In Situ Preparation and Analysis of Bimetal Co-doped Mesoporous Graphitic Carbon Nitride with Enhanced Photocatalytic Activity
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Co/Mo co-doped mesoporous graphitic carbon nitride (g-C3N4) exhibited enhanced photocatalytic performances with regard to H2 generation (8.6 times) and Rhodamine B degradation (10.1 times) compared with pristine g-C3N4.
The density functional theory calculations and optical simulation illustrate that Co/Mo co-doping and the created mesoporous structure can enhance light absorption.
The enhanced activity depends on the synergistic effect of Co and Mo co-doping.
KeywordsCo-doped g-C3N4 Porous g-C3N4 Photocatalysis Optical simulation Light absorption intensity
The energy crisis and environmental pollution are two significant global challenges [1, 2, 3]. Water-splitting to produce clean hydrogen through solar energy conversion is considered to be a sustainable and feasible solution to address these challenges [4, 5, 6]. In recent years, the emergent semiconductor photocatalytic technology has attracted substantial attention for realizing organic pollutant degradation and solar energy conversion [7, 8, 9, 10]. As a new non-metallic semiconductor, graphitic carbon nitride (g-C3N4) has attracted substantial attention; it exhibits advantages such as high thermal stability, non-toxicity, abundant precursors, and a suitable bandgap that could expand the absorbing threshold up to 460 nm [11, 12, 13]. However, drawbacks of pristine g-C3N4 (P-CN) such as the low specific surface area and rapid recombination rate of photoexcited charge carriers results in its low photocatalytic efficiency [14, 15, 16]. Various strategies have been adopted to modify photocatalyst in order to improve photocatalytic performance, such as metal doping, pore creating, and semiconductor coupling [17, 18, 19, 20]. Combining two or three of these strategies would improve photocatalytic activity more remarkably [21, 22, 23].
A mesoporous structure and high specific surface area also play vital roles in photocatalysis; they can result in improved photocatalytic efficiency owing to the promoted separation of photogenerated active charges and improved mass transfer. Generally, hard and soft template methods are applied to synthesize porous g-C3N4 with various pore structures [13, 24, 25]. However, certain drawbacks of the template methods limit their wide application. The synthesis process of the hard template method is complex and tedious, resulting in increased costs; moreover, the removal of a few silica templates by using the aqueous ammonium bifluoride (NH4HF2) or hydrogen fluoride (HF) negatively impacts the environment. The porous g-C3N4 prepared via soft template method would result in a higher carbon residue causing the “shielding effect”; this reduces light absorption and results in low catalytic activity [26, 27, 28]. Compared to hard or soft template, the template-free method does not require other substances as templates; furthermore, mesopores can be made by selecting suitable precursors [29, 30, 31]. Therefore, the template-free method as a convenient strategy to develop mesopores is highly desirable.
In addition, the g-C3N4 can function as electron donors to trap transition metal ions to form metal-N bonds owing to its heptazine ring with pyridinic nitrogen and six nitrogen lone pairs of framework . Several previous studies on transition metal-doped g-C3N4 demonstrated its superior photocatalytic performances [33, 34, 35]. Wang et al.  studied Fe-doped g-C3N4 with enhanced photocatalytic activity for Rhodamine B (RhB) degradation. The study by Oh et al.  demonstrated that metal-doped g-C3N4 (Cu, Co, and Fe) catalysts can improve the catalytic activity of sulfathiazole degradation; moreover, the Co-doped g-C3N4 presented the highest catalytic activity. However, few efforts have focused on the bimetallic doping of g-C3N4, which is likely to exhibit more efficiency in photocatalytic reaction owing to the possible synergistic effect of the doped bimetal . Inspired by the work that reported the synthesis of Cr3+ and Ce3+ co-doped N/TiO2 and its superior photocatalytic performance for the degradation of humic acid owing to the red shift of the bandgap energy , it could be speculated that bimetallic doping of g-C3N4 is of significant interest.
