Shanshan
Li
,
Chun
Hu
,
Yannan
Peng
and
Zhihong
Chen
*
Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou 510006, China. E-mail: chenzhihong@gzhu.edu.cn
First published on 14th October 2019
Integration of a nanostructure design with a sub-bandgap has shown great promise in enhancing the photocatalytic H2 production activity of g-C3N4via facilitating the separation of photogenerated charges while simultaneously increasing the active sites and light harvesting ability. However, the development of a synthetic route to generate an ordered g-C3N4 structure with remarkable sub-bandgap absorption via a scalable and economic approach is challenging. Herein, we report the preparation of a honeycomb-like structured g-C3N4 with broad oxygen sub-bandgap absorption (denoted as HOCN) via a scalable one-pot copolymerization process using oxamide as the modelling agent and oxygen doping source. The morphology and sub-bandgap position can be tailored by controlling the oxamide to dicyandiamide ratio. All HOCN samples exhibit remarkable enhancement of photocatalytic H2 production activity due to the synergistic effect between the sub-bandgap and honeycomb structure, which results in strong light absorption extending up to 1000 nm, fast separation of photogenerated charge carriers, and rich photocatalytic reaction sites. In particular, HOCN4 exhibits a remarkable photocatalytic H2 production rate of 1140 μmol h−1 g−1 under visible light irradiation (>420 nm), which is more than 13.9 times faster than the production rate of pristine g-C3N4. Moreover, even under longer wavelength light irradiation (i.e., >500 and >800 nm), HOCN4 still exhibits a high H2 production rate of 477 and 91 μmol h−1 g−1, respectively. In addition, HOCN4 possesses an apparent quantum yield (AQY) of 4.32% at 420 nm and 0.12% at 800 nm. These results confirm that the proposed synthesis strategy allow for scalable production of g-C3N4 with an ordered nanostructure via electronic modulation, which is beneficial for its practical application in photocatalytic H2 production.
Recently, integration of nanostructure design with a sub-bandgap has provided an efficient route for facilitating the separation of photogenerated electron–hole pairs while simultaneously increasing the number of active sites as well as the light harvesting of g-C3N4.29–37 Additionally, a synergistic effect between nanostructure design and sub-bandgap creation endowed the obtained g-C3N4 with remarkably improved photocatalytic H2 production activity. Tao et al. reported g-C3N4 nanoflakes with a phosphorus sub-bandgap that was prepared by post-calcination treatment of buck g-C3N4 with a phosphorus sub-bandgap and exhibited a strong sub-bandgap absorption over the entire visible region, enlarged surface area and improved charge separation and transfer efficiency. These properties led to remarkably enhanced visible-light photocatalytic H2 production.36 Yi et al. reported a combinative strategy involving hydrothermal pre-treatment and calcination to synthesize mesoporous g-C3N4 with an oxygen and sulfur sub-bandgap for outstanding photocatalytic hydrogen evolution.37 Apparently, the formation of an ordered structure along with the creation of a sub-bandgap is an effective route to further increase the photocatalytic activity of g-C3N4. However, an additional post- or pre-treatment procedure and salt addition are time consuming, expensive and complex operation processes that allow for facile heteroatom introduction, which severely limits the scalable synthesis of g-C3N4 with an ordered nanostructure and sub-bandgap modulation. To the best of our knowledge, the development of a scalable and simple approach for the synthesis of g-C3N4 with an ordered structure and sub-bandgap is challenge.
Haung et al. reported that the replacement of O atoms for lattice N atoms via the formation of C–OC bonds generated an acceptor level below the bottom of the conduction band, resulting in an oxygen sub-bandgap between the valence band and oxygen acceptor.38 Inspired by previous studies, honeycomb-like g-C3N4 with an oxygen sub-bandgap (denoted as HOCN) was fabricated using a scalable one-pot copolymerization method using oxamide as a modelling agent and oxygen doping source. The morphology and sub-bandgap position can be tuned by controlling the oxamide to dicyandiamide ratio. The HOCN samples exhibit a strong light absorption region up to 1000 nm due to the narrowed sub-bandgap, which is composed of a valence band (VB) and oxygen doping states. The HOCNs exhibit a honeycomb-like structure with a high surface area of 185.6 m2 g−1, leading to efficient charge transfer and separation. Therefore, all HOCN samples exhibit enhanced photocatalytic H2 evolution rates under visible light irradiation (λ > 420 nm). In particular, HOCN4 produced a remarkable enhanced photocatalytic H2 production rate of 1140 μmol h−1 g−1 under visible light irradiation, which is much higher than that of pure g-C3N4 (CN, 82 μmol h−1 g−1). Further, HOCN4 exhibits a high H2 production rate of 477 and 91 μmol h−1 g−1 under light illumination with longer wavelengths (i.e., λ > 500 nm and λ > 800 nm, respectively).
