is mainly due to the relatively negative LUMO position determined by the N 2p hybridization.[16a] To enhance the water oxidation activity, one of the preferable strategies is to down shift the LUMO position. To achieve this goal, a novel g-C3N4 polymer was developed via self-polymerization of thiocyanuric acid under N2 gas.[16b] It is interesting to notice that the valence edge of as-prepared solid was positive than that of traditional g-C3N4 ascribed to the S-mediated polycondensation process, and thus its photocatalytic activity for water oxidation was promoted. Besides, the oxygen evolution rate can be enormously promoted by loading Co3O4 nanoparticles (ca. 5 nm) as cocatalysts, which is mainly due to the fact that the oxygen evolution kinetic rate is greatly accelerated.[6c] This is no matter an exciting result, but further improvement in the activity is still in principle envisaged by the molecular engineering of cobalt species in the g-C3N4 framework, for example by coordination atomic cobalt into the nitrogen-rich carbon nitride network to create soft hybrid interface for water splitting chemistry, while still keeping the basic semiconductor characteristics of g-C3N4 to induce surface photocatalytic redox reactions. Generally, g-C3N4 can be regarded as a good Lewis base for the coordination reaction with metal ions (e.g. Fe3+, Zn2+) due to the existence of rich nitrogen lone-pair electrons in the covalent polymeric framework.[11] A model case of how nature progresses in such situations is also found in porphyrins and chloroplast, where metal centers (iron and Mg) are coordinated with the nitrogen atoms of the aromatic heterocycles. Such a metal modification can also enable g-C3N4 polymers for direct activation of H2O2 and molecular oxygen for organosynthesis.[11a,b] Following this line of inquiry, in this paper, Co2+ has been introduced into g-C3N4 framework in order to speed the charge carriers transfer rate, thus boost their water oxidation performance. To obtain the optimized polymer, a simple approach was carried out based on softchemical synthesis by using dicyandiamide (DCDA) and cobalt chloride as precursors (see details in Experimental section). Briefly, 1 g DCDA and a certain amount of CoCl2

g-C3N4

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ppm Figure 1. Powder XRD patterns (a) and Solid state 13C NMR (b) for pure and modified g-C3N4 samples.

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cobalt, cobalt oxide, cobalt nitrides, and cobalt carbides, even when cobalt contents increased to 10 wt%. This is a strong indication that cobalt was incorporated in the polymeric carbon nitride framework homogeneously. The proposed graphitic structure of cobalt modified carbon nitride can be further estimated by the solid state 13C CP-MAS NMR spectra (Figure 1b). Both g-C N and 3 4 Co-g-C3N4 show two distinct peaks located at δ = 164.5 and δ = 156.6 ppm, respectively. The first peak is attributed to the sp2 hybridized C in the form of N-C = N, which is determined as the major carbon of aromatic carbon nitride,[18] while the later one is related to the C atoms like CN3 in the melem. Also, no signals corresponding to cobalt can be observed in the picture, further expounded atomic cobalt coordinated with carbon nitride heterocycles. Furthermore, no typical peak ascribed to cobalt can be detected for the FT-IR and UV-Raman analysis (Figure S1). Since no evident alternation in the structure can be found after cobalt modification, the chemical state of cobalt is then desired to be made clear. XPS characterization was therefore carried out to check the oxidation state of cobalt. In Figure 2a, a typical spectrum reveals a binding energy (BE) peak related to Co 2p3/2 at 781.2 eV, which is lower than that of 783.2 eV present in the pure CoCl2. This is mainly due to the covalent contact between Co (II) and nitrogen in the conjugated system. The BE of 781.2 eV is close to the value reported for Co (II) porphyrin, where the metal center is coordinated to tetradentate N4 ligands and Cl (710.5 eV).[19] The high resolution of C1s and N 1s were also carried out. In Figure 2b, two single peaks of C 1s locating at 284.6 and 288.1 eV respectively can be checked. The former one is attributed to the sp2 hybridized C-C bonds, which is determined as the standard carbon. The later one is related to the sp2bonded carbon in the heterocycle (N-C = N) of aromatic g-C3N4, which is considered to be the major carbon in the skeleton of the conjugated system.[20] The largest peak of N1s in Figure 2c locating at 398.7 eV is corresponded to the sp2 bonded nitrogen in the form of C-N = C. This C-N = C linkage is viewed as the building block of the conjugated units, while the weak peak at 400.2 eV is caused by the tertiary nitrogen N–(C)3 groups. These two nitrogen together with the sp2 bonded carbon (N-C = N) make up the heptazine heterocyclic ring (C6N7) units.[21] All of the peaks of C 1s and N 1s are almost the same as those of the pristine carbon nitride (Figure S2), once again illustrating the fact that the cobalt modification at low levels do not alter the building structure of the polymers. We also conducted the control experiments. Only CoCl2·6H2O was treated at the same conditions just without the g-C3N4 precursor. As we expected, after the thermal treatment in the nitrogen atmosphere, the crystal water was gradually lose. This is a common phenomenon in the inorganic chemical reaction and can be certified by the XRD characterizations (Figure S3). Besides, the as-prepared Cog-C3N4 is very stable even when it was treated with strong acidic solution (2 M HCl). As shown in Figure S4, similar properties including DRS, XPS, and photocatalytic activities were obtained for the sample treated with 2M HCl and the non-treated sample.

