Insight into the Enhanced Photocatalytic Activity of Potassium and

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Insight into the Enhanced Photocatalytic Activity of Potassium and Iodine Co-doped Graphitic Carbon Nitride Photocatalysts Yarong Guo, Tianxiang Chen, Qiong Liu, Zhengguo Zhang, and Xiaoming Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06921 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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The Journal of Physical Chemistry

Insight into the Enhanced Photocatalytic Activity of Potassium and Iodine Co-doped Graphitic Carbon Nitride Photocatalysts Yarong Guo, Tianxiang Chen, Qiong Liu, Zhengguo Zhang, Xiaoming Fang* Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China.

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ABSTRACT: Potassium and iodine co-doped graphitic carbon nitride (CN-KI) photocatalysts were prepared via thermal polymerization of a mixture prepared by dissolving dicyandiamide into a KI solution. Several techniques were employed to characterize the CN-KI photocatalysts and elucidate the mechanisms of the K and I co-doping. Compared to pristine CN, CN-KI exhibited a red shift in optical absorption edge, and the electron spin resonance analysis revealed that the enlargement of the visible light response mostly originated from I doping. Then, photoluminescence spectra showed that retarded recombination of photogenerated carriers happened to CN-KI than that to pristine CN, and electrochemical impedance spectroscopy measurements suggested that the decrease in carrier recombination resulted from K doping because the doping with K into CN network improved the charge transportation efficiency. Consequently, the K and I co-doped g-C3N4 sample with an optimal mass fraction of KI exhibited superior photocatalytic activity over the only K doped or I doped ones owing to the synergistic effect of the K and I doping. This work might shed light on cation-anion co-doping for developing high-performance g-C3N4 photocatalysts.


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1. INTRODUCTION Photocatalysis technology is regarded as one of the most promising ways to deal with the dual crisis of resources and environment caused by fossil fuels. Semiconductor-based photocatalyst with high activity and good stability is the most important factor influencing photocatalytic efficiency.1 An ideal photocatalyst should have the ability to absorb light in the visible range and transfer charge carriers rapidly as well as being stable, non-toxic and abundant.2 Up to now, Substantial research efforts have been focused on developing a variety of photocatalysts, such as TiO23, TaO64, CdS5, etc.

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However, photocatalysts that meet all the above requirements have

not yet been developed because most of them suffer the limited absorption region or poor stability. Therefore, searching for a new efficient photocatalyst with high photocatalytic activity, good stability and low cost is urgently needed. Graphitic carbon nitride (g-C3N4), a newly developed polymer semiconductor with a band gap of 2.78 eV, has been recognized a promising material due to its good visible light response activity, suitable band positions as well as chemical stability.9 However, there still exists two main drawbacks limiting its photocatalytic activity.10 One is that the band gap of g-C3N4 is relatively a bit large for visible light absorption, making it only absorb the light with wavelength smaller than 450 nm.11 The other is the fast charge carrier recombination originated from its organic π-conjugated polymer structure, which results in the limited photocatalytic activity of gC3N4.12-14 Hence, it is of significance to develop a novel g-C3N4-based photocatalyst with narrower band gap and enhanced charge separation and immigration. To address these limitations, a series of strategies have been developed which mainly include morphology controlling, combination with other semiconductors, element and molecular doping,



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and dye-sensitization.15 Among all these approaches, element doping, including metal doping and non-metal doping, has been considered as one of the most effective methods. Specially, metal doping is usually realized by inserting metal ions into the framework of g-C3N4 via the attraction of N atoms with opposite charges, resulting in the acceleration of charge carrier separation and migration by the modification on the electronic structure of g-C3N4.16 Up to now, several metal elements, such as Zn, Ag, Ti and Fe, have been doped into g-C3N4 to improve its photocatalytic activity17-20 and alkali doping could also modulate the electronic structure of gC3N4.16,

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It has been reported that potassium-doped g-C3N4 possesses high photocatalytic

activity for both hydrogen production and photodegradation by effectively suppressing the recombination rate of photo-induced carrier and tuning the conduction and valence band potentials.22 On the other hand, it has been reported non-metal doping endows the function of tuning the conduction and valence band positions of g-C3N4 by replacing the N or C atoms, leading to the increase in visible light absorption.16 Except the common non-metal elements such as S, O, F and P have been doped into g-C3N4,13,

