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An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation Yuyang Kang, Yongqiang Yang, Li-Chang Yin, Xiangdong Kang, Gang Liu,* and Hui-Ming Cheng Depending on the degree of order in atomic arrangements, solid-state materials are divided into three forms: crystalline, quasi-crystalline, and amorphous. Compared to crystals and quasi-crystals with long-range atomic order, amorphous materials have unique atomic arrangements of short-range order but long-range disorder, giving them distinct mechanical, optical, electronic and magnetic properties. In photocatalysis, crystalline semiconductors are almost exclusively used to induce the target redox reactions.[1–7] The presence of long-range atomic order is typically considered to be important in enabling the efficient separation and diffusion of photoexcited charge carriers,[8,9] which is crucial for obtaining high photocatalytic efficiency. In contrast, amorphous semiconductors, which lack the long-range atomic order and have many defects, are usually considered to be photocatalytically inactive or have a low activity. Early attempts at using amorphous semiconductors as photocatalysts led to limited success in achieving high visible light activity,[10–14] particularly in solar fuel generation. Recent representative examples include the use of amorphous yellow TiO2 for the photobleaching of methylene blue under visible light reported by Randorn et al.[12] and disordered Co1.28Mn1.71O4 as a visible-light-responsive photocatalyst for hydrogen evolution by Chen et al.[14] A most remarkable advantage of amorphous semiconductors as photocatalysts is their much smaller bandgap (exactly, their smaller energy level difference between the highest occupied and lowest unoccupied electronic states) than their crystalline counterparts because of their band tails.[15–17] This enables them to have a wider light absorption range and thus harvest a larger portion of solar radiation, which is a prerequisite for achieving high-efficiency solar energy conversion. The exploration of efficient amorphous photocatalysts therefore remains desirable yet challenging. Here we show that amorphous carbon nitride (ACN) can be used as an effective visible light photocatalyst. ACN with a bandgap of 1.90 eV shows an order of magnitude higher photocatalytic activity in hydrogen evolution under visible light than the partially crystalline graphitic Y. Kang, Y. Yang, Dr. L.-C. Yin, Dr. X. Kang, Prof. G. Liu, Prof. H.-M. Cheng Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences 72 Wenhua Road, Shenyang 110016, China E-mail: [email protected]
DOI: 10.1002/adma.201501939
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carbon nitride (GCN) counterpart with a bandgap of 2.82 eV. ACN is determined to be active in hydrogen generation under visible light with wavelength beyond 600 nm. These findings may help to develop a class of amorphous photocatalysts for solar energy conversion. Semiconducting GCN with a certain amount of hydrogen in the form of NH/NH2 groups (it has been widely described as “g-C3N4” in the literature, although the real atomic ratio of carbon to nitrogen in GCN is smaller than 0.75 in stoichiometric g-C3N4 due to the incomplete condensation of NH/NH2 groups.) has emerged as a class of metal-free polymer photocatalysts.[18,19] It has attracted great research interest largely because of its relatively small bandgap of ≈2.7 eV for visible light absorption up to around 460 nm and suitable band edges for water splitting. To maximize the utilization of solar radiation and also satisfy the thermodynamic requirement of water splitting, the bandgap of GCN needs to be reduced to the ideal value of around 2 eV. The main method of narrowing the bandgap is to change the compositions of GCN by introducing heteroatoms[20–22] or vacancies.[23] Homogeneous amorphization has never been used to try to narrow the bandgap of GCN and deserves to be investigated. GCN has a similar layered structure to graphite with weak van der Waals forces between layers but a completely different planar structure.[24–26] In contrast to the planar pure covalent bonding of graphite, the planar bonding of GCN is partially due to hydrogen bonding between strands of polymeric melon units with NH/NH2 groups (Figure 1a). The presence of such planar cohesion together with the weak interplanar interaction makes it possible to significantly disrupt the long-range atomic order in directions both perpendicular and parallel to the GCN layers for homogeneous amorphization. On the other hand, the strong covalent C N bonds in the melem units (the part in the circle of Figure 1a) are more resistant to rupture than van der Waals and hydrogen bonds, which is important for the retention of short-range atomic order when GCN is subjected to treatment such as heating at a high temperature. All these features suggest that crystalline GCN might be used to prepare ACN. The above hypothesis was first examined by ab initio molecular dynamics (MD) simulation to test the geometric structure variations of monolayer GCN at 900 K in vacuum. Compared to its partially crystalline counterpart in Figure 1a, the monolayer shows a tendency to disorder after annealing for 4 ps (Figure 1b), including two significant structure changes: i) the breaking of in-plane hydrogen bonds between strands of
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polymeric melon units, and ii) the outward twisting of melon units due to large outward movement of the NH2 groups from the original GCN plane. As shown in Figure1b, both the atomic coordination within each melon unit and the C N C coordination between two nearest neighboring melon units within each polymeric strand remain unchanged. This implies that the long-range order of GCN could be destroyed by annealing at 900 K, while the short-range order of GCN might remain unchanged. To verify these theoretical predictions, GCN prepared by the polycondensation of dicyandiamide was used as the starting material to produce ACN by heating GCN in an atmosphere of argon (scanning electron microscopy images of GCN and ACN were given in Figure S1, Supporting Information). Representative X-ray diffraction (XRD) patterns of the original material and the product prepared at 620 °C are given in Figure 2a. The two materials show distinctively different XRD patterns. GCN gives two typical diffraction peaks at around 13.1° and 27.2° as reported previously,[27] which are respectively due to the inplane structural packing motif as depicted by the solid lines in Figure 1a and periodic stacking of layers along the c-axis. These two sharp peaks disappear in the ACN pattern and only one very broad weak peak appears near 27.2°, suggesting the absence of long-range order in the atomic arrangements in ACN. The microstructure of ACN was further revealed by Fourier transform infrared (FTIR) and Raman spectroscopies, which are sensitive to the local (or short-range) structure of materials. The
