Introduction
Photocatalysis is of great importance in the fields of energy and environmental applications, and is regarded as a revolutionary technology in the 21st Century. Most photocatalysts (such as TiO2 and GaN) exhibit wide band gaps within the range 2.0–3.4 eV (Fujishima et al., Reference Fujishima, Zhang and Tryk2008). This enables their photocatalytic activity under ultraviolet light (<385 nm) to be enhanced, which, meanwhile, accounts for only 3% of the solar energy (Herron et al., Reference Herron, Kim, Upadhye, Huber and Maravelias2015). In that regard, searching for a highly active photocatalyst with visible light response and its practical application has attracted the attention of many scholars(Dong et al., Reference Dong, Zhang, Pan and Qiu2014; Fatimah et al., Reference Fatimah, Wang and Wulandari2011; Thomas et al., Reference Thomas, Fischer, Frederic, Goettmann, Antonietti, Muller, Schlogl and Carlsson2008).
Graphite phase carbon nitride (g-C3N4) is a non-metal semiconductor photocatalytic composite with a band gap (2.7 eV) suitable for the initiation of catalytic reactions under visible light. Moreover, g-C3N4 is an ecofriendly, non-toxic, non-polluting material that possesses stable physical and chemical properties, thus offering promise for a wide range of photocatalytic applications (Dong et al., Reference Dong, Zhang, Pan and Qiu2014; Kumar et al., Reference Kumar, Karthikeyan and Lee2018; Wang et al., Reference Wang, Maeda, Thomas, Takanabe, Xin, Carlsson, Domen and Antonietti2009).
Methods of synthesis of g-C3N4 include physical vapor deposition (PVD), high-temperature chemical vapor deposition (CVD), and hot melt techniques (Thomas et al., Reference Thomas, Fischer, Frederic, Goettmann, Antonietti, Muller, Schlogl and Carlsson2008; Li, Y.P., et al., Reference Li, Zhan, Huang, Xu, Li, Zhang and Wu2014). However, all of them are costly and require stringent reaction conditions. In addition, unmodified g-C3N4 has no adsorption properties, and is easy to agglomerate, resulting in small specific surface area, high electron-hole recombination rate, and poor light utilization efficiency. These shortcomings reduce the photocatalytic efficiency of g-C3N4, thereby limiting its practical application (Xue et al., Reference Xue, Ma, Zhou, Zhang and He2015).
Recently, research has been focused on supporting the multi-layered silicate mineral matrix to solve the problem of easy agglomeration and small specific surface area of photocatalysts and to extend their potential application range (Cheng et al., Reference Cheng, Shi, Pu and Yan2011; Fatimah et al., Reference Fatimah, Wang and Wulandari2011; Peng et al., Reference Peng, Fu, Ouyang and Yang2016). Increasing attention has been paid to montmorillonite as a promising carrier for photocatalysts due to its large specific surface area and good adsorption performance (Li, C. et al., Reference Liang2016, Reference Liang2018; Li et al., Reference Liang2017; Sonawane et al., Reference Sonawane, Gumfekar, Kate, Meshram, Kunte, Ramjee, Mahajan, Parande and Ashokkumar2010; Sun et al., Reference Sun, Chen, Qiang, Ye and Yuan2002). Also, attempts have been made to combine Mnt with graphite phase carbon nitride and to obtain materials with increased photocatalytic properties. For example, Li, S. et al. (Reference Liang2016) prepared a g-C3N4/Mnt composite with a g-C3N4 mass fraction of 31.8%, whose photocatalytic degradation rate in relation to RhB was four times greater than that of pure g-C3N4. Furthermore, combining g-C3N4 with Mnt as a carrier allowed those authors to solve the problem of g-C3N4 agglomeration effectively. According to Li et al. (Reference Liang2017), the g-C3N4/Mnt composite possessed a large specific surface area, good temperature resistance, and easy recycling, which help to improve its photo absorption characteristics. Moreover, the photocatalytic activity of the g-C3N4/Mnt composite prepared exceeded that of pure g-C3N4.
When water molecules get into the interlayer space of the Mnt 2:1 layers through an aqueous solution, the distance between the Mnt lamellae enlarges because of increasing interlayer penetration depth and peeling away of hydration layers. In this respect, compared with bulk montmorillonite, the specific surface area of exfoliated two-dimensional Mnt layers is increased significantly. This improves noticeably the light transmittance of the lamellae due to the fact that a freshly exposed surface has a large number of active reaction sites.
