SYNTHESIS OF HIGH-ACTIVITY C AEROGELS-BASED g-C
3N
4PHOTOCATALYSTS DERIVED BY THE POLYMERIZATION OF PF
X. LIa, Q. YUb,*, Y. TAOb
aSchool of Mechanical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
bDepartment of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
This paper details the synthesis of high-activity carbon (C) aerogels-based g-C3N4 catalysts for photocatalytic degradation under visible light. We seal and heat a mixture of phenol (P), ZnCl2 and g-C3N4 solvated by formaldehyde, obtaining the mixture of an intimate interaction between phenol-formaldehude (PF) and g-C3N4 as precursor of C aerogels-based g-C3N4. The post thermal treatment results in not only carbon-doped g-C3N4 which improve the π-conjugation system to profit the separation of photo-induced carrier, but also obtaining C aerogels which possess high surface area. The resulted C aerogels-based g-C3N4 catalysts shown high adsorption capacity (78 mg/g), broadened light absorption, improved photoelectrochemical performance and higher photocatalytic properties compared with pristine g-C3N4.
(Received February 23, 2018;Accepted June 8, 2018)
Keywords: C aerogels-based g-C3N4, Phenol-formaldehude, Carbon doping, Surface area, Photocatalytic properties
1. Introduction
Graphitic carbon nitride (g-C3N4), as a organic semiconductor, has attracted considerable attention in photocatalytic degradation because of its suitable band structure, nontoxicity, abundance and highly physicochemical stability.[1,2] However, its development has disadvantages such as faster recombination of the electron−hole pairs, poor surface area, low visible light absorption, and seriously agglomerated in most solvents caused by the strong van der Waals attractions between sp2 carbon atoms.[3,4]
To overcome these short comings, previous researchers have employed strategies such as doping, nanostructure engineering, coupling with other materials, and introducing electrostatic repulsion among adjacent layers to achieve stable dispersion.[5-7] Yet most studies only consider one aspect of disadvantages, and ignore the other aspects. For example, Bu et al.[8] prepare uniformly dispersed g-C3N4 solution by treating it with oxygen plasma. This method on the one hand enhance the effective interaction between photocatalysts and target molecules, while on the other hand narrow light absorption, enhance the difficulty of recycling. Therefore, there is still an urgent call for a novel and efficient method to interconnect various aspects of performance.
As catalyst supports, porous carbon aergoles have drawn highly attention due to its large surface area, easy recycle, superior adsorption efficiency, and fast material transfer through broad interpenetrated channels.[9,10] But beyond that, carbon-based materials, using as coupling in photocatalysts, can also enhance the photoelectrochemical properties and conductivity of catalysts.[11] It is therefore desirable to develop photocatalysts that combine carbon aergoles with g-C3N4 resulting in the superior combination property, such as improving adsorption capacity, suppressing recombination of photogenerated charges, easy recycle, and high carbon-based isolation to prevent agglomerated of g-C3N4 in solvents.
In this work, the C aerogels-based g-C3N4 photocatalysts were prepaired based on Fig. 1.
The raw materials for C aerogels are phenol and formaldehyde. Firstly, formaldehyde segregates the stacked g-C3N4, obtained solvated g-C3N4 by forming hydrogen bonds. After adding phenol and
*Corresponding author: [email protected]
ZnCl2, the polymerization occurs in Teflon-lined autoclave. The obtained sample is the mixture of an intimate interaction between phenol-formaldehyde (PF) and g-C3N4. In addition the g-C3N4
insulated by PF in mixture, which prevent the agglomerated of g-C3N4. Then, the mixture was heated under flowing nitrogen atmosphere at 600℃ for 1.5h. In the formed C aerogels-based g-C3N4
photocatalysts, there are two processes. One, the C aerogels produced from PF annealing using ZnCl2 as foaming agent and porogen,[12] which can improve the adsorption capacity of photocatalysts. Second, the g-C3N4 may be incorporated with aromatic heterocycles in C by annealing, because of the π-π interaction between PF and g-C3N4.
Fig. 1. The process of the C aerogels-based g-C3N4 photocatalyst.
The PF-derived carbon (C) atoms repair the nitrogen (N) vacancies in g-C3N4. The C atom in triazine units and the attaching aromatic heterocycles may extend the delocalization of electrons in the π-conjugation system, promoting charge separation and transportation.[13] After purifying in 1 M HCl, the C aerogels-based g-C3N4 possesses a high surface area (800 m2 g-1). Thus, the obtained photocatalysts may be exhibit high photocatalytic activity because of bifunctional effect owing to C aerogels and higher dispersion g-C3N4 in C-based, such as extending π-conjugation, large surface area, high transmission of photogenerated charges, and preventing agglomerated of g-C3N4 in solvents.