Herein, co-doped mesoporous g-C3N4 with Co and Mo elements (Co/Mo-MCN) was prepared via a simple one-pot method by using cobalt chloride (CoCl2) as the Co source and MoS2 as the Mo source; its photocatalytic activity was evaluated by hydrogen evolution and RhB degradation under visible light irradiation. To our knowledge, this is the first study to prepare g-C3N4 with bimetal co-doping and to construct mesopores via a template-free approach. Compared with pristine g-C3N4 (P-CN) and mono-metal-doped g-C3N4, the as-prepared Co/Mo-MCN presented significantly higher photocatalytic activity. Furthermore, first principle calculations and optical simulations are carried out to analyze the electronic structure and optical absorption characteristics of the P-CN and doped g-C3N4. This study proposes a new method for the design and synthesis of a g-C3N4 with co-doping metals and structured mesopores; it also investigates the effect of the co-doping and the mesopores on the light absorption capacity, through finite-difference time-domain (FDTD) simulation and density functional theory (DFT) calculation. Furthermore, a feasible mechanism for enhanced photocatalytic of bimetal co-doped g-C3N4 is recommended.
2.1.1 Synthesis of Pristine g-C3N4
The P-CN was prepared by directly calcinating 8.0 g of guanidine hydrochloride at 550 °C for 60 min in a muffle furnace at a heating rate of 3 °C min−1.
2.1.2 Synthesis of Co-doped g-C3N4
Co-doped g-C3N4 (Co-CN) was synthesized by calcinating guanidine hydrochloride and Co precursor according to the modified method in the literature . Briefly, 8.0 g of guanidine hydrochloride and a certain amount of CoCl2·6H2O (5, 10, 30, and 100 mg) were dissolved in 5 mL of deionized water. After stirring for 1 h, the mixture was dried at 80 °C for 1 day. The solids obtained were then grinded into powder and transferred to a quartz crucible; the sintering process was similar to that for P-CN. The products were denoted as Co-CN-5, Co-CN-10, Co-CN-30, and Co-CN-100, respectively.
2.1.3 Synthesis of Mo-Doped Mesoporous g-C3N4
Mo-doped mesoporous g-C3N4 (Mo-MCN) was prepared by using MoS2 nanosheets as the Mo precursor, which were obtained via hydrothermal method. In a typical run, 1.694 g of sodium molybdate and 2.665 g of thiourea were dissolved in 24 mL of deionized water and then transferred to a stainless autoclave for 24 h at 220 °C. After being cooled naturally, the obtained black product was washed with ethanol and deionized water for several times until the supernatant was clear; then, it was dried at 60 °C. The final product was formulated into a 1 mg mL−1 MoS2 ethanol suspension and sonicated for 6 h. The typical synthesis of Mo-MCN samples was as follows: 8.0 g of guanidine hydrochloride and MoS2 solution (1, 2, and 4 mL; 1 mg mL−1) were mixed with 5 mL of deionized water. After stirring for 0.5 h and sonication treatment for 1 h, the mixture was dried at 80 °C for 24 h. Then, it was grinded into powder and heated at 550 °C for 1 h in air. The products were labeled as Mo-MCN-1, Mo-MCN-2, and Mo-MCN-4, respectively.
2.1.4 Synthesis of Co and Mo Co-doped Mesoporous g-C3N4
Co/Mo-MCN was prepared through the following typical experiment: 2 mL of MoS2 ethanol solution (1 mg mL−1), 8.0 g of guanidine hydrochloride, and a desired amount of CoCl2·6H2O (3, 5, 10, and 50 mg) were added to 5 mL of deionized water. After stirring for 0.5 h and sonication treatment for 1 h, the mixture was dried at 80 °C for 24 h. Then, it was grinded into powder and heated at 550 °C for 1 h in air. The products were denoted as Co/Mo-MCN-3, Co/Mo-MCN-5, Co/Mo-MCN-10, and Co/Mo-MCN-50, respectively.