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similar characteristics of the FTIR spectra of ACN and GCN in Figure 2b suggest good retention of both the heptazine ring and trigonal C N( C) C/bridging C NH C units of melon,[25] which are respectively indicated by the sharp peak at around 810 cm−1 and the peaks in the region 900 to 1800 cm−1 in ACN. Figure 2c compares UV Raman spectra of ACN and GCN. Both materials exhibit the typical Raman resonances of melon,[28] suggesting good retention of short-range atomic order in ACN. The compositions and chemical states of ACN compared to GCN were first studied by X-ray photoelectron spectroscopy (XPS) in Figure S2 (Supporting Information). The binding energies of C 1s and N 1s core electrons remain almost the same, suggesting similar chemical states for carbon and nitrogen. The dominant C 1s peak at 288.1 eV is the result of C N C coordination of melon. The three peaks centered at 398.7, 400.2, and 401.3 eV produced by deconvolution of the N 1s spectrum correspond to the C N C groups, N (C)3 groups and C N H groups of melon,[24] respectively. The atomic ratio of carbon to nitrogen is determined to be 3:4.29 in surface layer of ACN and 3:4.44 in GCN. This means the loss of some extra nitrogen atoms from the pristine GCN with amine groups during the amorphization, which is also supported by the variation of atomic ratio of C to N (3:4.16 in ACN vs 3:4.65 in GCN) determined by elemental analysis. Noted that, as a consequence of the removal of some extra nitrogen atoms from amine groups by the amorphization, the C/N atomic ratio in ACN gets closer to 3:4 of stoichiometric g-C3N4.
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Figure 2. Microstructural information on graphitic and amorphous carbon nitrides. a) XRD patterns; b) FTIR spectra; c,d) UV (266 nm excitation) and visible (632.8 nm excitation) Raman spectra.
Before concluding that the ACN sample exclusively consists of ACN, the possibility of forming (nitrogen-doped) carbon-like materials in the sample must be fully considered. It is known from Figure S3 (Supporting Information) that (doped/modified) carbon materials have two typical resonances, namely D and G bands with their respective centers at around 1320 cm−1 and 1590 cm−1 in their Raman spectra excited by visible light. As shown in Figure 2d, ACN has a very similar visible light Raman spectrum without the carbon related D and G bands to GCN, suggesting the absence of carbon-like materials in ACN. To doubly confirm this, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to monitor the decomposition behavior of ACN in an argon atmosphere. The ACN sample had a complete mass loss beyond 750 °C after experiencing two mass loss stages accompanied by two endothermic peaks with their centers at around 102 °C and 699 °C (Figure S4, Supporting Information). The first stage with 6 wt% mass loss was caused by the loss of surface hydroxyl groups and/or water molecules adsorbed. The high amount of hydroxyl groups/water molecules in ACN as a result of abundant active sites in the amorphous sample is consistently indicated by the strong signal in the region ranging from ≈3670 to ≈3270 cm−1 of the FTIR spectrum (Figure 2b). The second one was caused by the decomposition of C N bond-related framework into gaseous products identified by mass spectrometry. On the basis of the fact that carbon-like material cannot give a perceptible mass loss when heated under an identical inert atmosphere (Figure S5, Supporting Information gives the TGA–DSC curves of the activated carbon and nitrogen-doped carbon), the
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features of the TGA–DSC curves of ACN clearly suggest that carbon-like material is absent in the ACN sample. The presence of carbon-like materials in the ACN sample can be, therefore, safely ruled out. These structural characterizations have confirmed that heating partially crystalline GCN in an inert atmosphere of argon (similar results can be obtained in a nitrogen atmosphere) can lead to the production of amorphous ACN with the short-range atomic order and long-range atomic disorder. As a consequence, ACN with orange-red color exhibits distinct optical properties from GCN with yellow color, as shown in Figure 3. Compared to GCN, the absorption threshold of ACN is significantly shifted by 222 nm towards the low energy region from 460 to 682 nm. The extraordinary red-shift of the whole absorption edge indicates the band-to-band photon excitation of a narrowed intrinsic bandgap. The plots (Figure S6, Supporting Information) with steep edges of the transformed Kubelka–Munk function versus the light energy give a significant bandgap narrowing by 0.92 eV, from 2.82 eV for GCN to 1.90 eV for ACN. The radiative recombination process in ACN was studied by photoluminescence (PL) spectroscopy. As shown in Figure 3b, in contrast to a strong band-to-band emission peak at 462 nm for GCN, ACN shows no PL peak (Figure S7, Supporting Information). The quenching of the PL peak in ACN could be explained by the inhibition of radiative recombination pathways associated with the long-range order of atomic arrangements. To understand the origin of the bandgap narrowing of ACN, the band edges of ACN and GCN were studied by analyzing
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Figure 3. Optical properties of graphitic and amorphous carbon nitrides. a) UV–vis light absorption spectra and b) photoluminescence spectra (350 nm excitation at room temperature).