2. Experimental
2.1. Synthesis of C aerogels-based g-C3N4 catalysts
The g-C3N4 is prepared by heating dicyandiamide at 550℃ for 4h.[14] After that, we mixed 1.5 mL formaldehyde and 0.3 g g-C3N4 by ultrasonic to obtain solvated g-C3N4 by forming hydrogen bonds. Then, the 0.491 g phenol and 6 g zinc chloride (ZnCl2) were added to yield viscous sol. The sol was transferred to sealed autoclave, followed by heating at 160°C for 8 h. The geted mixture of phenol-formaldehude (PF) and g-C3N4 were dried at 100℃ and heated to 600℃ at a rate of 2.5 ºC min-1. After keeping this temperature for 1.5 h, the C aerogel-based g-C3N4 was purified in 1M HCl several times, and then filtered and dried. The obtained sample is signed as C aerogels-based g-C3N4(0.3). In order to compare the effect of the concentration in g-C3N4, variable quantity g-C3N4
(0.1 g and 0.2 g) were added, replacing 0.3 g g-C3N4 in reaction, while the other process is unchanged. The obtained samples is denoted as C aerogels-based g-C3N4(0.1) and C aerogels-based g-C3N4(0.2), respectively. In addition, we have calcined g-C3N4 again at 600℃ for 1.5 h. The sample is signed as g-C3N4(a).
2.2. Characterization
A UV–vis spectrophotometer (Varian CARY 100, USA) is use to characterize the UV–vis diffuse reflection spectra. The structure of the samples was measured using X-ray diffraction (XRD Rigaku RINT2000 diffractometer), Fourier transform spectrophotometer (FTIR), and X-ray photoelectron spectra (XPS Thermo ESCALAB 250). The morphology of the samples was characterized by field-emission scanning electron microscopy (SEM). The Q2000 thermogravimetric analyzer was used as thermo gravimetric analysis (TGA) in argon gas. The CHI 660E electrochemical workstation was using as electrochemical measurements by three electrode
cells.
2.3. Adsorption measurements
Methylene blue (MB) solutions of 31 and 62 μM concentrations were obtained in aqueous medium. 3 mg of sample was put in MB solutions and stayed for over 40 min to saturation. The adsorption capacity, qe (mg g-1), can be calculated according to the equation (1);
0 t
e
c c
q V
m
(1) where C0 is MB concentration (mg L-1) at the start, and Ct is the concentration after contact time t (min). V and m correspond to the solution volume and the amount of adsorbent added.
The photocatalytic activity
The photocatalytic ability of the samples were tested by photodegradation of MB under visible light irradiation (>420 nm). In detail, 3 mg sample was put in MB (62 μM) aqueous solution.
After saturated adsorption, UV-vis spectrophotometer (664 nm) is use to examine the degradation of MB solution.
3. Results and discussion
Fig. 2 displays the FTIR spectra of the samples. Compared with the C specimens (Fig. 2-2), the C aerogels-based g-C3N4 showed additional peak at 805 cm-1 (Fig. 2-1), which equivalent of the stretching vibration mode of the tri-s-triazine units in g-C3N4 (Fig. 2-3). The band at 1637 cm-1 equivalent of the breathing mode of C=C (Figure 2-2), become broad band at 1637-1588 cm-1 (Fig.
2-1). This FTIR analysis confirmed not only the existence of g-C3N4 in C aerogels-based g-C3N4, but also an interaction between C aerogels and g-C3N4.
Fig. 2. FTIR spectra of (1) C aerogels-based g-C3N4, (2) C and (3) g-C3N4.
XPS analysis of g-C3N4 and C aerogels-based g-C3N4 is displayed in Fig. 3. From Fig. 3a and Fig. 3c, the constituent of C 1s spectra is 284.7 eV, 285.2 eV, 286.4 eV and 288.5 eV for graphitic C=C or the cyano-group, adventitious carbon, C-NH2 species and SP2-hybridized carbon, repectively. The high-resolution N 1s spectra (Fig. 3b and 3d) and the constituent peaks are located at 399.4 and 400.9 eV for N atoms in N-(C)3 or sp2-bonded N in triazine system and the end of amino groups, respectively.[15,16] The peaks intensity of 284.7 eV and 285.2 eV in C 1s spectra (Fig. 3c) are increased comparing with Fig. 3a, while the peak at 397.7 eV (N 1s Fig. 3d) is decreased in C aerogels-based g-C3N4 sample comparing with Fig. 3b. In addition, the C/N ratio estimated from the XPS analysis implied an increase from 1.4 for g-C3N4 to 45.9 for C aerogels-based g-C3N4. The results indicated the deficiency of N in C aerogels-based g-C3N4
prompted by calcinations process and subsequent in situ doping of carbon from the residue of phenol-formaldehyde resin (PF) burned out.[17]
Fig. 3. XPS patterns (a) C 1s and (b) N 1s of g-C3N4 and (c) C 1s and (d) N 1s of C aerogels-based g-C3N4.