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iodine doped g-C3N4 which was

synthesized from dicyandiamide (DCDA) and ammonium iodine also exhibited an enhanced hydrogen evolution rate than that of pristine g-C3N4 mainly owing to its narrowed band gap.26 Therefore, apparently, the metal and non-metal doping strategies are both effective strategies for improving the photocatalytic performance of g-C3N4. One can expect that the co-doping with metal and non-metal elements has the potential to develop novel high-performance g-C3N4-based photocatalysts by integrating the functions of the metal and non-metal doping.12, 27-28 Although potassium or iodine doped g-C3N4 photocatalysts have been explored respectively, little work has been done on the potassium and iodine co-doped g-C3N4, and the effects of the potassium


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and iodine co-doping on its electronic structure, band gap and photocatalytic activity for hydrogen evolution have not been investigated yet.29 In the current work, potassium and iodine co-doped g-C3N4 photocatalysts were synthesized via the thermal polymerization of a mixture of dicyandiamide and potassium iodide (KI), which were pretreated by well dissolving dicyandiamide into a KI solution followed by drying. The characteristics of the co-doped g-C3N4 photocatalysts were analyzed, and the mechanisms of the K and I co-doping were elucidated. It was found that the doped potassium and iodine both play a role in the enhancement in photocatalytic activity, the former of which improves charge transport, and the latter contributes to the enlargement in visible light response. The K and I co-doped gC3N4 photocatalyst with an optimal mass fraction of KI exhibited superior photocatalytic activity over the K doped and I-doped ones owing to the synergistic effect of K and I doping. This work might shed light on cation-anion co-doping for developing high-performance g-C3N4 photocatalysts.

2. EXPERIMENTAL SECTION 2.1. Materials All chemicals used in the experiments were purchased from Richjoint Chemical Reagents Co., Ltd. in chemical grade purity without further purification. 2.2. Preparation Pristine g-C3N4 was prepared by calcining DCDA at 550 °C for 4 hours donated as CN.



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K and I co-doped g-C3N4 samples were prepared as follows. Typically, 5 g DCDA was mixed with different amounts (varied from 0.1, 0.2, 0.25, 0.3, 0.35, 0.4 and 0.5 g) of potassium iodine dissolved in 25 ml deionized water with stirring at 80 °C, followed by drying at 80 °C to remove water. The resultant solids were calcined at 550 °C for 4 h in a muffle furnace to obtain the final samples that were denoted as CN-KIx, where x (x=0.02, 0.04, 0.05, 0.06, 0.07, 0.08 and 0.10) refers to the initial mass ratio of potassium iodine to DCDA. For further comparison, CN-K0.06 and CN-I0.06 were synthesized using the same procedure as CN-KI, where DCDA (5 g) was used as precursor, KNO3 (0.3 g) and NH4I (0.3 g) served as the dopants, respectively. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were obtained from a Bruker D8 Advance X-ray diffractometer with Cu-Kα1 radiation (λ = 1.5418 Å). The infrared absorption spectra were recorded on a Vector 33 Fourier transform infrared spectrometer (FT-IR). X-ray photoelectron spectroscopy (XPS) data were obtained on KRATOS Amicus instrument with a monochromatized Al Kα line source (200 W). The Brunauer-Emmett-Teller (BET) surface area measurements were carried out at 77 K on a Quantachrome Nova 2000e surface area analyzer. The morphologies and microstructures of the obtained samples were observed using a Zeiss Merlin Compact fieldemission scanning electron microscope (SEM) and a JEOLJEM 2100F fieldemission electron microscope (TEM). UV-vis diffuse reflection spectra (DRS) were obtained from a U-3010 spectrophotometer. Photoluminescence (PL) spectra of the photocatalyst powders were obtained at room temperature using an F-4500 Fluorescence Spectrophotometer equipped with a solid sample holder, and the excitation wavelength is 375 nm. The time-resolved