Given the above information, Mnt has the prospect of being a good photocatalytic support. However, some aspects still need to be clarified. Among these are the need to confirm an increase in specific surface area of exfoliated Mnt as the support for the g-C3N4 photocatalyst, and to study the photocatalytic effect of g-C3N4 in the g-C3N4/Mnt composite and its related mechanism. The objective of the present study was, therefore, to find answers to these questions by measuring the effect of temperature on the photocatalytic performance of a series of g-C3N4/Mnt composites, prepared by a wet chemical method using exfoliated Mnt as the matrix.
Experimental
Raw Materials and Reagents
The Mnt was prepared and exfoliated as follows. Bentonite (Santai City, Sichuan Province, China) was ground as a dry powder and passed through a 200-mesh sieve (74 μm), 10 g of which was combined with 0.6 g of anhydrous sodium carbonate in a 500 mL beaker, mixed by stirring, and then 160 mL of deionized water (with a resistivity of 18.2 MΩ • cm) was added, giving a a solid-liquid ratio of 1:10. The mixture was then stirred for 30 min in a water bath at 80°C to effect dispersion and sodium saturation. After that, the solid:liquid ratio was adjusted to 1:20 using deionized water, and the suspension was stirred in a high-speed homogenizer at a rotational speed of 10,000 rpm for 8 h. The resulting suspension was centrifuged at 600 rpm (30 g) and the resulting sedimented gel was frozen (–80°C) and freeze dried. The resulting dry powder was labeled “M”. A portion of the powder was also resuspended in water to give a 20% suspension.
Raw materials were chemically pure melamine, analytically pure anhydrous sodium carbonate, and RhB, all purchased from Chengdu Kelong Chemical Reagent Factory (Chengdu, China).
Catalyst Preparation
Melamine (9.0 g) was added to 1000 mL of deionized water and stirred with a magnetic stirrer at 80°C to prepare a hot melamine solution, which was then dropped into 1000 mL of the Mnt suspension using a peristaltic pump at a speed of 1.2 mL/min. The combined suspension was stirred for 12 h at 80°C in a water bath, then dried in a vacuum freeze dryer to obtain a precursor composite. To obtain the g-C3N4/Mnt composites, the precursor was placed in a 50 mL corundum crucible, sealed with tin foil, and calcined for 4 h at 490, 520, 550, 580, 610, or 650°C. The heating rate was 2.3°C/min. After natural cooling, the composites were labeled as CNM-T, where CNM stands for g-C3N4/Mnt and T is the corresponding calcination temperature.
The same steps were taken to prepare a pure g-C3N4 specimen at 550°C, referred to as CN-550°C.
For the comparative study, 0.1 g of M and 1.68 g of CN-550°C were mixed and ground to obtain a physically mixed composite CN+M.
Photocatalysis Experiment
A 300 W xenon lamp, purchased from Beijing China Education Au-Light Co., Ltd, was used as the visible light source within a wavelength range of 400–800 nm. The photocatalyst (50 mg) was dispersed in RhB solution (20 mg/L, 100 mL) and then stirred for 1 h in the dark on a magnetic stirrer to reach the adsorption-desorption equilibrium. After that, the light source was turned on. Aliquots (6 mL) of the RhB solution were taken every 10 min. An Evolution 300 UV-Visible spectrophotometer (Shimadzu, Kiya-cho, Nijiyanan, Japan) was used to test the absorbance of the supernatant at the maximum absorption wavelength of RhB (554 nm), and the degradation rate of RhB was calculated according to the Beer-Lambert law (Swinehart, Reference Swinehart1962) as follows:
where A is the absorbance; T is the transmittance; K is the molar absorption coefficient; b is the thickness of the absorbing layer; c is the concentration of the light-absorbing substance (mol/L); A 0 is the absorbance of the solution in the dark at adsorption equilibrium; A t is the absorbance of the solution at time t of light radiation; C 0 is the solution concentration in the dark at adsorption equilibrium (mg/L); and C t is the solution concentration at time t of the photocatalytic reaction (mg/L).
Characterization
Thermal performance analysis of samples was conducted in ambient atmospheric air using a STA449F5 synchronous thermal analyzer (TG-DSC) (NETZSCH, Berlin, Germany). Specimens were heated within a temperature range of 35–800°C at a heating rate of 20°C/min, and α-Al2O3 powder was taken as the reference material.
X-ray diffraction (XRD) patterns of samples were collected within a scanning angular range of 3 to 80°2θ by an Ultima IV X-ray diffractometer (PANalytical B.V., Eindhoven, The Netherlands) with a CuKα radiation source operating at a voltage of 40 kV and a tube current of 40 mA. The XRD patterns were analyzed using Jade 6.0 software (Huang & Li, Reference Huang and Li2012).