their XPS valence band spectra shown in Figure 4a. Although the extrapolated dominant edge of the valence band of ACN is very close to that of GCN (2.11 eV vs 2.07 eV), ACN gives two distinctly different features from GCN. One is a less sharp peak at around 6 eV, which is mainly contributed by N 2p states. The other is a prolonged band tail with its end at 1.43 eV, which is higher than the band tail of GCN by 0.31 eV. These features are associated with the absence of long-range atomic order and the simultaneous presence of dangling bonds in ACN. Considering that the bandgap of ACN is narrowed by 0.92 eV, it is reasonable to infer that its conduction band edge is shifted by 0.61 eV towards the valence band. It is known that traditional GCN usually has conduction and valence band edges that are respectively higher by around 0.9 and 0.6 eV than the redox potentials of H+/H2 and O2/OH−.[21] The band alignments of ACN and GCN referred to the redox potentials of water reduction and oxidation are schematically given in Figure 4b. It is important to note that the narrowed bandgap of ACN as a result of the shifts of the band edges still satisfies the thermodynamic requirements for water reduction and oxidation. The combined advantages of a wide visible light absorption range and quenched radiative recombination of photoexcited electron and hole pairs, together with suitable edges for photocatalytic hydrogen and oxygen generation from water splitting, make ACN a promising visible light photocatalyst. Its photocatalytic activity was estimated by detecting hydrogen generation from an aqueous solution containing triethanolamine
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Figure 4. Band structures of graphitic and amorphous carbon nitrides. a) XPS valence band spectra; b) Band alignments referring to the redox potential of water reduction and oxidation for hydrogen and oxygen generation. CB: conduction band; VB: valence band.
as electron donor under visible light. As shown in Figure 5a, ACN exhibits stable hydrogen generation to GCN, much superior by a factor of 12.4. The average hydrogen generation rate of ACN is 157.9 µmol h−1 g−1 and that of GCN is only 12.7 µmol h−1 g−1. Figure 5b gives the dependence of the wavelength of the incident light on the hydrogen generation rate of ACN together with its UV–vis absorption spectrum. The fact that the hydrogen generation is wavelength dependent is consistent with the optical absorption spectrum and suggests that the observed hydrogen generation of ACN comes from a photocatalysis process. The maximum wavelength of visible light to excite ACN for photocatalytic hydrogen generation is determined to be greater than 600 nm, which is one of the largest ones for carbon nitride-based photocatalysts. Moreover, ACN shows no obvious change in the photocatalytic activity after storing in air for over 10 months. The structure change from partially crystalline to amorphous by heating is unusual. It is known that heating usually results in an improved crystallinity of solid state materials with strong bonds. The key to this reverse process is that weak interactions (hydrogen bonds and van der Waals forces) are present not only between the layers but also in the layer planes of GCN so that the 3D long-range atomic order maintained by the weak interactions can be totally disrupted when the energy input is enough to destroy these weak interactions. The unique bonding of GCN allows this change in the microstructure and thus the
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Figure 5. Photocatalytic activity estimation. a) Photocatalytic hydrogen generation from a water/triethanolamine mixture of 9:1 by volume with graphitic carbon nitride and amorphous carbon nitride under visible light (λ > 440 nm); b) Wavelength dependent hydrogen generation from a water/triethanolamine mixture with amorphous carbon nitride (the intensity of the monochromatic light of each wavelength was normalized to be 10 mW cm−2).