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fluorescence spectra were measured using FLSP920 steady/transient fluorescence spectrometer. The solid electron spin resonance (ESR) measurements were conducted using a Bruker model A300 spectrometer with a 300W Xe lamp equipped with a UV-cutoff (>420 nm) as the visible light source. 2.4. Photocatalytic activity evaluation Photocatalytic water splitting for hydrogen evolution was performed in an outer top-irradiation reaction vessel connected to a closed glass gas circulation system. Typically, 50 mg of catalyst powder was dispersed in an aqueous solution (100 mL) containing methanol (5 vol.%) as sacrificial electron donor and 1 wt.% Pt was loaded on the surface of the catalyst by in-situ photodeposition method using H2PtCl6 as co-catalyst. The reaction solution was evacuated 1 h to remove air completely prior to irradiation with a 300 W Xenon lamp (PerfectLight, China, LX300F) with a 420 nm cutoff filter. The temperature of the reaction solution was maintained at 5 °C by cold water circulation during the reaction. The evolved gases were analyzed by gas chromatography with N2 as the carrier gas. The stability of the K and I co-doped g-C3N4 photocatalyst CN-KI0.06 was test as follows. Firstly, CN-KI0.06 (1wt% Pt) was used to catalyze the photocatalytic hydrogen evolution for 3 h using the method described above. Secondly, the reaction solution was evacuated 1h to remove the generated H2 gas completely. Then, the photocatalytic reaction was continued for another 3 h. That reaction was circulated four times (12 h in total). 2.5. Photoelectrochemical analysis



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The film electrodes for photoelectrochemical measurements were fabricated as follows. 0.1 g photocatalyst and 0.01 g ethylcellulose were grinded into fine slurries with ethanol solution. The slurry was then coated onto fluoride-tin oxide (FTO) conductive glass sheet by the doctorblading method followed by drying at 120 °C for 1 h. The photocurrent response measurements were performed in a conventional three-electrode cell using an electrochemical analyzer (CHI660E), in which a Pt sheet and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrodes were immersed in a sodium sulfate electrolyte solution (0.2 M), and the exposed effective area of the working electrodes under illumination was 1 cm2. The periodic on/off photocurrent response of each film electrode was measured at 0.5 V bias vs. Ag/AgCl. For electrochemical impedance spectroscopy (EIS) experiments, the perturbation signal was 0.5 V, and the frequency ranged from 100 kHz to 10 mHz.

3. RESULTS AND DISCUSSION 3.1. Structure and morphology of CN-KIx Figure 1A shows the XRD patterns of pristine CN and the doped CN samples. It can be found that all the CN-KIx samples exhibit a diffraction peak at around 27.1°, corresponding to the (002) plane. It represents the inter-planar graphitic stacking with an inter-layer distance of 0.33 nm.30 While, with the amount of KI increasing, the (001) diffraction peak at around 13.0°, representing an inter-planar separation, is getting lower and lower, and finally disappears. It is indicated that the in-plane structure is destroyed or changed.31-32 And the intensity of both the diffraction peaks at 27.1° and 13.0° are weakened after doped with KI, which might be attributed to the defects arising from the incomplete polymerization. For comparison, the XRD patterns of CN-KI0.06,


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CN-K0.06 and CN-I0.06 are displayed in Figure 1B. It is obvious that the intensity of the diffraction peak at around 27.1° varies from dopant to dopant. CN-K0.06 exhibits the lowest intensity as compared with CN-KI0.06 and CN-I0.06, implying that the doping with K plays a more effective role in modifying the structure of CN than that with I. Similar FT-IR spectra in regard to the typical CN heterocycles skeletal vibration are observed for CN and CN-KIx (Figure 2A), which have been offset for clarity. The peaks located at 810 cm1

and 1200-1600 cm−1 are related to the typical breathing and stretching vibration modes of the

