Djoko Hartanto*a,
Grace Yuhanekaab,
Wahyu Prasetyo Utomo
ac,
Ade Irma Rozafiaa,
Yuly Kusumawatia,
Wiwik Dahanid and
Ani Iryanie
aDepartment of Chemistry, Faculty of Science and Data Analytics, Institut Teknologi Sepuluh Nopember (ITS), Sukolilo, Surabaya 60111, Indonesia. E-mail: djokohar@its.ac.id
bStudy Program of Laboratory Testing Analysis, SMK Negeri 1 Driyorejo, Gresik 61177, Indonesia
cSchool of Energy and Environment, City University of Hong Kong, Kowloon 999077, Hong Kong SAR
dDepartment of Mining Engineering, Trisakti University, Jakarta, Indonesia
eDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Pakuan University, Bogor, Indonesia
First published on 16th February 2022
ZSM-5/graphitic carbon nitride (g-C3N4) composites were successfully prepared using a simple solvothermal method. By varying the amount of ZSM-5 and g-C3N4 in the composites, the charge carrier (electrons and holes) transfer within the materials, which contributes to the enhanced photocatalytic performance, was unraveled. The X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and scanning electron microscopy (SEM) analysis revealed that more ZSM-5 component leads to a stronger interaction with g-C3N4. The photocatalytic performance test toward methylene blue (MB) degradation shows that more ZSM-5 in the composites is beneficial in enhancing photocatalytic activity. Meanwhile, the impedance electron spectroscopy (EIS) and photoluminescence (PL) analysis revealed that ZSM-5 facilitates the charge carrier transfer of photogenerated electrons and holes from g-C3N4 to the catalyst surface due to its lower charge transfer resistance. During the charge carrier migration, the interface between g-C3N4 and ZSM-5 particles may induce higher resistance for the charge carrier transfer, however after passing through the interface from g-C3N4 to ZSM-5 particles, the charge carrier can be efficiently transferred to the surface, hence suppressing the charge carrier recombination.
Several semiconductors have been employed as photocatalyst materials for the photodegradation of organic pollutants. For example, WO3 was reported to be an efficient photocatalyst due to its tunable bandgap, unique band positions, and excellent electronic properties that promoted its stability and photoactivity.9 Generating heterojunction in WO3, such as in Bi2WO6/Fe2O3/WO3 S-scheme ternary system, will further enhance the performance toward bisphenol-A degradation due to the efficient charge carriers separation and its ability to maintain redox reaction.10 Another material such as Cu2−xSe microsheet was reported to be an efficient photocatalyst for rhodamine B degradation, showing a complete discoloration within 30 minutes due to the highly accessible active sites and wider light absorption ranges as a result of the narrower bandgap.11 In terms of rhodamine B degradation, recently MoS2/tourmaline composite was also reported to have enhanced performance due to the increased dispersion of MoS2 nanosheet on the tourmaline support, which exposed more active sites. Moreover, the strong electric field in the tourmaline surface suppressed the recombination of charge carriers.12
The aforementioned photocatalysts are based on metal catalysts. Nowadays, nonmetal photocatalysts were also obtained considerable attention due to their synthesis practicality and abundant source. Among nonmetal photocatalysts, graphitic carbon nitride (g-C3N4) has been considered as one of the promising materials for photocatalytic process13 because it has a narrow bandgap (∼2.7 eV), good chemical and thermal stability, nontoxicity, and relatively easy large-scale production with readily raw materials.14–16 Recently, g-C3N4 is also considered as an emerging material for biomedical applications, such as for biosensing, diagnosis, therapy, and antibacterial materials due to its good biocompatibility and unique fluorescence properties.17 However, pristine g-C3N4 still suffers from the rapid charge carriers (electrons and hole pairs) recombination. Hence, some modifications can be applied to suppress this limitation, such as by preparing porous g-C3N4 and g-C3N4-based composites. Porous g-C3N4 was reported to enhance photocatalytic activity due to the increased surface area and narrowed bandgap, which shortened the charge carriers transfer and improves the light absorption.18–20 Moreover, the preparation of g-C3N4-based composites can efficiently suppress the charge carriers recombination.21,22 Among supported materials, ZSM-5 is considered as an appropriate support for g-C3N4 due to its chemical, mechanical, and thermal stability, superior adsorption ability, large surface area, and unique pore structure. Furthermore, ZSM-5 has also been reported as suitable photocatalyst support due to its ability to delocalize the bandgap energy of the excited electrons and to decrease the recombination rate.23–25
Some researchers have utilized ZSM-5 as support material for g-C3N4. For example, Prakash et al. reported the use of ZSM-5/g-C3N4 as a photocatalyst for rhodamine-B and crystal violet degradation.25 Preparing ZSM-5/g-C3N4 consists of ZSM-5 and g-C3N4 with a mass ratio of 4:
1; it was observed that the ZSM-5 was decorated on the surface of g-C3N4 particles with strong interaction. Also, the charge recombination can be suppressed as indicated by the lower intensity in the photoluminescence (PL) spectra of ZSM-5/g-C3N4 compared to bare g-C3N4. The enhanced photocatalytic performance was attributed to the efficient separation of charge carriers and the formation of heterojunction between ZSM-5 and g-C3N4.25 Similar results were also reported by Pham et al. in ZSM-5 decorated O-doped g-C3N4.26 Despite the enhanced performance of ZSM-5 and g-C3N4 composites, the mechanism of charge carriers transfer within the composite's components and how the charge carrier recombination can be suppressed are still not well understood.