The formation of a honeycomb-like structure was observed and confirmed via SEM and TEM analyses. As shown in Fig. 2A and B, in the absence of oxamide, CN exhibits the typical morphology with aggregated and stacked layers. However, with the addition of oxamide, HOCN exhibits a honeycomb-like morphology, which is accompanied by the formation of many irregular pores (Fig. 2C and D). Further observation of the TEM images indicates that HOCN consists of dense thick nanosheets with irregular pore sizes on its surface (Fig. 2E). However, the surface of CN is smooth with no nanopores (Fig. 2F). The specific surface area of CN and HOCN was further investigated based on their nitrogen adsorption and desorption isotherms. As shown in Fig. 2G, the BET surface area of HOCN4 is approximately 185.8 m2 g−1, which is approximately 12.8 times higher than that of pristine CN (14.6 m2 g−1). As shown in the inset of Fig. 2G, the pore size of HOCN4 is 60–110 nm and 5–20 nm, which is consistent with the diameter observed from the TEM analysis. Only small mesopores (5–15 nm) were observed in the pristine CN samples. These results suggest that the additional oxamide results in the formation of CN with a honeycomb structure.
The crystallinity of the as-prepared CN and HOCN4 were characterized by XRD, as shown in Fig. 3. The HOCN4 samples possess XRD patterns that are similar to CN due to the graphitic-like layered structure. The (002) peak was indexed to the strong interplanar stacking peak of the aromatic systems at 27.1°, confirming the sheet-like structure.39 The minor peak at 13.2° corresponds to the (100) diffraction peak, arising from the in-plane graphene structure. In comparison to that of CN, the characteristic (100) and (002) peaks of the HOCN4 samples were weaker and broader due to the formation of a honeycomb structure.
As shown in Fig. 4, the Fourier transform infrared (FTIR) spectra for CN and HOCN4 contained several bands in the 1250–1700 cm−1 region for the typical stretching vibrations of C–N heterocycles as well as a characteristic peak at 809 cm−1 corresponding to the tri-s-triazine units. It is important to note that HOCN4 exhibits –OH groups at 2900–3300 cm−1 and C–O stretching vibrations at 1130 and 1206 cm−1. However, the peak at 980 cm−1, which corresponds to the N–O groups, was not observed, implying that the O inset into the carbon nitride framework may occur via replacement of the N atom in the C–NC bonds of the carbon nitride framework.40 To further investigate the location of the O atoms in the carbon nitride framework, X-ray photoelectron spectroscopy (XPS) of CN and HOCN was carried out, and the results are shown in Fig. 5. The results can be fitted to two peaks at 531.6 eV and 532.8 eV, corresponding to C–O and surface-adsorbed water, respectively. The peak of the C–O bond for HOCN is more intense than that for CN. In the C 1s high-resolution XPS spectra, the peak at 284.6 eV is due to graphitic C–C bonds, and the peaks at 286.5 eV and 288.2 eV are due to C–N bonds and hybridized C–CN, respectively. The intensity of the peaks at 284.6 eV and 288.7 eV, which correspond to C–C and C–O bonds, in the C 1s high-resolution spectra is much stronger than those for CN, demonstrating that more C–C and C–O bonds exist in HOCN. In the N 1s high-resolution XPS spectra, no signals corresponding to N–O species were observed for the CN and HOCN samples, suggesting that O is not bound to N. The FTIR and XPS results indicate that the O atoms are incorporated into the framework of CN by replacing the coordinated N atoms (i.e., C–NC).
Fig. 5 (A) Full XPS spectra of CN and HOCN4; (B) high-resolution XPS spectra of O; (C) high-resolution XPS spectra of C 1s; (D) high-resolution XPS spectra of N 1s. |
Based on these results, the possible reaction for the thermal polymerization of oxamide and dicyandiamide as well as the role of oxamide in the formation of the honeycomb structure are shown in Scheme 1. At a medium–high temperature, the carbon atoms of the OA nucleophilically attack the amino groups of DCDA by releasing ammonia to form a similar structure to that of melem (prepolymer 1) followed by polymerization with dicyandiamide to form prepolymer 2. Simultaneously, the polymerization between two dicyandiamides occurs to form melem.16,41 At a high temperature, further polymerization between prepolymer 2 and melem occurs, leading to the introduction of a hetero oxygen atom into the CN framework. Simultaneously, some of prepolymer 2 would decompose into H2O and NH3 and CO2 gases at a high temperature, and then, these gases could expand the packing layers, resulting in the formation of pores in the CN bulk to afford the honeycomb structure. As a result, an oxygen co-doped carbon nitride with a honeycomb structure was obtained.