heptazine heterocyclic ring (C6N7), respectively, confirming the formation of a triazine phase in all samples. The broad peak at 2900-3500 cm−1 ascribes to the O-H vibration, originating from the absorbed water molecules and the N-H vibration caused by the surface uncondensed amine groups.33 The peak located at around 2104 cm−1, corresponding to the stretch vibration of azide group,34 increases with the mass fraction of KI, suggesting the doping with KI lead to the formation of N-K bonding. Figure 2B shows the FT-IR spectra of CN-KI0.06, CN-K0.06 and CNI0.06, which have been offset for clarity. Interestingly, different from CN-KI0.06 and CN-K0.06, CN-I0.06 does not exhibit the peak located at around 2104 cm−1, which further confirms that the emergence of azide group is attributed to the doping with K rather than with I. The formation of N-K bond suggests that K has been inserted into the framework of g-C3N4. The compositions and chemical states of CN and the doped samples were examined by XPS. As illustrated in Figure 3A, C, N, and O are detected in the survey spectra of all the samples, where O originated from the high temperature treatment in air during the synthesis. For CNKI0.06 and CN-KI0.10, both K and I are detected (Figure 3B and C), indicating the successful incorporation of K and I into CN network. The K and I contents in the samples have been calculated from the XPS data and are listed in Table 1. It can be seen that the K and I contents in



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the as-prepared samples increase with KI content increasing in the precursor. Meanwhile, K and I can also be detected from CN-K0.06 and CN-I0.06, respectively, which the contents are similar to those in CN-KI0.06. The K2p3/2 and K2p1/2 peaks of CN-KI0.06 are located at around 292.3 eV and 295.0 eV, which of the binding energy are lower than 293 eV and 296 eV of potassium salts, respectively.35 Interestingly, the peak of K2p1/2 is similar to that for potassium azide (KN3),36 suggesting the formation of the N-K bond in CN-KIx, in accordance with the aforementioned FTIR results. Furthermore, the binding energy of K2p3/2 and K2p1/2 for CN-KI0.10 are 292.5 eV and 295.2 eV, higher than 292.3 eV and 295.0 eV for CN-KI0.06, respectively. The peak locations of K2p increasing with the increasing KI content in the precursor, further verify the existence of the covalent bond between K and g-C3N4.29 These results reveal that K ions have been doped into g-C3N4. Similarly, the peaks of I3d3/2 and I3d5/2 at around 629.2 eV and 617.6 eV in CNKI0.06 are lower than 630.3 eV and 618.8 eV in iodine salts, implying the existence of the covalent bond between I and C3N4.37 It is indicated that I ions have been doped into g-C3N4. As for the doping sites of I, the sp2-bonded N tends to be substituted by I atoms according to a previous report.26 Thus the possible doping sites for K and I ions in CN-KI are illustrated in Figure 4. In addition, the XPS measurement has been also conducted to the samples after being etched with Ar for 10 nm, and the C, N, O, K and I contents in the etched samples have been also obtained and listed in Table 1. It can be seen that the C, N and O contents in the etched samples are close to those in the as-prepared ones, and obvious decreases in the K and I contents are not found after the samples have been etched with Ar. It is verified that K and I atoms exist in the lattice of g-C3N4, not just on the surface. SEM has been employed to observe the microstructures of pristine CN and the doped samples. As seen from the SEM images (Figure 5), all samples possess the similar layered structure,


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implying these doped samples maintained the original morphology of g-C3N4. Simultaneously, TEM images (Figure S1) display similar layered structures of these g-C3N4 samples. Figure S2 displays nitrogen adsorption and desorption isotherms of the as-prepared samples. The BET specific surface areas (SBET) of CN, CN-KI0.06, CN-KI0.10, CN-K0.06 and CN-I0.06 are calculated to be 8.31, 30.97, 40.12, 50.59 and 69.81 m2/g, respectively, as listed in Table 1. Compared to pristine CN, all the doped samples exhibit an increase in SBET, probably owing to the hindered crystal growth caused by doping. In addition, the SBET of CN-K0.06 and CN-I0.06 are larger than those of CN-KI0.06 and CN-KI0.10, respectively, which originated from the release of NO2 or NH3 during their syntheses. What’s more, the SEM-EDS element mapping images of CN-KI0.06 (Figure S3) prove the uniform distribution of K and I atoms in the g-C3N4 lattice. 3.2. Optical properties and band structure of CN-KIx Figure 6A shows the DRS of pristine CN and the doped CN samples. It can be seen that all the CN-KIx samples exhibit similar optical absorption as compared to CN, indicating that the intrinsic backbone structure of g-C3N4 has not been changed after the doping with KI. However, a remarkable red-shift in absorption edge emerges among the doped CN samples, designating a narrowing of band gap after the KI modification. In addition, the band gap of these samples has been obtained from (Ahν)2-hν spectrums (Figure S4) transformed from the UV-vis diffuse reflection spectra. As listed in Table 1, the band gap decreases from 2.78 eV for pristine CN to 2.67 eV for CN-KI0.06 and 2.64 eV for CN-KI0.10, indicating the visible light response is extended by the KI doping. Significantly, as displayed in Figure 6B, CN-KI0.06 exhibits the largest redshift as compared with CN-K0.06 and CN-I0.06, indicating that CN-KI0.06 has the narrowest band gap. According to the XPS results (listed in Table 1), the K and I contents in CN-KI0.06 are almost equal to those in CN-K0.06 and CN-I0.06, respectively. Thus it can be inferred that the 11 Environment ACS Paragon Plus