In this present study, we investigate the charge carrier transfer behavior within ZSM-5/g-C3N4 composites, which determines their photocatalytic performances. In this regard, three ZSM-5/g-C3N4 composites with significant differences of ZSM-5 and g-C3N4 composition were prepared to obtain noticeable differences of each composite. The photocatalytic test was performed toward methylene blue (MB) degradation as a reaction model. The study revealed that the photogenerated electrons and holes generated by g-C3N4 migrate to the catalyst surface facilitated by ZSM-5 particles due to their higher conductivity. The facilitated charge carriers transfer by ZSM-5 particles suppressed the charge carriers recombination, hence enhancing the catalytic performance of the ZSM-5/g-C3N4 composites compared to bare g-C3N4.
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The detection of reactive species was performed by trapping experiment using different scavengers, namely DMSO, methanol, IPA, and L-ascorbic acid as scavengers for electrons (e−), holes (h+), hydroxyl radicals (˙OH), superoxide radicals (˙O2−), respectively. The experiments were similar to the photodegradation experiments with addition of the corresponding scavenger.15
The FTIR analysis was then performed to further investigate the chemical composition of the GZ composite. As shown in FTIR spectra (Fig. 2), the GZ composites exhibit a mixed characteristic feature of g-C3N4 and ZSM-5 FTIR spectra. A broad peak around 3200 cm−1 can be assigned to the N–H bond stretching from amino groups (–NH2 or NH groups).30 The peaks at 1234, 1319, 1404, 1543, and 1627 cm−1 can be attributed to the vibration modes of the C–N stretching and C
N stretching in the aromatic ring.31 The peak at 810 cm−1 corresponds to the out-of-plane skeletal bending modes of the tri-s-triazine ring.16,32 In the FTIR spectra of ZSM-5, the peak at 450 cm−1 refers to the buckling vibration of T–O–T, where T can be Si or Al atoms. The peak at 546 cm−1 can be observed, attributing to the formation of the pentasil ring by the tetrahedral SiO4 and AlO4 units.33 The symmetric stretching of the Si–O–Si group can be indicated by the peak at 789 cm−1. The peak at 1049 cm−1 and 1218 cm−1 can be attributed to the symmetric stretching of the Si–O–T group from the intra-tetrahedral mode of the zeolite framework.34 Moreover, the slight shifting in some peaks, especially at wavenumber of 1234 cm−1, 1404 cm−1, and 1543 cm−1, can also be observed in the GZ-3 samples, which indicates the strong chemical interaction between ZSM-5 and g-C3N4 particles in GZ-3.25,26 This result is in good agreement with the XRD results.