The micromorphology of specimens was probed by means of an Ultra 55 field emission scanning electron microscope (Carl ZeissNTS GmbH, Oberkochen, Germany) with a MnKα radiation source, operating at a voltage of 15 kV and an energy resolution >127 eV. The optical images were recorded at a count rate of 20,000 cps.
The specific surface areas of samples were determined using an Autosorb-1MP aperture analyzer (American Kangta Instruments Co., Ltd, Florida, USA). The experiments were made at a degassing temperature of 200°C for 6 h.
The optical properties were analyzed using a Shimadzu UV-3700 solid ultraviolet-visible-near-infrared spectrophotometer (Shimadzu, Kiya-cho, Nijiyanan, Japan) within the scanning range 200–800 nm at a resolution of 0.1 nm. BaSO4 was employed as a reference sample.
The spectroscopic analysis was performed on powder specimens using a Frontier Fourier-transform Mid/Far Infrared spectrometer (PerkinElmer, Waltham, Massachusetts, USA). The data were collected over the range 4000–400 cm–1.
The absorbance of RhB solution was analyzed by an Evolution 300 UV spectrophotometer (Shimadzu, Kiya-cho, Nijiyanan, Japan) using a CEL-HXF300F3 xenon lamp as the visible light source (300 W, 400–800 nm), purchased from Beijing China Education Au-Light Co., Ltd.
Results and Discussion
Thermal Performance
The differential thermal analysis curves of the M, CN-550°C, and CNM-550°C samples (Fig. 1) revealed an exothermic peak at 118°C associated with the evaporation exotherm of adsorbed water and exchangeable cation coordination water molecules on the surface and between the layers of Mnt (Ralph & Richards, Reference Ralph and Richards1942) The mass loss rate of M from 25 to 180°C was 13.6%. A similar result was obtained from the exothermic peak at 112°C in the DTA curve, caused by the removal of physisorbed water of Mnt in Ref. (Shmuel et al., Reference Shmuel, Müller-Vonmoos, Gunter and Anton1989).
The mass loss of CN-550°C from 25 to 600°C was 5.3%, which was due to the evaporation of water adsorbed on the surface and the release of volatile substances (Chang et al., Reference Cheng, Shi, Pu and Yan2013). This could indicate that the structure of g-C3N4 underwent no noticeable changes up to 600°C. However, the emergence of the exothermic peak at 735°C in the DTA curve was a testimony to the decomposition of g-C3N4 into NH3 and CO2 gases (Yan et al., Reference Yan, Li and Zou2009), thus meaning the deterioration of the g-C3N4 quality at temperatures above 600°C. Moreover, almost no loss occurred between 750 and 800°C, which could indicate the complete decomposition of g-C3N4 into gases had already occurred.
The mass loss of CNM-550°C from 25 to 600°C was 7%, greater than that of pure g-C3N4. The appropriate peak was associated with the evaporation of adsorbed water and the dehydroxylation reaction of Mnt in the composite. Meanwhile, unlike the CN-550°C sample, a further increase in the temperature to 750°C did not impact the quality of CNM-550°C because of the significantly improved CN thermal stability after compounding with M.
Phase Analysis
The XRD patterns of a series of samples at various calcination temperatures are shown in Fig. 2. Almost all the specimens, except that labeled as M, revealed the presence of (100) and (002) reflections at 12.97 and 27.53°2θ, respectively, which were associated with a g-C3N4 phase (Shi et al. Reference Shi, Jiang, Zhou, Wang, Gui, Hu and Yuen2013). No diffraction peaks of melamine were observed, meaning that the latter was decomposed and converted into g-C3N4. At 490°C, the full-width at half maximum (FWHM) of the (002) peak was larger and the peak intensity was weaker, indicating a decrease in the crystallinity of g-C3N4. With a further increase in the calcination temperature, the FWHM of (100) and (002) XRD peaks of g-C3N4 gradually decreased at rising peak intensities, suggesting the improvement in crystallinity of g-C3N4 with increasing calcination temperature. On the other hand, a lack of peaks from Mnt indicated that Mnt was stripped and dispersed in g-C3N4 without forming a periodic structure (Zhao & Tan, Reference Zhao and Tan2018). This meant that the g-C3N4/Mnt nanolayer composite had been prepared successfully.