related properties of carbon nitride. As a consequence, the ACN demonstrates a significant band-to-band redshift by 222 nm of the light absorption edge. The steep light absorption edge and high absorbance in visible light region for ACN are intrinsically different from those of the conventionally modified photocatalysts (including GCN) with heteroatoms or defects, which usually have an extended shoulder-like absorption band and low absorbance.[29,30] For instance, several reports concerned the effect of structural point defects on optical absorption property of GCN without changing the nature of being graphitic structure of material.[31–34] In all these cases, only an additional shoulder-like low-energy absorption band with much lower absorbance than the pristine absorption band was formed as a consequence of the redshift of the partial absorption edge. At the electronic structure level, the former is attributed to bandgap narrowing and the latter is caused by the formation of dopant/defect-related localized states in the bandgap. At the atomic level, the bandgap narrowing by introducing heteroatoms/defects in photocatalysts intrinsically requires the homogeneous distribution of heteroatoms/defects throughout the whole particle,[22,29,30,35,36] 29 which is hard to realize in most cases and consequently only some localized states are formed in the bandgap. To a large extent, amorphization can be considered a homogeneous self-modification of the microstructure by disrupting their long-rang atomic order. This leads to the effective modification of the electronic structure of carbon nitride, demonstrated by the different band edges, narrowed bandgap, and quenched radiative recombination of photoexcited charge carriers. With the increase of the visible light absorption range to around 680 nm, the responsive wavelength range of ACN for photocatalytic hydrogen generation was extended to around 650 nm. The consistency between the absorption spectrum and photocatalytic action spectrum is closely related to the nature of band-to-band visible light excitation in ACN. In this situation, the properties of conduction band electrons and valence band holes generated by the absorption of photons with energies higher than the bandgap and their subsequent rapid relaxation to the band edges could be the same. On the other hand, the redox potentials of photocatalysts are determined by band edges. The down-shift of the conduction band edge and the up-shift of the valence band edge to produce a wide
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visible light absorption range simultaneously lower the redox potentials. It is known that the charge transfer rate between photocatalyst and reactant is closely related to the energy level difference between their redox potentials.[37] The large energy level difference favors the transfer. This means that the photocatalytic activity of ACN is controlled by a compromise between the wide visible light absorption and lowered redox potentials, which is actually a common feature that must be addressed for all other photocatalysts with a wide visible light absorption range. In summary, an ACN with a bandgap of 1.90 eV was obtained by simply heating partially crystalline GCN with a bandgap of 2.82 eV. As a visible light photocatalyst, this material exhibits a much superior activity in hydrogen generation than does the GCN as a result of the combination of a wider light absorption range and quenched radiative recombination. These results could open up a way to develop a new class of visible light-driven amorphous photocatalysts.
Experimental Section Preparation of Amorphous Carbon Nitride: Graphitic carbon nitride powder was prepared by the polycondensation of dicyandiamide (6 g) at 500 °C for 4 h in static air in a muffle furnace. The ramping rate was 2 °C min−1. The sample was cooled naturally to room temperature. To prepare ACN, the resultant GCN powder was heated at 620 °C for 2 h in an argon atmosphere with a flow rate of 50 mL min−1. Characterization: X-ray diffraction patterns were recorded on a Rigaku diffractometer using Cu Kα irradiation. FTIR spectra were recorded on a Bruker Tensor 27. Raman spectra (266 nm excitation) were recorded on a home-assembled UV Raman spectrometer using a Jobin-Yvon T64000 triple stage spectrograph with spectral resolution of 2 cm−1. Raman spectra (632.8 nm) were collected with LabRam HR 800. The compositions and chemical states of the samples were analyzed using XPS (Thermo Escalab 250, a monochromatic Al Kα X-ray source). Elemental analysis was performed on a Vario MICRO instrument (Elementar, Germany). All binding energies were referenced to the C 1s peak (284.6 eV) that arises from adventitious carbon. The synchronous thermogravimetry/differential scanning calorimetry/ mass spectrometer (TG/DSC/MS) analyses were carried out using a Netzsch 449C jupier/QMS 403C at a ramping rate of 10 °C min−1 under high-purity Ar (99.9999 purity) as purge gas. The optical absorption spectra were recorded on a UV–vis spectrophotometer (JASCO-550) in the diffuse reflectance mode. Photoluminescence emission spectra
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Y.K. and Y.Y. contributed equally to this work. The authors thank the Major Basic Research Program, Ministry of Science and Technology of China (2014CB239401), NSFC (No. 51422210, 51002160, 51221264, 51202255), the Key Research Program of Chinese Academy of Sciences (Grant No. KGZD-EW-T06). The authors acknowledge Dr. Fengtao Fan from Dalian Institute of Chemical Physics, Chinese Academy of Sciences for UV Raman measurements. The authors thank the Shenyang Supercomputing Center, Chinese Academy of Sciences, for their support. Part of the simulation work was carried out at the National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 (A). Received: April 22, 2015 Revised: May 25, 2015 Published online: July 6, 2015
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(350 nm excitation) were measured at room temperature with a fluorescence spectrophotometer (Edinburgh Instruments, FLSP-920). Theoretical Calculations: The ab initio MD simulation was conducted using a microcanonical ensemble with the Vienna ab initio simulation package. The details of the simulation were given in Supporting Information. Photocatalytic Activity Estimation: Photocatalytic hydrogen generation reactions were carried out in a top-irradiation vessel connected to a glass gas circulation system. 50 mg of the photocatalyst powder was dispersed in 300 mL aqueous solution containing 10 vol% triethanolamine as electron donor. The deposition of 6 wt% Pt as a reducing co-catalyst was achieved by dissolving H2PtCl6 in the above 300 mL reaction solution. The reaction temperature was kept around 10 °C. The amount of H2 generated was determined using a gas chromatograph (Agilent 6890). The light source was a 300 W Xe lamp (Beijing Trusttech Co. Ltd, PLS-SXE-300UV). The wavelength of incident light in the visible light photocatalytic reactions was obtained by using a 440 nm long-pass glass filter.