404.1 eV

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BE / eV Figure 2. High resolution XPS spectra of Co 2p, C 1s, and N 1s for Co-gC3N4 sample.

To further estimate the existence of Co (II) species in the polymeric network, low temperature (liquid nitrogen, 77K) EPR spectra was carried out to characterize both pure and modified g-C3N4. In Figure 3, the weak and broad band of

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g-C3N4 Co-g-C3N4

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Co-g-C3N4 peaking at 1300 G with a g-value about 6 is determined as the Co (II) species in the material.[22] Besides, the strong peak l at 3400 G for both samples with a g-value of 2.0034 is attributed to an unpaired electron on the carbon atoms of the aromatic rings within p-bonded nanosized clusters, which is considered as the typical signal of g-C3N4.[14a] This once again illustrates the cobalt impregnation does not change the basic structure and texture properties of g-C3N4, but modifying the surface properties of the polymer for heterogeneous catalytic reactions. To give a direct exhibition of the texture, TEM characterization was performed. In Figure 4 e and f, a similar typical graphitic stacking structure can be checked for both pure and modified g-C3N4. This indicates that the basic morphology does not change after modification. In addition, there is no accumulation of particles on the surface of g-C3N4, which suggests that the cobalt was coordinated in the conjugated system of g-C3N4. This can be further proved by the elemental mapping characterizations. The elemental mapping pictures of C, N and Co elements for the selected area are shown in Figure 4b-d. This indicates that the atomic cobalt is highly dispersed in the soft carbon nitride framework, enabling to tune optical/electric and surface properties. The effect of metal modification on the electronic structure of g-C3N4 can be identified by the optical absorption spectra (Figure 5a). It is clear to observe that the absorption band edge gradually shift to the red wavelength region when increasing the Co content in the Co-g-C3N4 materials. Besides, an obvious enhanced optical absorption was found in the visible light spectrum range (450–700 nm). This change is similar to ligand-to-metal charge transition in molecular systems, once again illustrating the fierce host-guest interaction between metal and g-C3N4.[11a] This optimization in the electronic structure of the g-C3N4 tends to strengthen the

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Figure 4. HR-TEM in dark (a) and elemental mapping of Co, C, and N (b-d) for 5wt-Co-g-C3N4; HER-TEM (e, f) for pure and modified g-C3N4 samples.

charge carrier transfer in the conjugated system of aromatic CN heterocycles. The generation, separation, and migration rate of the photogenerated carriers of g-C3N4 was revealed by the room temperature photoluminescence (PL) under 400 nm light excitation (Figure 5b). Clearly, the PL intensity was gradually decreased when increasing the metal doping content. This is a good illustration that the recombination rate of the photogenerated charge carriers is enormously restrained.[14] The accelerated charge carrier transfer rate can also be determined by the photoelectrochemical experiments. In Figure 6, it shows the transient photocurrent generation from g-C3N4/ FTO electrodes at 0.4 V vs. Ag/AgCl in 0.2 M Na2SO4 under visible light irradiation (λ > 420 nm). Evidently, the current density of Co-g-C3N4 is about two times higher than that of pure g-C3N4. This suggests the electrical conductivity was improved at the electrode interface.[20] The photocatalytic behavior of the cobalt modified carbon nitride is evaluated in an assay of water oxidation in the presence of AgNO3 as electrons trapping agent and thus leaving the holes with sufficient chemical potentials to oxidize water. Firstly, we checked the effect arise from the doping content.

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As the metal density increased, the activity is firstly increased and then decreased (shows in Figure 7a). When the doping density is determined as 1.0, the oxygen evolution rate is 10.5 µmol h−1, which is 2 times faster than that of the pure g-C3N4 (4.5 µmol h−1). Although the oxygen evolution rate is still low, this indeed provides a promising strategy to enhance

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the photocatalytic water oxidation activity of g-C3N4 after optimization of reaction conditions and of synthetic conditions of the materials. We therefore checked the effect of thermal polymerization temperature of g-C3N4 on the photocatalytic activity for water oxidation reaction. In the Figure 7b, when the temperature increased, the water oxidation rate is gradually improved. The highest water oxidation rate is determined as 13 µmol h−1 as the temperature is raised to 650 °C. Further increasing the temperature will lead to the decomposition of g-C3N4. The enhanced activity is mainly due to the optimized texture as the thermal treatment temperature increased (Figure S5). This temperature dependence for the water oxidation activities is also found in previous studies.[16b] When the irradiation time is prolonged, the gases were evolved gradually. After 7 h continuous operation, the evolved oxygen amounts were reached to 27 and 15 µmol for Co-g-C3N4 and g-C3N4, respectively (Figure 7c). The decrease of the gas evolution rate as the time prolonged is mainly due to the reduced optical absorption resulted from the reduction of Ag+ on the surface of the catalyst, creating a light-shielding effect.[6b] The evolved nitrogen gas is originated from the self-oxidation of g-C3N4, but after the cobalt-modification the catalytic selectivity towards nitrogen was remarkably reduced in term of the molar ratio of N2/(N2+O2). The enhanced water oxidation activity and reduced nitrogen evolution selectivity are mainly attributed to the formation of surface cobalt junctions on g-C3N4 networks to accelerate charge-carriers separation at the soft materials/interface.