2.2 Photocatalytic Activity Measurement
The photocatalytic activity of the samples was examined by RhB degradation and hydrogen generation under visible light irradiation (> 400 nm). The H2 production was performed in a closed system. Typically, a 100 mg of photocatalyst was dispersed in 300 mL of aqueous solution containing TEOA (10 vol%), and Pt (1.5 wt%). After 5 min of sonication, the solution was degassed under flowing N2. A 300-W Xe lamp equipped with UV filter (> 400 nm) was used as a light source. Evolved gas of 1 mL was sampled for analysis by gas chromatography with a TCD detector. For degrading the RhB solution, 50 mg of photocatalyst was dispersed in 50 mL of RhB solution under stirring. Considering the adsorption of the photocatalysts, the suspension was kept in dark for 30 min to attain an adequate adsorption–desorption equilibrium prior to irradiation.
3 Results and Discussion
Elemental analysis data of all the as-prepared materials are exhibited in Table S3. The C/N ratio is 0.65 for Mo-MCN and 0.64 for Co/Mo-MCN; these values are marginally lower than that of P-CN (0.68). This implies that Co- and/or Mo-doping inhibits the deamination in thermal polymerization, resulting in further defects .
The photocatalytic activity was also assessed by RhB degradation under visible irradiation. P-CN can degrade 19.5% RhB in 15 min; in contrast, Co/Mo-MCN-5 exhibits the highest photocatalytic activity and can degrade 94.7% RhB in 15 min (Fig. 6c). The photocatalytic activities of Co-CN and Mo-MCN are illustrated in Fig. S14a. Co-CN-5 and Mo-MCN-2 could degrade 71.2% and 54.4% RhB in 15 min. The results demonstrate that Co/Mo-MCN exhibits superior photocatalytic performance than those of P-CN, Co-CN, and Mo-MCN. In addition, the photocatalytic activities of Co-CN and Co/Mo-MCN decrease with the increase in the Co content; this can be attributed to the low crystallinity degree of g-C3N4 according to the XRD results. The cycling test of RhB degradation also verifies the high stability of Co/Mo-MCN-5 (Fig. 6d). Figures S14b and S15 display the kinetic curves of the RhB photodegradation of the as-prepared photocatalysts; these satisfy the pseudo-first-order rate law. The k (apparent reaction rate constant) of Co/Mo-MCN-5, Co-CN-5, and Mo-MCN-2 are 0.193, 0.134, and 0.065 min−1, which are 10.1, 7.0, and 3.4 times, respectively, than that of P-CN (0.0192 min−1).
The bandgaps of both P-CN and Co/Mo-MCN-5 are 2.77 eV. Furthermore, the Mott–Schottky measurement was carried out to investigate the conduction band (CB) edge of the materials (Fig. 7b). The positive slopes of the curves indicate the typical n-type characteristics of both the semiconductors. Therefore, the flat band potential can be approximately considered as the CB potential. The upshift of CB can be determined from − 1.19 eV for P-CN to − 1.32 eV for Co/Mo-MCN-5; moreover, Co-CN-5 and Mo-MCN-2 are -1.40 and -1.24 eV, respectively (Fig. S18). Based on the results of the bandgap and CB potential, the valence band (VB) potentials were calculated to be 1.58, 1.36, 1.55, and 1.45 eV for P-CN, Co-CN-5, Mo-MCN-2, and Co/Mo-MCN-5, respectively. This result demonstrates that the increase in CB caused by the bimetal co-doping can dramatically enhance the reduction capability of the photocatalyst; this can significantly improve the photocatalytic performance of hydrogen evolution. Furthermore, the trapping of the photogenerated electrons by transition metals can effectively inhibit the recombination of carriers; this is favorable to direct hole oxidation (RhB degradation) .
The photocurrent was carried out to investigate the capability to generate and transfer photogenerated charge carriers under visible light irradiation (Fig. 7c). One can conclude that the photocurrent density of Co/Mo-MCN-5 is significantly higher than that of Co-CN-5 and P-CN. Therefore, Co/Mo-MCN-5 can effectively facilitate the separation of photogenerated charge carriers owing to the mesopore structure, high SBET, and the formation of Co–N and Mo–N bonds.
The PL spectra are closely related to the recombination of photo-induced electrons and holes. A lower PL intensity generally implies lower recombination of photo-induced charge carries, resulting in an improved photocatalytic performance. Co/Mo-MCN-5 displays significantly lower PL intensity than those of Co-CN-5 and P-CN (Fig. 7d). This can be attributed to the transition of electrons in the mesopores and the trapping of electrons by Co and Mo bonds; these cause the low recombination rate of photogenerated electron–hole pairs, which are favorable to the improvement of photocatalytic activity.