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An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation Yuyang Kang, Yongqiang Yang, Li-Chang Yin, Xiangdong Kang, Gang Liu,* and Hui-Ming Cheng Depending on the degree of order in atomic arrangements, solid-state materials are divided into three forms: crystalline, quasi-crystalline, and amorphous. Compared to crystals and quasi-crystals with long-range atomic order, amorphous materials have unique atomic arrangements of short-range order but long-range disorder, giving them distinct mechanical, optical, electronic and magnetic properties. In photocatalysis, crystalline semiconductors are almost exclusively used to induce the target redox reactions.[1–7] The presence of long-range atomic order is typically considered to be important in enabling the efficient separation and diffusion of photoexcited charge carriers,[8,9] which is crucial for obtaining high photocatalytic efficiency. In contrast, amorphous semiconductors, which lack the long-range atomic order and have many defects, are usually considered to be photocatalytically inactive or have a low activity. Early attempts at using amorphous semiconductors as photocatalysts led to limited success in achieving high visible light activity,[10–14] particularly in solar fuel generation. Recent representative examples include the use of amorphous yellow TiO2 for the photobleaching of methylene blue under visible light reported by Randorn et al.[12] and disordered Co1.28Mn1.71O4 as a visible-light-responsive photocatalyst for hydrogen evolution by Chen et al.[14] A most remarkable advantage of amorphous semiconductors as photocatalysts is their much smaller bandgap (exactly, their smaller energy level difference between the highest occupied and lowest unoccupied electronic states) than their crystalline counterparts because of their band tails.[15–17] This enables them to have a wider light absorption range and thus harvest a larger portion of solar radiation, which is a prerequisite for achieving high-efficiency solar energy conversion. The exploration of efficient amorphous photocatalysts therefore remains desirable yet challenging. Here we show that amorphous carbon nitride (ACN) can be used as an effective visible light photocatalyst. ACN with a bandgap of 1.90 eV shows an order of magnitude higher photocatalytic activity in hydrogen evolution under visible light than the partially crystalline graphitic Y. Kang, Y. Yang, Dr. L.-C. Yin, Dr. X. Kang, Prof. G. Liu, Prof. H.-M. Cheng Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences 72 Wenhua Road, Shenyang 110016, China E-mail: [email protected]
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carbon nitride (GCN) counterpart with a bandgap of 2.82 eV. ACN is determined to be active in hydrogen generation under visible light with wavelength beyond 600 nm. These findings may help to develop a class of amorphous photocatalysts for solar energy conversion. Semiconducting GCN with a certain amount of hydrogen in the form of NH/NH2 groups (it has been widely described as “g-C3N4” in the literature, although the real atomic ratio of carbon to nitrogen in GCN is smaller than 0.75 in stoichiometric g-C3N4 due to the incomplete condensation of NH/NH2 groups.) has emerged as a class of metal-free polymer photocatalysts.[18,19] It has attracted great research interest largely because of its relatively small bandgap of ≈2.7 eV for visible light absorption up to around 460 nm and suitable band edges for water splitting. To maximize the utilization of solar radiation and also satisfy the thermodynamic requirement of water splitting, the bandgap of GCN needs to be reduced to the ideal value of around 2 eV. The main method of narrowing the bandgap is to change the compositions of GCN by introducing heteroatoms[20–22] or vacancies.[23] Homogeneous amorphization has never been used to try to narrow the bandgap of GCN and deserves to be investigated. GCN has a similar layered structure to graphite with weak van der Waals forces between layers but a completely different planar structure.[24–26] In contrast to the planar pure covalent bonding of graphite, the planar bonding of GCN is partially due to hydrogen bonding between strands of polymeric melon units with NH/NH2 groups (Figure 1a). The presence of such planar cohesion together with the weak interplanar interaction makes it possible to significantly disrupt the long-range atomic order in directions both perpendicular and parallel to the GCN layers for homogeneous amorphization. On the other hand, the strong covalent C N bonds in the melem units (the part in the circle of Figure 1a) are more resistant to rupture than van der Waals and hydrogen bonds, which is important for the retention of short-range atomic order when GCN is subjected to treatment such as heating at a high temperature. All these features suggest that crystalline GCN might be used to prepare ACN. The above hypothesis was first examined by ab initio molecular dynamics (MD) simulation to test the geometric structure variations of monolayer GCN at 900 K in vacuum. Compared to its partially crystalline counterpart in Figure 1a, the monolayer shows a tendency to disorder after annealing for 4 ps (Figure 1b), including two significant structure changes: i) the breaking of in-plane hydrogen bonds between strands of
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polymeric melon units, and ii) the outward twisting of melon units due to large outward movement of the NH2 groups from the original GCN plane. As shown in Figure1b, both the atomic coordination within each melon unit and the C N C coordination between two nearest neighboring melon units within each polymeric strand remain unchanged. This implies that the long-range order of GCN could be destroyed by annealing at 900 K, while the short-range order of GCN might remain unchanged. To verify these theoretical predictions, GCN prepared by the polycondensation of dicyandiamide was used as the starting material to produce ACN by heating GCN in an atmosphere of argon (scanning electron microscopy images of GCN and ACN were given in Figure S1, Supporting Information). Representative X-ray diffraction (XRD) patterns of the original material and the product prepared at 620 °C are given in Figure 2a. The two materials show distinctively different XRD patterns. GCN gives two typical diffraction peaks at around 13.1° and 27.2° as reported previously,[27] which are respectively due to the inplane structural packing motif as depicted by the solid lines in Figure 1a and periodic stacking of layers along the c-axis. These two sharp peaks disappear in the ACN pattern and only one very broad weak peak appears near 27.2°, suggesting the absence of long-range order in the atomic arrangements in ACN. The microstructure of ACN was further revealed by Fourier transform infrared (FTIR) and Raman spectroscopies, which are sensitive to the local (or short-range) structure of materials. The