The surface morphology of the composites was then observed using SEM.35 As shown in Fig. 3, the bare g-C3N4 displays a wrinkled-interplanar structure with sheet-like morphology. The rough surface of the sheet particles indicates the formation of pores due to the utilization of acetic acid as a pore-forming agent during g-C3N4 synthesis.19 Meanwhile, the ZSM-5 shows a cubic-like or prism-like structure with some rough surface due to the desilication and dealumination process. The SEM images of the composite materials exhibit mixed particles between the g-C3N4 and ZSM-5 particles. In particular, the GZ-1 sample shows the dominance of g-C3N4 particles decorated with ZSM-5 particles on the surface. Unfortunately, it seems that the contact area between the ZSM-5 and of g-C3N4 is still limited. The GZ-2 shows more ZSM-5 particles together with g-C3N4 particles. As expected, the GZ-3 samples were dominated by the ZSM-5 particles. In this case, the g-C3N4 particles were surrounded by many ZSM-5 particles, creating more contact area and strong interaction between g-C3N4 and ZSM-5 particles. The more contact area between g-C3N4 and ZSM-5 particles are favorable in terms of the photogenerated charge carrier transport from g-C3N4 to the surface of the catalyst to further contact with the reactant.25
The elemental composition of GZ composites was also confirmed using EDX analysis (Fig. 4 and Table 1).36 As presented in Fig. 4a and Table 1, all GZ samples composed of major elements such as carbon (C) and nitrogen (N) as representative elements for g-C3N4 as well as silicon (Si), aluminium (Al), and oxygen (O) as representative elements for ZSM-5. The amount of C and N elements decrease from GZ-1 to GZ-3 as the proportion of g-C3N4 decreases (Table 1), which is in a good agreement with the SEM analysis. The elemental mapping of GZ-3 (Fig. 4b–g) show overlapping elemental distribution between elements from g-C3N4 and ZSM-5, which suggest a uniform distribution between the two components.
Element | GZ-1 | GZ-2 | GZ-3 | |||
---|---|---|---|---|---|---|
Weight (%) | Atomic (%) | Weight (%) | Atomic (%) | Weight (%) | Atomic (%) | |
Carbon (C) | 15.28 | 20.22 | 8.02 | 11.91 | 3.92 | 6.10 |
Nitrogen (N) | 28.33 | 32.15 | 13.66 | 17.40 | 9.91 | 13.21 |
Silicon (Si) | 18.29 | 10.35 | 32.58 | 20.69 | 36.95 | 24.56 |
Aluminium (Al) | 1.39 | 0.82 | 2.20 | 1.45 | 2.73 | 1.89 |
Oxygen (O) | 36.71 | 36.46 | 43.54 | 48.54 | 46.48 | 54.24 |
The surface area measurement was carried out using the N2 adsorption/desorption method on bare g-C3N4 and ZSM-5. As shown in Fig. 5, bare g-C3N4 and ZSM-5 show typical type IV isotherms with hysteresis loops according to the Brunauer–Deming–Teller classification. The specific surface area for g-C3N4 and ZSM-5 are 13.97 m2 g−1 and 281.97 m2 g−1, respectively. As typical type IV isotherms, both samples possess mesopores structure, with pore radii being distributed around ∼15.7 nm and ∼3.6 nm for g-C3N4 and ZSM-5, respectively. Based on the surface area measurement, the ZSM-5 may increase the specific surface area of the GZ composites, where the more ZSM-5 content in the GZ composites, the higher the surface area.25,26 In this regard, the higher specific surface area and the larger mesopores in g-C3N4 makes the GZ composites promising candidates for MB photodegradation with higher efficiency due to the facilitated mass transport and enhanced multilight scattering.37,38
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Fig. 5 N2 adsorption/desorption isotherms of (a) g-C3N4 and (b) ZSM-5. Inset is the corresponding pore size distribution. |
EVB = X − E0 + 0.5Eg | (2) |
ECB = EVB − Eg | (3) |
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Fig. 6 (a) DRS spectra of bare g-C3N4, ZSM-5, GZ-1, GZ-2, and GZ-3 catalysts. (b) Tauc plot of bare g-C3N4. |
Prior to the photocatalytic degradation, the adsorption experiment using 50 mg samples was performed to reveal the adsorption capacity of the photocatalyst toward MB. In this regard, the adsorption capacity of the photocatalyst may differ due to the different proportions of g-C3N4 and ZSM-5. As presented in Fig. 7, the adsorption capacity of all samples increases rapidly within 30 min of adsorption time, followed by the relatively stable adsorption capacity up to 90 min. These results suggest that after 30 min, the adsorption process has reached equilibrium. The rapid adsorption process within 30 min was attributed to the presence of mesopores in the samples, which allows the diffusion of MB molecules into the pores.41 Moreover, the ZSM-5 seems to be the component, which determines the adsorption ability due to the porous nature. However, when high portion of ZSM-5 is combined with g-C3N4, as in GZ-3, the adsorption capacity was further enhanced, due to the combined effect between the porosity from ZSM-5 and the larger mesopores in g-C3N4. The lesser amount of ZSM-5 lead to the decrease on the adsorption capacity. Since the adsorption reached the equilibrium after 30 min, performing the stirring process for 1 h in dark before irradiating the reactor with UV light is sufficient to exclude the contribution of the adsorption process during the photocatalytic reaction. Hence, after 1 h of stirring in dark, the decrease of MB concentration under UV irradiation is contributed by the photocatalytic process.