The light absorption properties of CN and HOCN were investigated using UV-vis diffuse reflectance spectroscopy (DRS). As shown in Fig. 6A, the light absorption edge of CN was located at approximately 442 nm. Comparatively, HOCN exhibits a light absorption edge at approximately 476 nm with a strong absorption tail (Urbach tail) over the entire visible light region, and the absorption tail edge gradually red shifts to longer wavelengths with increasing oxamide addition, which is beneficial for enhancing the utilization of solar light. As reported, the Urbach tail arises from the sub-bandgap, which originates from the VB to doping states and is located within the band gap of CN.24 The band gap (Eg) of CN and HOCN4 can be determined by fitting the Tauc plots. CN exhibits only one characteristic band gap of 2.80 eV. However, HOCN4 exhibits a characteristic band gap (Eg) of 2.6 eV with a sub-band gap (ES) of 2.13 eV (Fig. 6B). To estimate the band position and structure of CN and HOCN4, the valence band (VB) XPS spectra were recorded (Fig. 6C), and the VB positions for CN and HOCN4 were 1.70 and 1.51 eV, respectively. The conduct band (CB) and doping states (Ed) could be determined as follows: Eg = ECB − EVB, ES = Ed − EVB. The ECB of CN was calculated to be −1.10 V, and the ECB and Ed of HOCN4 was calculated to be at −1.09 and −0.63 V vs. NHE, respectively (Fig. 6D). Both the ECB and Ed of HOCN4 are more negative than the water reduction level, indicating that the photogenerated electron on the CB and doping states can thermodynamically reduce H2O to H2.
To further confirm the beneficial role of the sub-band gap and honeycomb-like structure for facilitating separation of the photogenerated electron–hole pairs, electron paramagnetic resonance (EPR) spectroscopy, photoluminescence spectroscopy (PL), photoresponse current and electrochemical impedance spectroscopy (EIS) were performed, and the results are shown in Fig. 7. As shown in Fig. 7A, HOCN4 exhibits a much higher EPR signal intensity than CN both under dark and light irradiation. Fig. 7B shows the PL spectrum of CN and HOCN4. CN exhibits a high fluorescence emission peak at approximately 458 nm, corresponding to band-to-band recombination of photogenerated electron–hole pairs. However, the intensity of this emission peak for HOCN4 decreased, suggesting that the loss of photogenerated electron–hole pairs due to band-to-band recombination is effectively prevented. Moreover, in comparison to CN, the PL signal of HOCN4 was red shifted due to narrowing of the band gap as well as the existence of the sub-band gap, which is consistent with the DRS results. HOCN4 exhibits a much higher photoresponse current and smaller diameter than CN (Fig. 7C and D), implying much easier generation of electron–hole pairs and efficient electron transport at the interfaces. Based on these results, the sub-band gap and honeycomb-like structure efficiently facilitated the generation of electron–hole pairs and inhibited their recombination in CN, which is advantageous for photocatalytic water reduction. In addition, when the oxamide to dicyandiamide molar ratio exceeds 4% (HOCN5 samples), a higher PL signal intensity, lower photocurrent and larger diameter were observed, corresponding to a higher recombination probability for the photogenerated charge carriers in HOCN5. Therefore, the excess oxamide results in the formation of recombination sites and a decreased HER rate.
Fig. 7 EPR spectra of CN and HOCN4 (A), PL spectra (B), I–T (C) and Nyquist (D) plots of CN, HOCN4 and HOCN5 under visible light. |
Based on these results, the proposed photocatalytic H2 evolution mechanism is shown in Scheme 2. Engineering of textural and electronic structures can enable synergetic tuning and optimization of photocatalytic water reducing reactions, resulting in maximum enhancement of the photocatalytic H2 production performance. The three most important features are as follows: (1) the oxygen sub-bandgap contributes to enhanced visible light absorption, (2) the oxygen acceptor states further expedite the transfer of photogenerated charge carriers, and (3) the honeycomb structure increases the number of available active sites and results in more efficient visible light absorbance due to the reflection effect. First, pristine CN can only absorb visible light with wavelengths less than 450 nm through an excited bandgap. However, the HOCN samples can generate electron–hole pairs due to the bandgap as well as the excited oxygen sub-bandgap. Therefore, this material can utilize both short and long wavelength visible light to generate photoelectrons from the VB to the CB as well as oxygen acceptor states while leaving the photogenerated holes in the VB. Second, because the CB position is more negative than that of the oxygen acceptor states, the photoelectrons on the CB of HOCN would transfer to the oxygen acceptor states, which is beneficial for separation of photogenerated electron–hole pairs. Finally, the photoelectrons transferred to the oxygen acceptor and the original photogenerated electrons on the oxygen acceptor states will shift to Pt and reduce H2O to H2. In addition, the photo-holes on the valence band of HOCN will react with sacrificial TEOA.
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