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band-gap narrowing results from both the K and I doping, revealing a synergistic effect between the K and I doping. Furthermore, CN-I0.06 has a narrower band gap than CN-K0.06, as shown in Figure 6B, suggesting that I doping plays a more effective role in narrowing band gap than the K doping. The mechanism for the red shift in the absorption spectra has been elucidated through XPS valence band spectra, as shown in Figure 7. The valence band position of CN-KI0.06 is close to that of CN-I0.06, which is lower than those of CN-K0.06 and CN. And the valence band position of CN-K0.06 is very close to that of CN. It is revealed that the red shift of CN-KI0.06 is ascribed to its lower valence band position, which mainly originated from the iodide ion doping. In order to further elucidate the electronic band structures of the samples, ESR technology has been employed, as shown in Figure 8. A symmetrical Lorentzian line centered at a g value around 2.0103, which is assigned to lone pair electrons in sp2-carbon in a typical heptazine graphitic carbon nitride, is detected both in dark and visible light irradiation for the obtained samples in the magnetic field from 3440 G to 3570 G. The single Lorentzian lines demonstrate the well-established CN semiconductor structure for all the samples.32, 38 Obviously, the ESR intensities of CN-KIx are extremely amplified, elucidating the more effective extension in delocalized systems of CN heterocycles after doped with KI. Moreover, an enhanced ESR signal is observed for CN and CN-KIx under visible light irradiation, and the enhancement increases with the KI content. It is suggested that the CN-KIx photocatalysts could be much easier to be inspired by visible light than pristine CN. Interestingly, the ESR intensity of CN-I0.06 under visible light is close to that of CN-KI0.06, while that of CN-K0.06 is approaching to that of pristine CN. These results reveal that the enhancement in visible light activity originates from the I doping rather than the K doping, consistent with the DRS results (Figure 6). In addition, a signal at g value of 1.9930 is observed over ESR at 77 K for both CN-KI0.06 and CN-K0.06, as shown in


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Figure 9, which might be ascribed to the electrons trapped by K+ centers. The intensities under visible light for CN-KI0.06 and CN-K0.06 are almost the same and much higher than that in dark. These results indicate that the doping of K can reduce recombination rate of charges since electrons transfer to K+, promoting charge separation. Figure 10 displays the room temperature photoluminescence (PL) spectra of pristine CN and the doped CN samples at the excitation wavelength of 375 nm. It is obvious that the emission peaks of all the CN-KIx samples are lower than that of pristine CN, suggesting that the doping with KI greatly reduces the recombination of photo-generated charge carriers. Moreover, the emission intensity of the CN-KIx samples decreases at first and then increases as the amount of KI increases from 0.02 to 0.10, of which the lowest is the amount of 0.06. It is indicated that CNKI0.06 has the lowest emission peak and excess KI contents might form recombination centers for carriers. Significantly, as compared with CN-K0.06 and CN-I0.06 in Figure 10B, CN-KI0.06 still exhibits the lowest emission peak, suggesting the least recombination of photogenerated carriers in CN-KI0.06. Since the K and I contents in CN-KI0.06 are almost equal to those in CN-K0.06 and CN-I0.06, respectively, according to the XPS results (listed in Table 1), it can be inferred that the reduction in carrier recombination results from both K and I doping, revealing a synergistic effect between the K and I doping. Moreover, the emission intensity of CN-K0.06 is lower than that of CN-I0.06 (Figure 10B), suggesting that the K doping plays a more effective role in reducing carrier recombination than the I doping. The lifetimes of charge carriers for pristine CN and doped CN were further investigated by the time-resolved fluorescence spectra, as shown in Figure 11. And the fitting parameters of the radiative lifetimes with different percentages are listed in Table 2. The short life time of charge carriers in CN-KI0.06 is 2.36 ns with a percentage of 45.86% compared to 1.25 ns and 68.32% in 13 Environment ACS Paragon Plus