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similar characteristics of the FTIR spectra of ACN and GCN in Figure 2b suggest good retention of both the heptazine ring and trigonal C N( C) C/bridging C NH C units of melon,[25] which are respectively indicated by the sharp peak at around 810 cm−1 and the peaks in the region 900 to 1800 cm−1 in ACN. Figure 2c compares UV Raman spectra of ACN and GCN. Both materials exhibit the typical Raman resonances of melon,[28] suggesting good retention of short-range atomic order in ACN. The compositions and chemical states of ACN compared to GCN were first studied by X-ray photoelectron spectroscopy (XPS) in Figure S2 (Supporting Information). The binding energies of C 1s and N 1s core electrons remain almost the same, suggesting similar chemical states for carbon and nitrogen. The dominant C 1s peak at 288.1 eV is the result of C N C coordination of melon. The three peaks centered at 398.7, 400.2, and 401.3 eV produced by deconvolution of the N 1s spectrum correspond to the C N C groups, N (C)3 groups and C N H groups of melon,[24] respectively. The atomic ratio of carbon to nitrogen is determined to be 3:4.29 in surface layer of ACN and 3:4.44 in GCN. This means the loss of some extra nitrogen atoms from the pristine GCN with amine groups during the amorphization, which is also supported by the variation of atomic ratio of C to N (3:4.16 in ACN vs 3:4.65 in GCN) determined by elemental analysis. Noted that, as a consequence of the removal of some extra nitrogen atoms from amine groups by the amorphization, the C/N atomic ratio in ACN gets closer to 3:4 of stoichiometric g-C3N4.
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Figure 2. Microstructural information on graphitic and amorphous carbon nitrides. a) XRD patterns; b) FTIR spectra; c,d) UV (266 nm excitation) and visible (632.8 nm excitation) Raman spectra.
Before concluding that the ACN sample exclusively consists of ACN, the possibility of forming (nitrogen-doped) carbon-like materials in the sample must be fully considered. It is known from Figure S3 (Supporting Information) that (doped/modified) carbon materials have two typical resonances, namely D and G bands with their respective centers at around 1320 cm−1 and 1590 cm−1 in their Raman spectra excited by visible light. As shown in Figure 2d, ACN has a very similar visible light Raman spectrum without the carbon related D and G bands to GCN, suggesting the absence of carbon-like materials in ACN. To doubly confirm this, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to monitor the decomposition behavior of ACN in an argon atmosphere. The ACN sample had a complete mass loss beyond 750 °C after experiencing two mass loss stages accompanied by two endothermic peaks with their centers at around 102 °C and 699 °C (Figure S4, Supporting Information). The first stage with 6 wt% mass loss was caused by the loss of surface hydroxyl groups and/or water molecules adsorbed. The high amount of hydroxyl groups/water molecules in ACN as a result of abundant active sites in the amorphous sample is consistently indicated by the strong signal in the region ranging from ≈3670 to ≈3270 cm−1 of the FTIR spectrum (Figure 2b). The second one was caused by the decomposition of C N bond-related framework into gaseous products identified by mass spectrometry. On the basis of the fact that carbon-like material cannot give a perceptible mass loss when heated under an identical inert atmosphere (Figure S5, Supporting Information gives the TGA–DSC curves of the activated carbon and nitrogen-doped carbon), the
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features of the TGA–DSC curves of ACN clearly suggest that carbon-like material is absent in the ACN sample. The presence of carbon-like materials in the ACN sample can be, therefore, safely ruled out. These structural characterizations have confirmed that heating partially crystalline GCN in an inert atmosphere of argon (similar results can be obtained in a nitrogen atmosphere) can lead to the production of amorphous ACN with the short-range atomic order and long-range atomic disorder. As a consequence, ACN with orange-red color exhibits distinct optical properties from GCN with yellow color, as shown in Figure 3. Compared to GCN, the absorption threshold of ACN is significantly shifted by 222 nm towards the low energy region from 460 to 682 nm. The extraordinary red-shift of the whole absorption edge indicates the band-to-band photon excitation of a narrowed intrinsic bandgap. The plots (Figure S6, Supporting Information) with steep edges of the transformed Kubelka–Munk function versus the light energy give a significant bandgap narrowing by 0.92 eV, from 2.82 eV for GCN to 1.90 eV for ACN. The radiative recombination process in ACN was studied by photoluminescence (PL) spectroscopy. As shown in Figure 3b, in contrast to a strong band-to-band emission peak at 462 nm for GCN, ACN shows no PL peak (Figure S7, Supporting Information). The quenching of the PL peak in ACN could be explained by the inhibition of radiative recombination pathways associated with the long-range order of atomic arrangements. To understand the origin of the bandgap narrowing of ACN, the band edges of ACN and GCN were studied by analyzing
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Figure 3. Optical properties of graphitic and amorphous carbon nitrides. a) UV–vis light absorption spectra and b) photoluminescence spectra (350 nm excitation at room temperature).