The Co/Mo-MCN-5 conduction band (− 1.32 eV) is more negative than the hydrogen electrode potential (H+/H2). Therefore, H+ can be reduced to H2 directly. The radical species trapping experiments were performed to investigate the mechanism of RhB degradation by Co/Mo-MCN-3 by using isopropanol (IPA, as ⋅OH scavenger), p-benzoquinone (BQ, as ⋅O2− scavenger), and triethanolamine (TEOA, as h+ scavenger). As demonstrated in Fig. S19, the photodegradation efficiency of RhB abruptly decreased from 92.2% to 16.5% when the BQ was added; this indicates that ⋅O2− was the predominant active species for RhB degradation by Co/Mo-MCN. In contrast, the negligible inhibition by the addition of IPA indicates that ⋅OH does not positively affect the degradation of RhB. In addition, the degradation efficiency was marginally decreased with the introduction of TEOA. Therefore, these results demonstrate that ⋅O2− plays an important role in the degradation of RhB and that h+ is also involved in the Co/Mo-MCN catalyst.
The improved photocatalytic activity of Co/Mo-MCN compared with P-CN and mono-metal-doped g-C3N4 should be ascribed mainly to the synergistic effect of bimetallic doping, including the highly efficient charge separation, the fast mass transfer, and the higher SBET and larger pore volumes derived from the abundant mesoporous structure. Moreover, the formation of Co–N and Mo–N bonds by the doping with Co and Mo is another cause. The heteroatom Co or Mo can trap photogenerated electrons to reduce the recombination of charge carriers effectively, thereby increasing the photocatalytic activity. In order to further verify the reasons for the improved photocatalytic performance, the DFT calculations were applied.
For further analysis of the effects of Co-doping and Mo-doping on the photocatalytic activity, the charge density difference between P-CN and both the metal-doped materials was investigated; and the results are shown in Fig. 8d, e. Either of doped Co and Mo atoms could form chemical bonds between two layers and construct channels to deliver charges, thereby promoting the exciton dissociation .
Another factor that substantially affects the photocatalytic performance is the optical absorption characteristics. This can be estimated by the bandgap and optical absorption coefficients . The bandgap determines the optical spectral range where light can be absorbed to excite the photogenerated charge carriers. The optical absorption coefficients determine the optical absorption intensity at different wavelengths.
The above discussions are mainly on the change in the inherent properties of materials. However, there are certain nanoscale structures in the inner part of the photocatalyst. They can result in near-field effects, which modify the optical absorption properties. Owing to the constraint of optical measurement, the optical characteristics in the inner part of the photocatalyst is challenging to measure. Thus, optical simulation was utilized in this work for a better illustration of the optical absorption characteristics in the inner part of the photocatalyst. Moreover, all the optical simulations in this study were carried out using finite difference time domain (FDTD) method implemented on the FDTD Solutions software developed by Lumerical Solutions.
In summary, Co/Mo-MCN with high specific surface area was synthesized via template-free method. DFT calculations and FDTD simulations revealed that the bimetal co-doping can change the inherent optical properties of the material, thus extending the absorption region and increasing the absorption intensity. Moreover, it was revealed that the intrinsic mesoporous channels and wrinkle structure can improve the light absorption capability at the edge of the pore and reduce reflectance. Co/Mo-MCN exhibited superior photocatalytic performance for RhB degradation and hydrogen evolution than that of P-CN and mono-metal-doped g-C3N4. The improved photocatalytic performance is mainly ascribed to the synergistic effect of Co and Mo bimetallic doping, which resulted in larger specific surface area, narrowed bandgap, more negative CB potential, and abundant metal–N bonds. These brought about extended visible light absorption region, stronger reduction capability, improved separation efficiency of charge carriers, and accelerated mass transfer. This study provides a fresh perspective for the design and synthesis of high performing photocatalysts with suitable electronic structures and high specific surface areas by applying bimetallic co-doping.
We thank the financial support from National Natural Science Foundation of China (51472062).
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