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CN. The long life time and percentage of carriers increase from 4.05 ns and 31.38% in CN to 7.51 ns and 54.14% in CN-KI0.06, respectively. And the weighted mean lifetime of 5.14 ns for CN-KI0.06 is larger than 2.13 ns for CN. In addition, CN-K0.06 and CN-I0.06 also exhibits longer life time than CN, while CN-K0.06 shows a longer one than CN-I0.06. These results of the extended lifetime of charge carriers further illustrate the introduction of K more than I can efficiently retard the carriers recombination thus enhance the possibility of carriers participating in photocatalytic hydrogen evolution. However, the shorter lifetime of CN-KI0.10 suggests the formation of recombination center due to excessive K doping. Photoelectrochemical measurements have been conducted to investigate the charge transport property of pristine CN and the doped CN samples, as shown in Figure 12. CN-KI0.06 exhibits the highest photocurrent density as compared with CN, CN-KI0.10, CN-K0.06 and CN-I0.06, owing to the least recombination of photogenerated carriers in CN-KI0.06. The photocurrent density verifies the results of PL (Figure 10). Moreover, the photocurrent density of CN-K0.06 is larger than that of CN-I0.06, verifying that the K doping plays a more effective role in improving charge transport than the I doping. More significantly, the current density of pristine CN and the doped CN samples does not decrease after four times’ run, indicating the good stability of all these samples. Figure 12B displays the EIS plots of pristine CN and doped CN samples in the dark. Significant decrease in semicircular Nyquist plots are observed for CN-KI0.06, clearly demonstrating that the doping with KI can indeed effectively improve the electronic conductivity of polymer matrix to promote charge separation. Thus charge carriers of CN-KI0.06 have longest lifetime. While, the Nyquist plot diameter of CN-KI0.10 is larger than that of CN-KI0.06, suggesting that excessive KI content might restrict the electronic conductivity. It is noted that CN-K0.06 has a much smaller Nyquist plot diameter than CN-I0.06, further demonstrating the K


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doping plays the main role in improving the electronic conductivity of CN-KIx, conforming to the results of time-resolved fluorescence. 3.3. Photocatalytic activity of CN-KIx Photocatalytic hydrogen evolution of the as-prepared samples were evaluated by loading 1wt% Pt as co-catalyst and using methanol (5 wt%) as the sacrificial reagent under visible light irradiation (>420 nm). The hydrogen evolution rates of the catalysts are displayed in Figure 13. From Figure 13A, it can be seen that the hydrogen evolution rate of CN-KIx gradually increases first and then decreases when the doping content of KI is increased from 0.02 to 0.10. CN-KI0.06 has the highest photocatalytic activity, which average hydrogen evolution rate reaches 41.23 µmol/h, about 4.9 times as high as that of CN. It has been revealed that, in a H2O/methanol system, the effective protons for hydrogen evolution reaction are mainly from H2O, and CH3OH evidently serves as an outstanding sacrificial agent reacting with holes.39 Thus, the evolved hydrogen was almost produced by H2O and no CO2 was produced by oxidation of methanol. The decrease in photocatalytic activity with the further increase in the KI content originates from the increase in carrier recombination (Figure 10) and the decline in charge transport (Figure 12), though CN-KI0.10 exhibits higher optical absorption than CN-KI0.06 (Figure 6). It can be inferred that the reduction in carrier recombination and the increase in charge transport plays a more important role in improving photocatalytic activity than the enhancement in optical absorption for the g-C3N4 based photocatalysts due to the π-conjugated structure. As shown in Figure 13B, the photocatalytic activity of CN-KI0.06 is higher than that of CN-I0.06. Based on the above descriptions, CN-I0.06 exhibits higher optical absorption than CN-K0.06, while CN-K0.06 shows more reduced carrier recombination and faster charge transport than CN-I0.06. These results further verify that the determining factor for the photocatalytic reaction of g-C3N4 is the carrier 15 Environment ACS Paragon Plus