their XPS valence band spectra shown in Figure 4a. Although the extrapolated dominant edge of the valence band of ACN is very close to that of GCN (2.11 eV vs 2.07 eV), ACN gives two distinctly different features from GCN. One is a less sharp peak at around 6 eV, which is mainly contributed by N 2p states. The other is a prolonged band tail with its end at 1.43 eV, which is higher than the band tail of GCN by 0.31 eV. These features are associated with the absence of long-range atomic order and the simultaneous presence of dangling bonds in ACN. Considering that the bandgap of ACN is narrowed by 0.92 eV, it is reasonable to infer that its conduction band edge is shifted by 0.61 eV towards the valence band. It is known that traditional GCN usually has conduction and valence band edges that are respectively higher by around 0.9 and 0.6 eV than the redox potentials of H+/H2 and O2/OH−.[21] The band alignments of ACN and GCN referred to the redox potentials of water reduction and oxidation are schematically given in Figure 4b. It is important to note that the narrowed bandgap of ACN as a result of the shifts of the band edges still satisfies the thermodynamic requirements for water reduction and oxidation. The combined advantages of a wide visible light absorption range and quenched radiative recombination of photoexcited electron and hole pairs, together with suitable edges for photocatalytic hydrogen and oxygen generation from water splitting, make ACN a promising visible light photocatalyst. Its photocatalytic activity was estimated by detecting hydrogen generation from an aqueous solution containing triethanolamine
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Figure 4. Band structures of graphitic and amorphous carbon nitrides. a) XPS valence band spectra; b) Band alignments referring to the redox potential of water reduction and oxidation for hydrogen and oxygen generation. CB: conduction band; VB: valence band.
as electron donor under visible light. As shown in Figure 5a, ACN exhibits stable hydrogen generation to GCN, much superior by a factor of 12.4. The average hydrogen generation rate of ACN is 157.9 µmol h−1 g−1 and that of GCN is only 12.7 µmol h−1 g−1. Figure 5b gives the dependence of the wavelength of the incident light on the hydrogen generation rate of ACN together with its UV–vis absorption spectrum. The fact that the hydrogen generation is wavelength dependent is consistent with the optical absorption spectrum and suggests that the observed hydrogen generation of ACN comes from a photocatalysis process. The maximum wavelength of visible light to excite ACN for photocatalytic hydrogen generation is determined to be greater than 600 nm, which is one of the largest ones for carbon nitride-based photocatalysts. Moreover, ACN shows no obvious change in the photocatalytic activity after storing in air for over 10 months. The structure change from partially crystalline to amorphous by heating is unusual. It is known that heating usually results in an improved crystallinity of solid state materials with strong bonds. The key to this reverse process is that weak interactions (hydrogen bonds and van der Waals forces) are present not only between the layers but also in the layer planes of GCN so that the 3D long-range atomic order maintained by the weak interactions can be totally disrupted when the energy input is enough to destroy these weak interactions. The unique bonding of GCN allows this change in the microstructure and thus the
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Figure 5. Photocatalytic activity estimation. a) Photocatalytic hydrogen generation from a water/triethanolamine mixture of 9:1 by volume with graphitic carbon nitride and amorphous carbon nitride under visible light (λ > 440 nm); b) Wavelength dependent hydrogen generation from a water/triethanolamine mixture with amorphous carbon nitride (the intensity of the monochromatic light of each wavelength was normalized to be 10 mW cm−2).