3. Conclusion A facile soft-chemical synthesis based on the impregnation of cobalt ions in g-C3N4 framework has been investigated to liberate oxygen from water at soft materials interface. Interestingly, the optical absorption, charge carriers transfer rate, as well as electronic conductivity have been promoted by the cobalt modification, while keeping the basic graphitic semiconductor structure that allows for generation and separation of energized charge carriers upon light illumination. Such a homogeneous modification of sustainable carbon nitride photocatalysts at molecular levels enhances the light-induced water oxidation capability of pristine g-C3N4. Integration of this enhanced strategy to the structure and morphology engineering of carbon nitride substrates using already-known tools[11–15] would allow for further improvement in both lightharvesting capability and photocatalytic activity towards the generation of a renewable source of protons and electrons from water for energy-generation reactions like CO2 fixation, while releasing dioxygen to atmosphere.

650 °C) under a nitrogen flow atmosphere and then tempering at this temperature for another 4h. The samples thus obtained were denoted as x-Co-g-C3N4, where x (0.3, 0.5, 1, 3 and 5) is the weightin amount of CoCl2.6H2O (3, 5, 10, 30, and 50 mg). The pristine sample g-C3N4 was obtained following the same procedure. Characterization: X-ray diffraction measurements were collected on a Bruker D8 Advance diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). Fourier transformed infrared (FTIR) spectra were recorded using a Nicolet Magna 670 FTIR spectrometer. Uv-Raman scattering measurements were performed with a multichannel modular triple Raman system (Renishaw Co.) with confocal microscope at room temperature using a 325 nm laser. The solid-state 13C NMR experiments were performed on a Bruker Advance 600 spectrometer. The UV/Vis diffuse reflectance spectra (DRS) were measured on a Varian Cary 500 Scan UV/Vis system. Photoluminescence (PL) spectra were recorded on an Edinburgh FI/FSTCSPC 920 spectrophotometer. Electron paramagnetic resonance (EPR) measurements were recorded using a Bruker model A300 spectrometer. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Thermo Scientific ESCA Lab250 spectrometer. Electrochemical measurements were conducted with a BAS Epsilon Electrochemical System in a conventional three electrode cell, using a Pt plate as the counter electrode and an Ag/AgCl electrode (3 M KCl) as the reference electrode. Photocatalytic Activity for Water Oxidation: Reactions were carried out in a Pyrex top-irradiation reaction vessel connected to a glass closed gas system. Photocatalytic O2 production was carried out in a Pyrex top-irradiation reaction vessel connected to a glass closed gas circulation system. For each reaction, 50 mg catalyst powder was well dispersed in an aqueous solution (100 mL) containing AgNO3 (0.01M) as an electron acceptor and La2O3 (0.2g) as a pH buffer agent. The reaction solution was evacuated several times to remove air completely prior to irradiation with a 300 W Xeon lamp with a working current of 15 A (Shenzhen ShengKang Technology Co., Ltd, China, LX300F). The wavelength of the incident light was controlled by applying some appropriate long-pass cut-off filters. The temperature of the reaction solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed by gas chromatography equipped with a thermal conductive detector (TCD) and a 5Å molecular sieve column, using Argon as the carrier gas.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

4. Experimental Section Synthesis of Co-g-C3N4 Photocatalysts: For catalysts synthesis, 1 g DCDA was mixed with different amounts of CoCl2.6H2O in 10 mL water with stirring at 80 °C to remove water. The resultant solids were calcined at different temperatures (500, 550, 600, and

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This work is financially supported by National Basic Research Program of China (2013CB632405), the National Natural Science Foundation of China (21033003, 21273038, and 21173043), the State Key Laboratory of NBC Protection for Civilian (SKLNBC2013– 04K), the Specialized Research Fund for the Doctoral Program of

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small 2014, DOI: 10.1002/smll.201402636

Cobalt in Graphitic Carbon Nitride Frameworks for Photocatalytic Water Oxidation

Higher Education (20133514110003), and the Department of Education of Fujian Province in China.

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Dispersing molecular cobalt in graphitic carbon nitride frameworks for photocatalytic water oxidation.

The development of water oxidation catalysts (WOCs) to cooperate with light-energy transducers for solar energy conversion by water splitting and CO2 ...
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