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recombination and charge transport. More significantly, in contrast to CN-K0.06 and CN-I0.06, CN-KI0.06 has a much higher hydrogen evolution, revealing the synergistic effect of the potassium and iodine co-doping. Note that CN-KI0.06 exhibits the best optical absorption property and the least carrier recombination, leading to its highest photocatalytic activity. Furthermore, the action spectra of photocurrent response and hydrogen evolution were analyzed with the aid of four band-pass filters (420 ± 20,465 ± 20, 510 ± 20 and 550 ± 20 nm), and the obtained results are shown in Figure 14. It is clearly observed that the photocurrent response value and photocatalystic hydrogen evolution rate are both well matched with the optical absorption spectra, indicating the absorption of the visible photons plays a primary role in the photocatalytic performance. CN-KI0.06 still preserves the ability for photocurrent response and hydrogen evolution even when the irradiation wavelength is extended to as long as 550 nm, showing its advanced optical property for photocatalysis. Moreover, the quantum efficiency of hydrogen evolution was calculated to be 0.85% at 420±20 nm, 0.68% at 465±20 nm, 0.10% at 510±20 nm and 0.06% at 550±20 nm, respectively, according to the following equation. 40

QE =

2 × the number of evolved H 2 molecules × 100% the number of incident photons

In order to evaluate the stability and reusability of the CN-KIx photocatalyst, hydrogen production catalyzed by CN-KI0.06 was carried out under visible light cycled for four times. As shown in Figure 15, under 4 h continuous illumination, the amount of H2 release shows a linear. In addition, no obvious deactivation is observed after four consecutive cycle’s reactions, indicating that CN-KI0.06 has good stability under visible light irradiation.


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4. CONCLUSIONS In summary, K-I co-doped g-C3N4 has been successfully prepared by a simple thermal polymerization method using dicyandiamide and KI as precursor and dopant, respectively. Compared to pristine CN, CN-KI possesses a similar structure and morphology with higher surface area, narrower band gap and enhanced charge separation efficiency. Thus CN-KI has higher photocatalytic activity. The optimal one, CN-KI0.06, exhibits superior photocatalytic activity with the hydrogen evolution rate of 41.23 µmol/ h under visible light, about 4.9 times as high as that of CN. Further studies reveal that the broadened visible light absorption mostly comes from iodine doping, which results in a much narrower bandgap. On the other hand, the decreased carrier recombination mainly derives from potassium doping, optimizing the electronic structure of g-C3N4 and attracting electrons thus promoting charge separation. This paper highlights the importance of the K-I co-doping to increase both optical and electrical properties so as to increase photocatalytic activity of this newly-developed carbon nitride semiconductor. It is still expected that the innovative cation-anion co-doping strategy presented herein could provide an effective method for improving the photocatalytic activity of g-C3N4. ASSOCIATED CONTENT Supporting Information. Additional data, including TEM and (Ahν)2-hν spectrums of CN and doped CN; elemental mapping images of CN-KI0.06. AUTHOR INFORMATION Corresponding Author *Tel: 86 20 87112997, Fax: 86 20 87113870, Email: [email protected]



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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21276088 and 60976053). REFERENCES (1) Dalrymple, O. K.; Stefanakos, E.; Trotz, M. A.; Goswami, D. Y., A Review of the Mechanisms and Modeling of Photocatalytic Disinfection. Applied Catalysis B: Environmental 2010, 98, 27-38. (2) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat Mater 2009, 8, 76-80. (3) Liu, L.; Chen, X., Titanium Dioxide Nanomaterials: Self-Structural Modifications. Chem Rev 2014, 114, 9890-9918. (4) Boltersdorf, J.; Wong, T.; Maggard, P. A., Synthesis and Optical Properties of Ag(I), Pb(II), and Bi(III) Tantalate-Based Photocatalysts. Acs Catalysis 2013, 3, 2943-2953. (5) Yao, K.; Lu, W.; Wang, J., Ionic Liquid-Assisted Synthesis, Structural Characterization, and Photocatalytic Performance of CdS Nanocrystals. Materials Chemistry and Physics 2011, 130, 1175-1181. (6) Kisch, H., Semiconductor Photocatalysis--Mechanistic and Synthetic Aspects. Angew Chem Int Ed Engl 2013, 52, 812-847. (7) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J., Nano-Photocatalytic Materials: Possibilities and Challenges. Adv Mater 2012, 24, 229-251. (8) Hernández-Alonso, M. D.; Fresno, F.; Suárez, S.; Coronado, J. M., Development of Alternative Photocatalysts to TiO2: Challenges and Opportunities. Energy & Environmental Science 2009, 2, 1231. (9) Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Z., Graphitic Carbon Nitride Materials: Controllable Synthesis and Applications in Fuel Cells and Photocatalysis. Energy & Environmental Science 2012, 5, 6717.


