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Synergic Effect between Adsorption and Photocatalysis of Metal-Free g-C3N4 Derived from Different Precursors.

Xu HY, Wu LC, Zhao H, Jin LG, Qi SY - PLoS ONE (2015)

Bottom Line: After 120 min reaction time, the blue color of MB solution disappeared completely.Subsequently, based on the measurement of the surface Zeta potentials of CN-M500 at different pHs, an active anionic dye, Methyl Orange (MO) was selected as the contrastive target pollutant with MB to reveal the synergic effect between adsorption and photocatalysis.Finally, the photocatalytic mechanism was discussed.

View Article: PubMed Central - PubMed

Affiliation: School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, P. R. China.

ABSTRACT
Graphitic carbon nitride (g-C3N4) used in this work was obtained by heating dicyandiamide and melamine, respectively, at different temperatures. The differences of g-C3N4 derived from different precursors in phase composition, functional group, surface morphology, microstructure, surface property, band gap and specific surface area were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-visible diffuse reflection spectroscopy and BET surface area analyzer, respectively. The photocatalytic discoloration of an active cationic dye, Methylene Blue (MB) under visible-light irradiation indicated that g-C3N4 derived from melamine at 500°C (CN-M500) had higher adsorption capacity and better photocatalytic activity than that from dicyandiamide at 500°C (CN-D500), which was attributed to the larger surface area of CN-M500. MB discoloration ratio over CN-M500 was affected by initial MB concentration and photocatalyst dosage. After 120 min reaction time, the blue color of MB solution disappeared completely. Subsequently, based on the measurement of the surface Zeta potentials of CN-M500 at different pHs, an active anionic dye, Methyl Orange (MO) was selected as the contrastive target pollutant with MB to reveal the synergic effect between adsorption and photocatalysis. Finally, the photocatalytic mechanism was discussed.

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Evaluation of adsorption capacity and photocatalytic activity for g-C3N4 samples.(a) Sample derived from dicyandiamide at different temperatures (b) Sample derived from melamine at different temperatures. (c) Comparative analysis of adsorption capacity and photocatalytic activity between CN-D500 and CN-M500. All the experiments were conducted at the pristine pH value of MB solution with g-C3N4 dosage of 1.0g/L and initial MB concentration of 5mg/L.
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pone.0142616.g008: Evaluation of adsorption capacity and photocatalytic activity for g-C3N4 samples.(a) Sample derived from dicyandiamide at different temperatures (b) Sample derived from melamine at different temperatures. (c) Comparative analysis of adsorption capacity and photocatalytic activity between CN-D500 and CN-M500. All the experiments were conducted at the pristine pH value of MB solution with g-C3N4 dosage of 1.0g/L and initial MB concentration of 5mg/L.

Mentions: The photocatalytic discoloration of MB over different g-C3N4 samples is illustrated in Fig 8, where it can be observed that MB can be efficiently discolored by g-C3N4 photocatalysts and the discoloration ratio of MB can reach above 90% within 120 min reaction. Blank experiments show that MB dye has a process of decomposition itself under visible light irradiation. The MB discoloration ratio by decomposition itself is near 23.5% at the irradiation time of 120 min, much lower than that by photocatalysis. Fig 8(a) clearly indicates that MB discoloration ratio is 92.9%, 95.2%, 90.2% and 96.5% for the sample CN-D460, CN-D500, CN-D540 and CN-D580, respectively, within 120 min reaction, with the highest adsorption capacity for CN-D580 and the best photocatalytic activity for CN-D500. It has been generally accepted that adsorption played an important role in the heterogeneous photocatalytic process [54]. Therefore, in this study, the specific surface area of as-obtained g-C3N4 was determined and is listed in Table 1. For the same precursor, the surface area of g-C3N4 increases when the pyrolysis temperature ascends, similar to previous report [48]. Before the photoreaction, about 12.3%, 22.8%, 35.2% and 59.5% of MB are adsorbed on the surface of CN-D460, CN-D500, CN-D540 and CN-D580, respectively, suggesting that larger sample’s surface area makes higher MB adsorption capacity. Although the adsorption capacity of CN-D580 is the highest, the reaction rate of photocatalytic discoloration driven by CN-D580 is not the fastest. The possible reason for this might be that the increase in the amount per unit area of dye molecules adsorbed onto the catalyst would reduce the number of active sites on the photocatalyst surface, which consequently hindered the generation of hydroxyl and superoxide radicals. It can be calculated that the amount per unit area of MB molecules adsorbed onto g-C3N4 sample is 3.88×10−4 mmol/m2, 3.36×10−4 mmol/m2, 5.12×10−4 mmol/m2 and 4.00×10−4 mmol/m2 for CN-D460, CN-D500, CN-D540 and CN-D580, respectively. As thus, it is not difficult to understand why the sample CN-D500 exhibits the best photocatalytic activity, instead of CN-D580. Fig 8(b) reveals a similar photocatalysis behavior of MB discoloration over g-C3N4 samples prepared from melamine at different temperatures. Likewise, the sample CN-M580 has the highest adsorption capacity and CN-M500 shows the best photocatalytic activity. Subsequently, CN-D500 and CN-M500 were selected to compare the adsorption capacity and photocatalytic activity of g-C3N4 under the identical conditions and the results are depicted in Fig 8(c). The data distinctly indicate that CN-M500 has higher adsorption capacity and better photocatalytic activity than CN-D500, attributed to its larger surface area. This result encourages us to prefer melamine to dicyandiamide as the precursor for the preparation of g-C3N4 photocatalyst. Therefore, CN-M500 was selected as the photocatalyst for the follow-up studies.