Infrared Spectroscopy Data
The FTIR spectroscopic data in Fig. 3 reflected the chemical structure of the CNM-T samples. The peaks at 1640, 1569, 1326, 1412, and 1240 cm–1 corresponded to the tensile vibration modes of C=N and C-N heterocycles (Li, Y., et al., Reference Li, Zhan, Huang, Xu, Li, Zhang and Wu2014). Their intensities increased gradually with rising temperature, indicating the increase in the degree of crystallinity of g-C3N4. The broad band in the range 3000–3400 cm–1 was due to the N–H stretching vibration and the adsorption of H2O molecules (Li, C. et al., Reference Liang2016). A strong absorption peak at 805 cm–1 represented the out-of-plane bending vibration of the triazine ring (Liu et al., Reference Liu, Shi, Yu, Tai, Wang, Feng, Liu, Wen, Yuan and Hu2015).
Surface Topography
Scanning electron microscopy images of Mnt nanolayers, g-C3N4, and g-C3N4/Mnt composites at different calcination temperatures (Fig. 4) revealed that the Mnt was completely exfoliated into flakes with a thickness of 5–10 nm and their diameters were 2–5 um. (Fig. 4a). The thinner the flakes, the larger the specific surface area and the surface energy, causing the flakes to wrinkle and bulge. In turn, the adsorption performance of pollutants was also enhanced. However, a block structure of the pure g-C3N4, caused by the large surface energy, led to the formation of pure g-C3N4 agglomerates. Moreover, this was not conducive to the absorption of light, resulting in the poor photocatalytic performance of g-C3N4.
The Mnt nanolayers were covered evenly and densely by g-C3N4. (Fig. 4c-h). The CN-490 sample exhibited a multi-layered stacked structure with relatively loose interlayers. According to the XRD data, this was due to the low crystallinity of g-C3N4 at 490°C. Furthermore, the sample surface possessed a fine granular structure owing to the impurities that remained unconverted into g-C3N4.
As the calcination temperature increased from 520 to 550°C, the layered structure of the composite became clear, dense, and smooth, indicating that the crystallinity of g-C3N4 increased. However, the compact stacking of the lamellae led to a block structure of g-C3N4, thereby reducing the specific surface area and hindering the light absorption and the photocatalytic effect. At 580°C, the laminate structure of g-C3N4 became loose due to the fact that the heat generated by the high-temperature calcination destroyed the interlayer van der Waals forces between the g-C3N4 particles. As a result, the layers were separated from each other and the spacing between them increased, thereby leading to the enlargement of the specific surface area. An increase in the calcination temperature to 610°C caused a further composite peel off and the emergence of large voids on the sample surface, further increasing the specific surface area. At 650°C, the composite material exhibited a fluffy layered structure, in which flakes were peeled off into the 1–5-nm-thick layers. Moreover, the excessively high temperature caused distortion of the triazine rings in g-C3N4, which would affect the photocatalytic performance of CNM-650.
Specific Surface Area
The specific surface areas and the pore volumes of CNM-T, M, and CN-550°C specimens (Table 1) showed that these values increased gradually with increasing temperature. For example, the specific surface area and pore volume of CN-550°C were 5.61 m2/g and 0.008 cm3/g and those of CNM-550°C were 13.24 m2/g and 0.113 cm3/g, respectively. The maximum specific surface area and pore volume of CNM-610°C were 29.10 m2/g and 0.251 cm3/g, respectively. Therefore, more adsorption sites could be provided for pollutants, thereby increasing the reaction rates (Wang et al., Reference Wang, Zhao, Zhang, Chen, Yuan, Zhou and Huang2018).
Optical Absorption Performance
The UV-Vis diffuse reflectance spectra (DRS) and the Kubelka-Munk function curves of CNM-t composites at various temperatures (Fig. 5) exhibited pronounced absorption within the visible range of 460–530 nm, which was interpreted as the electronic transition in the g-C3N4 semiconductor (Shi et al., Reference Shi, Chang, Zhang, Hai, Yang, Wang and Ye2016). Furthermore, the light absorption edge of samples gradually redshifted (to higher wavelength) with rising calcination temperature from 484 nm for CNM-490°C to 523 nm for CNM-610°C, which was due to increased pore size of specimens. This could be explained in terms of multiple reflections induced by the coarse-pore structure under light irradiation, thus allowing the catalyst to absorb more light energy and, respectively, to generate more electron-hole pairs (Liang, Reference Liang2015).