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Figures and Tables Figures Figure 1 XRD patterns of CN and CN-KIx (A) and comparison on XRD patterns of samples with different dopants (B) Figure 2 FT-IR spectra of CN and CN-KIx (A) and comparison in FT-IR of samples with different dopants (B) Figure 3 XPS spectra of pristine CN and doped CN samples: Survey (A), K2p (B) and I3d (C) Figure 4 Possible doping sites for K and I ions in CN-KI Figure 5 Typical SEM images of CN (A), CN-KI0.06 (B), CN-K0.06 (C) and CN-I0.06 (D) Figure 6 UV-vis diffuse reflection spectra of CN and CN-KIx (A) and comparison in optical absorption of the samples with different dopants (B) Figure 7 XPS valence band spectra of CN and doped CN samples Figure 8 ESR spectra of CN and CN-KIx in the dark (A) and under visible light irradiation (B) (>420nm) Figure 9 ESR spectra of CN-KI0.06 and CN-K0.06 at 77 K Figure 10 Photoluminescence spectra of CN and CN-KIx (A) and comparison of different dopants (B) Figure 11 Time-resolved fluorescence spectra of CN and doped-CN Figure 12 Periodic on/off photocurrent response under visible light irradiation (A) and electrochemical impedance spectroscopy plots in the dark (B)



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Figure 13 Photocatalytic H2 evolution rate under visible light irradiation for CN-KIx samples (A) and comparison in H2 evolution rate of CN-KI0.06, CN-K0.06 and CN-I0.06 (B) Figure 14 Action spectra of photocurrent response (A) and hydrogen evolution (B) of CN-KI0.06 in visible region Figure 15 Cycle runs of CN-KI0.06 for H2 evolution


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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Figure 9



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Figure 10


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Figure 11



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Figure 12


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Figure 13



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Figure 14


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Figure 15



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Tables Table 1 Surface area, chemical compositions and band gap of the samples. Table 2 The fluorescence decay lifetimes and their percentages of photo-introduced carriers of CN and doped CN.


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Table 1

Materials

Surface areaa/m2·g-1

CN

CN-KI0.06 CN-KI0.10

CN-K0.06

CN-I0.06

8.31

30.97

40.12

50.59

69.81

Atomicb/%（ Ar etching） C N O K I

Band gap / eV

56.78

40.39

2.84

-

-

2.78

53.60

44.16

2.24

-

-

55.46

38.25

2.66

2.38

0.11

50.28

45.05

1.56

2.99

0.12

56.29

37.34

3.36

2.91

0.18

49.58

45.43

1.81

3.01

0.17

52.77

41.82

2.93

2.39

-

51.45

43.95

1.64

2.96

-

55.33

42.04

2.51

-

0.12

52.71

43.79

3.38

-

0.12

a. Calculated from N2 absorption–desorption isotherms. b. Calculated from XPS test.


2.67

2.64

2.76

2.70


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Table 2 Materials

τ1 /ns

Rel. / %

τ2 /ns

Rel. / %

τ / ns

CN

1.25

68.62

4.05

31.38

2.13

CN-KI0.06

2.36

45.86

7.51

54.14

5.14

CN-KI0.10

1.71

48.51

5.93

51.49

3.88

CN-K0.06

1.93

49.56

6.35

50.44

4.16

CN-I0.06

1.44

46.17

5.43

53.83

3.59


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Table of Contents


Insight into the Enhanced Photocatalytic Activity of Potassium and

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