Synergic Effect between Adsorption and Photocatalysis of Metal-Free g-C3N4 Derived from Different Precursors.

Xu HY, Wu LC, Zhao H, Jin LG, Qi SY - PLoS ONE (2015)

Evaluation of adsorption capacity and photocatalytic activity for g-C3N4 samples.(a) Sample derived from dicyandiamide at different temperatures (b) Sample derived from melamine at different temperatures. (c) Comparative analysis of adsorption capacity and photocatalytic activity between CN-D500 and CN-M500. All the experiments were conducted at the pristine pH value of MB solution with g-C3N4 dosage of 1.0g/L and initial MB concentration of 5mg/L.
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getmorefigures.php?uid=PMC4643995&req=5

pone.0142616.g008: Evaluation of adsorption capacity and photocatalytic activity for g-C3N4 samples.(a) Sample derived from dicyandiamide at different temperatures (b) Sample derived from melamine at different temperatures. (c) Comparative analysis of adsorption capacity and photocatalytic activity between CN-D500 and CN-M500. All the experiments were conducted at the pristine pH value of MB solution with g-C3N4 dosage of 1.0g/L and initial MB concentration of 5mg/L.
Mentions: The photocatalytic discoloration of MB over different g-C3N4 samples is illustrated in Fig 8, where it can be observed that MB can be efficiently discolored by g-C3N4 photocatalysts and the discoloration ratio of MB can reach above 90% within 120 min reaction. Blank experiments show that MB dye has a process of decomposition itself under visible light irradiation. The MB discoloration ratio by decomposition itself is near 23.5% at the irradiation time of 120 min, much lower than that by photocatalysis. Fig 8(a) clearly indicates that MB discoloration ratio is 92.9%, 95.2%, 90.2% and 96.5% for the sample CN-D460, CN-D500, CN-D540 and CN-D580, respectively, within 120 min reaction, with the highest adsorption capacity for CN-D580 and the best photocatalytic activity for CN-D500. It has been generally accepted that adsorption played an important role in the heterogeneous photocatalytic process [54]. Therefore, in this study, the specific surface area of as-obtained g-C3N4 was determined and is listed in Table 1. For the same precursor, the surface area of g-C3N4 increases when the pyrolysis temperature ascends, similar to previous report [48]. Before the photoreaction, about 12.3%, 22.8%, 35.2% and 59.5% of MB are adsorbed on the surface of CN-D460, CN-D500, CN-D540 and CN-D580, respectively, suggesting that larger sample’s surface area makes higher MB adsorption capacity. Although the adsorption capacity of CN-D580 is the highest, the reaction rate of photocatalytic discoloration driven by CN-D580 is not the fastest. The possible reason for this might be that the increase in the amount per unit area of dye molecules adsorbed onto the catalyst would reduce the number of active sites on the photocatalyst surface, which consequently hindered the generation of hydroxyl and superoxide radicals. It can be calculated that the amount per unit area of MB molecules adsorbed onto g-C3N4 sample is 3.88×10−4 mmol/m2, 3.36×10−4 mmol/m2, 5.12×10−4 mmol/m2 and 4.00×10−4 mmol/m2 for CN-D460, CN-D500, CN-D540 and CN-D580, respectively. As thus, it is not difficult to understand why the sample CN-D500 exhibits the best photocatalytic activity, instead of CN-D580. Fig 8(b) reveals a similar photocatalysis behavior of MB discoloration over g-C3N4 samples prepared from melamine at different temperatures. Likewise, the sample CN-M580 has the highest adsorption capacity and CN-M500 shows the best photocatalytic activity. Subsequently, CN-D500 and CN-M500 were selected to compare the adsorption capacity and photocatalytic activity of g-C3N4 under the identical conditions and the results are depicted in Fig 8(c). The data distinctly indicate that CN-M500 has higher adsorption capacity and better photocatalytic activity than CN-D500, attributed to its larger surface area. This result encourages us to prefer melamine to dicyandiamide as the precursor for the preparation of g-C3N4 photocatalyst. Therefore, CN-M500 was selected as the photocatalyst for the follow-up studies.

Bottom Line: After 120 min reaction time, the blue color of MB solution disappeared completely.Subsequently, based on the measurement of the surface Zeta potentials of CN-M500 at different pHs, an active anionic dye, Methyl Orange (MO) was selected as the contrastive target pollutant with MB to reveal the synergic effect between adsorption and photocatalysis.Finally, the photocatalytic mechanism was discussed.

View Article: PubMed Central - PubMed

Affiliation: School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, P. R. China.

ABSTRACT
Graphitic carbon nitride (g-C3N4) used in this work was obtained by heating dicyandiamide and melamine, respectively, at different temperatures. The differences of g-C3N4 derived from different precursors in phase composition, functional group, surface morphology, microstructure, surface property, band gap and specific surface area were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-visible diffuse reflection spectroscopy and BET surface area analyzer, respectively. The photocatalytic discoloration of an active cationic dye, Methylene Blue (MB) under visible-light irradiation indicated that g-C3N4 derived from melamine at 500°C (CN-M500) had higher adsorption capacity and better photocatalytic activity than that from dicyandiamide at 500°C (CN-D500), which was attributed to the larger surface area of CN-M500. MB discoloration ratio over CN-M500 was affected by initial MB concentration and photocatalyst dosage. After 120 min reaction time, the blue color of MB solution disappeared completely. Subsequently, based on the measurement of the surface Zeta potentials of CN-M500 at different pHs, an active anionic dye, Methyl Orange (MO) was selected as the contrastive target pollutant with MB to reveal the synergic effect between adsorption and photocatalysis. Finally, the photocatalytic mechanism was discussed.

Show MeSH