In order to study further the optical properties of the composite materials, their band gap values (E g ) were calculated according to the Tauc approach (Tauc, Reference Tauc1974) and the Davis-Mott model (Davis & Mott, Reference Davis and Mott1970) as follows:
Here, A is the adsorption coefficient; h is Planck's constant; ν is the photon frequency; and E g represents the band gap energy; k is a constant; and n is related to the type of semiconductor.
In this respect, the band gaps of CNM-490°C, CNM-550°C, CNM-610°C, and CNM-650°C were as large as 2.56, 2.5, 2.37, and 2.4 eV, respectively, proving that the E g of CNM-t could be reduced effectively by increasing the calcination temperature.
Photocatalytic Performance
The photocatalytic degradation rates evaluated for CN-550°C, CN+MC, and CNM-550°C (Fig. 6) showed that the self-degradation rate of RhB solution, assessed through a blank experiment under visible light irradiation, was only 6.7%, indicating the light irradiation stability of RhB. After the dark reaction for 1 h, the absorption rate of the pure g-C3N4 was only 1%, but increased as soon as g-C3N4 was compounded with Mnt nanolayers. This was because of the electrostatic attraction that made the negatively charged Mnt adsorb a large number of positively charged RhB molecules (Al-Ghouti et al., Reference Al-Ghouti, Khraisheh, Ahmad and Allen2009). In turn, the degradation rate of CNM-550°C (97.8%) was higher than that of CN-550°C (81.5%) and CN+M (76.2%). This meant that the photocatalytic performance of composites was improved due to the loading of Mnt, causing the interaction between the g-C3N4 interface and the Mnt (Li, J. et al., Reference Li, Liu, Ma, Hu, Wan, Sun and Fan2016).
The photocatalytic degradation rate of CNM-490°C after a 2-h exposure to visible light irradiation was only 65% (Fig. 7). Such a low value was due to the poor crystallinity of g-C3N4. The best photocatalytic activity was found for CNM-610°C, the degradation rate of which also reached >95% after 60 min of visible light irradiation. The improvement of photocatalytic efficiency within the temperature range of 490 to 610°C seemed to be on account of a gradual increase in the crystallinity of the composite along with a small band gap of g-C3N4. The changes in both the crystallinity degree and the band gap width arose from a porous sheet structure of g-C3N4 with a loose surface after thermal peeling. The degradation rate of the composites under higher calcination temperatures, however, underwent a sharp drop, achieving a value of 68.2% only for the CNM-650°C sample. This could be explained by the onset of decomposition of g-C3N4 above a threshold temperature, where a partial collapse of g-C3N4 nanolayers resulted in a rapid decrease in the specific surface area and pore volume, thus reducing the degradation rate and photocatalytic performance. The excellent photocatalytic effect of the composite could be attributed to two phenomena. First, the introduction of Mnt layers improved the adsorption capacity of the composite material in relation to target pollutants, promoting the interplay of g-C3N4 with pollutants and thereby improving the degradation efficiency of the whole composite (Li et al., Reference Liang2017). Second, close contact between g-C3N4 and Mnt could be achieved, inducing a positive synergistic effect. As a result, the electrons generated by light excitation of g-C3N4 formed an electrostatic attraction between the negatively charged Mnt and the positively charged RhB molecules, which further promoted the electron exchange reaction.
Summary and Conclusions
A series of g-C3N4/Mnt nanolayers was prepared at various calcination temperatures, using melamine as the precursor and Mnt nanolayers as the matrix. The Mnt nanolayers in the composites were as thin as 5–10 nm and their diameters were 2–5 um. Their surface was uniformly and densely coated by g-C3N4 in a block-based manner, forming a multi-layered stacked structure.
An increase in temperature led to a gentle increase of the crystallinity of g-C3N4 along with a decrease in the band gap of the composite. Besides this, the light absorption edge gradually redshifted, and the multi-layered stack structure became dense. At 580°C, the interlayer van der Waals forces of g-C3N4 were destroyed by the thermal energy and the structure of the composite material began to loosen until it was peeled off.
The conclusion was, therefore, that the combination of Mnt nanolayers and g-C3N4 was beneficial to the transfer of photogenerated electrons, increasing the photocatalytic effect and the degradation rate of RhB of the composite material at temperatures up to 610°C. A further increase in temperature led to a structural collapse and decomposition of g-C3N4; as a result, the specific surface area and the pore volume of CNM-650°C decreased rapidly along with a reduction in the photocatalytic performance.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 42072048, 41772036).
Funding
Funding sources are as stated in the Acknowledgments.
Declarations
Conflict of interest
The authors declare that they have no conflict of interest.