We further examined the doping of carbon in C aerogels-based g-C3N4 by the degradation curve and XRD analysis. From Fig. 4a, both showed similar XRD curve for the g-C3N4 and g-C3N4(a). The stacking peak of conjugated aromatic systems is exhibited at around 27.4°. It conform the g-C3N4 calcined again at 600℃ do not change the stacking structure. However, the C aerogels-based g-C3N4, obtained by calcinating the mixture of PF and g-C3N4 at 600℃, have similar XRD curve with C (Fig. 4b). It has not the XRD peak (002) at around 27.4°. The reason may be the structural damage in g-C3N4 causing by doping of carbon by calcination of the PF, which act as in turn the crosslinking of PF, improved crosslinking density.
Fig. 4. XRD spectra of (a) g-C3N4 (1) and g-C3N4 (a); (b) C (1) and C aerogels-based g-C3N4 (2).
From Fig. 5a, both of the TGA followed similar weight loss pattern for PF, g-C3N4 and the mixture of PF and g-C3N4. However, the mixture of PF and g-C3N4 fabricated by polymerization
(Fig. 5a-1), shows a slow weight loss comparing with PF and g-C3N4 (Fig. 5a-2 and 5a-3), implying thermally more stable. The DTA curve showed a sharp peak at 655℃, 679℃ and 526℃ for the mixture of PF and g-C3N4, g-C3N4 and PF, respectively (Fig. 5b). The decomposition for the mixture of PF and g-C3N4 occurs at higher temperature compared with PF, implying the superior thermal stability for the main chains of PF.[18] Thus it can be interpreted that the residue of PF burn out improve in situ doping of carbon in C aerogels-based g-C3N4, which cause more crosslinking in interchain of PF，hence an increase in stability. This is consistent with the above XRD analysis.
Fig. 5. TGA (a) and DTG (b) curves of the mixture of PF and g-C3N4 (1), g-C3N4 (2) and PF (3).
In fact, the replacement of bridging nitrogen by carbon due to doping, can create large delocalized π bonds, which enhances the electrical conductivity and impedes the electron hole recombinations. All of those may provide novel optoelectronic properties. The UV-vis spectra of the sample is shown in Fig. 6a, the C aerogels-based g-C3N4 reveals broad light absorption intensity in the whole regions comparing with g-C3N4, which is a typical behavior of carbon-based materials.[19]
It is attributed to narrower gap of the sp2 carbon cluster embedded in C, which possess the superior light absorption in the entire wavelength.[20]
The photoelectrochemical properties are investigated by I-V curves (Fig. 6b). The photocurrent density of C aerogels-based g-C3N4 shows higher at potentials from -2 to 2 V than that of pristine g-C3N4. These results illustrate that the unique structure increase the contact area, which shorten the diffusion distance of electron shuttling between reactive sites, suppresses recombination ration of photoinduce electron-hole. In addition, the C in C aerogels-based g-C3N4 increases the lightharvesting capability, which enhancement the ability of photogenerated electron-hole pairs.
Fig. 6. UV-vis spectra (a) and I-V curves of C aerogels-based g-C3N4 (1) and g-C3N4 (2).
Fig. 7 display SEM images of g-C3N4, C and C aerogels-based g-C3N4. From Fig. 7a, g-C3N4 consisted of the stacking sheets. And the carbon obtained by calcinations of PF, is composed of adhesion spheres (Fig. 7b). After calcination of the mixture of PF and g-C3N4 at 600℃, the
morphology of obtained C aerogels-based g-C3N4 is different from g-C3N4 and C, which consist of separate spheres coated by bulk g-C3N4 (Fig. 7c). The g-C3N4 in C aerogels-based g-C3N4 is smaller than the original g-C3N4, indicating that annealing on the PF that adhered to the g-C3N4 may have modified polymeric melon units to reduce substantially the size[16].
Fig. 7. SEM of g-C3N4 (a), C (b) and C aerogels-based g-C3N4 (c).