related properties of carbon nitride. As a consequence, the ACN demonstrates a significant band-to-band redshift by 222 nm of the light absorption edge. The steep light absorption edge and high absorbance in visible light region for ACN are intrinsically different from those of the conventionally modified photocatalysts (including GCN) with heteroatoms or defects, which usually have an extended shoulder-like absorption band and low absorbance.[29,30] For instance, several reports concerned the effect of structural point defects on optical absorption property of GCN without changing the nature of being graphitic structure of material.[31–34] In all these cases, only an additional shoulder-like low-energy absorption band with much lower absorbance than the pristine absorption band was formed as a consequence of the redshift of the partial absorption edge. At the electronic structure level, the former is attributed to bandgap narrowing and the latter is caused by the formation of dopant/defect-related localized states in the bandgap. At the atomic level, the bandgap narrowing by introducing heteroatoms/defects in photocatalysts intrinsically requires the homogeneous distribution of heteroatoms/defects throughout the whole particle,[22,29,30,35,36] 29 which is hard to realize in most cases and consequently only some localized states are formed in the bandgap. To a large extent, amorphization can be considered a homogeneous self-modification of the microstructure by disrupting their long-rang atomic order. This leads to the effective modification of the electronic structure of carbon nitride, demonstrated by the different band edges, narrowed bandgap, and quenched radiative recombination of photoexcited charge carriers. With the increase of the visible light absorption range to around 680 nm, the responsive wavelength range of ACN for photocatalytic hydrogen generation was extended to around 650 nm. The consistency between the absorption spectrum and photocatalytic action spectrum is closely related to the nature of band-to-band visible light excitation in ACN. In this situation, the properties of conduction band electrons and valence band holes generated by the absorption of photons with energies higher than the bandgap and their subsequent rapid relaxation to the band edges could be the same. On the other hand, the redox potentials of photocatalysts are determined by band edges. The down-shift of the conduction band edge and the up-shift of the valence band edge to produce a wide
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visible light absorption range simultaneously lower the redox potentials. It is known that the charge transfer rate between photocatalyst and reactant is closely related to the energy level difference between their redox potentials.[37] The large energy level difference favors the transfer. This means that the photocatalytic activity of ACN is controlled by a compromise between the wide visible light absorption and lowered redox potentials, which is actually a common feature that must be addressed for all other photocatalysts with a wide visible light absorption range. In summary, an ACN with a bandgap of 1.90 eV was obtained by simply heating partially crystalline GCN with a bandgap of 2.82 eV. As a visible light photocatalyst, this material exhibits a much superior activity in hydrogen generation than does the GCN as a result of the combination of a wider light absorption range and quenched radiative recombination. These results could open up a way to develop a new class of visible light-driven amorphous photocatalysts.
Experimental Section Preparation of Amorphous Carbon Nitride: Graphitic carbon nitride powder was prepared by the polycondensation of dicyandiamide (6 g) at 500 °C for 4 h in static air in a muffle furnace. The ramping rate was 2 °C min−1. The sample was cooled naturally to room temperature. To prepare ACN, the resultant GCN powder was heated at 620 °C for 2 h in an argon atmosphere with a flow rate of 50 mL min−1. Characterization: X-ray diffraction patterns were recorded on a Rigaku diffractometer using Cu Kα irradiation. FTIR spectra were recorded on a Bruker Tensor 27. Raman spectra (266 nm excitation) were recorded on a home-assembled UV Raman spectrometer using a Jobin-Yvon T64000 triple stage spectrograph with spectral resolution of 2 cm−1. Raman spectra (632.8 nm) were collected with LabRam HR 800. The compositions and chemical states of the samples were analyzed using XPS (Thermo Escalab 250, a monochromatic Al Kα X-ray source). Elemental analysis was performed on a Vario MICRO instrument (Elementar, Germany). All binding energies were referenced to the C 1s peak (284.6 eV) that arises from adventitious carbon. The synchronous thermogravimetry/differential scanning calorimetry/ mass spectrometer (TG/DSC/MS) analyses were carried out using a Netzsch 449C jupier/QMS 403C at a ramping rate of 10 °C min−1 under high-purity Ar (99.9999 purity) as purge gas. The optical absorption spectra were recorded on a UV–vis spectrophotometer (JASCO-550) in the diffuse reflectance mode. Photoluminescence emission spectra
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Y.K. and Y.Y. contributed equally to this work. The authors thank the Major Basic Research Program, Ministry of Science and Technology of China (2014CB239401), NSFC (No. 51422210, 51002160, 51221264, 51202255), the Key Research Program of Chinese Academy of Sciences (Grant No. KGZD-EW-T06). The authors acknowledge Dr. Fengtao Fan from Dalian Institute of Chemical Physics, Chinese Academy of Sciences for UV Raman measurements. The authors thank the Shenyang Supercomputing Center, Chinese Academy of Sciences, for their support. Part of the simulation work was carried out at the National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 (A). Received: April 22, 2015 Revised: May 25, 2015 Published online: July 6, 2015
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(350 nm excitation) were measured at room temperature with a fluorescence spectrophotometer (Edinburgh Instruments, FLSP-920). Theoretical Calculations: The ab initio MD simulation was conducted using a microcanonical ensemble with the Vienna ab initio simulation package. The details of the simulation were given in Supporting Information. Photocatalytic Activity Estimation: Photocatalytic hydrogen generation reactions were carried out in a top-irradiation vessel connected to a glass gas circulation system. 50 mg of the photocatalyst powder was dispersed in 300 mL aqueous solution containing 10 vol% triethanolamine as electron donor. The deposition of 6 wt% Pt as a reducing co-catalyst was achieved by dissolving H2PtCl6 in the above 300 mL reaction solution. The reaction temperature was kept around 10 °C. The amount of H2 generated was determined using a gas chromatograph (Agilent 6890). The light source was a 300 W Xe lamp (Beijing Trusttech Co. Ltd, PLS-SXE-300UV). The wavelength of incident light in the visible light photocatalytic reactions was obtained by using a 440 nm long-pass glass filter.
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