In addition, this structure of separate spheres can increase the surface area of the C aerogels-based g-C3N4 photocatalyst, which probably improve the adsorption capacity. It is investigated by estimating the samples in the MB concentrations changing from 31 to 62 μM (Fig.
8a). The adsorption capacity increases when the MB concentration increases both of the two samples. However, no matter what the MB concentration change, the adsorption capacity of C aerogels-based g-C3N4 is higher than pristine g-C3N4. It is a universal phenomenon that the π-conjugate in samples induce the adsoption of π-conjugated carbon clusters in MB. But, the enhanced adsorption capacity in C aerogels-based g-C3N4 may be caused by the enhanced π-conjugate produced by the carbon doping in g-C3N4. The C atoms take place of the bridging N atoms. Then, enlarging delocalized π bonds are induced among the substituted C and the hexatomic rings, improving the adsorption capacity in C aerogels-based g-C3N4.[21] In addition, it is important that C aerogels produce high surface area.
Fig. 8. (a) Adsorption capacity of samples; (b) Pseudo second order kinetics of C aerogels-based g-C3N4.
The rate of adsorption followed pseudo-second order kinetics where the linear form is represented as Eq. 2.
2 2
1 1
t e e
t t
q q K q
(2)where, qt (mg g-1) is the amount of MB adsorption at the contact time of t on per unit mass of C aerogels-based g-C3N4. K2 is the pseudo-second order rate constants. The typical pseudo-second
order kinetics plot is showed in Fig. 8b. The details of pseudo-second order kinetics are summarized in Table 1. The value of regression correlation coefficient (R2) is close to 1. The experimentally determined value of qe is comparable with the estimated qe value calculated by this model. So the pseudo-second order kinetics is suited for the adsorption of MB on the C aerogels-based g-C3N4. This result supply the higher adsorption capacity in C aerogels-based g-C3N4 (78.8 mg g-1) can be due to enhanced π-π interaction produced by the carbon doping in g-C3N4 and high surface area owing to C aerogels. The enhanced adsorption is an important factor for photocatalytic efficiency.
Table 1. Summary of the parameters of pseudo second order kinetics.
concentration kinetics
31.3 μM Pseudo-second order
r2 0.99609
K2 0.0087
qe (calculated) 80
qe (experimental) 78.8
Fig. 9. Photocatalytic degradation of g-C3N4 (1), C aerogels-based g-C3N4(0.1) (2), C aerogels-based g-C3N4(0.2) (3), C aerogels-based g-C3N4(0.3) (4).
The photocatalytic experiments of the as-synthesized sample were carried out under visible light irradiation using MB as a model pollutant. From Fig. 9, the C aerogels-based g-C3N4 (0.3) shows higher photocatalytic degradation in comparison with pristine g-C3N4, due to not only small amount doped carbon and high surface area caused by C aerogels, but also preventing agglomerated of g-C3N4 in solvents. However, the photocatalytic degradation decrease when the concentration of C increase without limit. Especially, the C aerogels-based g-C3N4(0.1) shows lower photocatalytic degradation comparing with pristine g-C3N4. The reason may be too few concentration of g-C3N4.
4. Conclusions
In conclusion, we developed a facile route to prepare C aerogels-based g-C3N4, which possess suppressing recombination of photogenerated charges due to enhancing π-conjugation system, improving adsorption capacity owing to C aerogels structure, enhancing the effective interaction between photocatalysts and target molecules caused by separation of g-C3N4 in C aerogels. The process in preparing the mixture of PF and g-C3N4 play the key role in doped carbon process, in which formaldehyde molecules were anchored on g-C3N4 to segregate the sheet and flow polymerization to attach PF with g-C3N4 by π-conjugation. In annealing treatment, on the one hand
PF-derived carbon (C) atoms repair the nitrogen (N) vacancies in g-C3N4, while on the other ZnCl2
adding in process of preparing mixture of PF and g-C3N4 was used as foaming agentand porogen, and crucial to directly obtain the C aerogels.
The use of the devised method not only increase the π-conjugation system and the surface area of C aerogels-based g-C3N4, but also effectively separate g-C3N4 in C aerogels, which facilitated charge separation, higher adsorption capacity, and available material contact. The catalyst show high photocatalytic degradation. This finding exhibits the high potential of extending the π-conjugation systems and improving surface areas by PF annealing, and offer an effective path for the preparation of highly active photocatalysis.
This work was supported by the National Nature Science Foundation of China (21401001), Postdoctoral Science Foundation of China (2015M571913), Anhui Province Natural Science Foundaion (160808085MB25) and Postdoctoral Fund in Anhui (2016B090 and 2016B108)
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