Without light irradiation, the photocatalyst is not able to degrade the MB as presented by the degradation curve of MB using bare g-C3N4 in the dark, as a primary photoactive material (Fig. 8a). Under UV light illumination, all materials are able to degrade the MB, in which the photodegradation trends of bare g-C3N4, GZ-1, GZ-2, and GZ-3 samples, are very comparable. Moreover, the degradation rates are also comparable, as indicated by the rate constant values (Fig. 8b and Table 2). Even though the degradation rate of all samples shows a similar trend and values, it does not mean that there is no enhancement in the photocatalytic activity upon the compositing of ZSM-5 and g-C3N4. In fact, it shows an indirect evidence of the enhancement and indicates the determining component which responsible for catalytic activity. Considering the composition of the catalysts, in 50 mg of GZ composites, the portion of g-C3N4 and ZSM-5 in each sample is different. As tabulated in Table 2, in 50 mg of tested catalysts, the amount of g-C3N4 are 50 mg, 33.3 mg, 25.0 mg, and 16.7 mg (or equal to 100%, 66.6%, 50.0%, and 33.3%) for each bare g-C3N4, GZ-1, GZ-2, and GZ-3, respectively. The remaining components are ZSM-5. Based on this component composition, we know that even with the lower amount of g-C3N4 in GZ-1, GZ-2, and GZ-3, the exhibited photocatalytic performances are still comparable with the pure g-C3N4. These results indirectly indicate the enhancement of photocatalytic activity since the photocatalytic activity is generally decreases along with the significantly lower photocatalyst dose.42 Please note that the degradation experiments were started after the adsorption process reached equilibrium (after 1 hour of adsorption in the dark), hence the contribution of the adsorption process can be excluded.
Sample | Rate constant (k, min−1) | Component portion (%) | |
---|---|---|---|
g-C3N4 | ZSM-5 | ||
g-C3N4 | 0.0083 | 100.0 | 0.0 |
GZ-1 | 0.0092 | 66.7 | 33.3 |
GZ-2 | 0.0086 | 50.0 | 50.0 |
GZ-3 | 0.0073 | 33.3 | 66.7 |
To further verify the contribution of ZSM-5 and the bare g-C3N4 in the GZ composites, we performed the MB photodegradation process using GZ-2 as representative GZ samples. 50 mg of GZ-2 consists of 25 mg of g-C3N4 and 25 mg of ZSM-5. Thus, performing the MB degradation experiment using 25 mg of g-C3N4 and 25 mg of ZSM-5 can give information on each component contribution and how much the contribution is. As presented in Fig. 8c and d, the 25 mg bare g-C3N4 exhibits a slower MB degradation rate compared to the GZ-2. Moreover, the MB degradation over 25 mg of ZSM-5 is negligible. These results verify that the determining component for the photocatalytic activity is the g-C3N4. It is reasonable considering the nature of g-C3N4, which can generate electrons and holes upon light irradiation. In the photocatalytic process, the photogenerated electrons and holes are two species that initiate the photodegradation process via direct or indirect degradation by forming radicals from water (˙OH and ˙O2−). In this regard, the ZSM-5 can serve as the support for g-C3N4, where the supporting mechanism will be discussed in the following sections.
Unveiling that the g-C3N4 is the determining component in GZ composites, we then performed the MB photodegradation using the same amount of g-C3N4 component for each sample. In this regard, the mass of total photocatalyst is 50 mg, 75 mg, 100 mg, and 150 mg for bare g-C3N4, GZ-1, GZ-2, and GZ-3, since all samples will have 50 mg of g-C3N4. In this case, the influence of the ZSM-5 amount in the GZ-composites can be investigated. The portion of ZSM-5 increases in the following order: GZ-3 > GZ-2 > GZ-1. The MB photodegradation plots of all samples are depicted in Fig. 9a. GZ-2 and GZ-3 show enhancement of the MB degradation rate compared to GZ-1 and bare g-C3N4. Surprisingly, the GZ-1 shows a comparable MB degradation with bare g-C3N4. Based on these results, the addition of more ZSM-5 into g-C3N4 is favorable for enhancing photocatalytic performance. The low amount of ZSM-5 as the support seems to have a small effect on the catalytic performance. Among the three GZ composites, the GZ-3 shows the highest MB degradation rate. The same trends are also observed and become more obvious using the higher initial concentration of MB (i.e. 40 ppm) as presented in Fig. 9c and d. To verify these results, the control experiments were conducted (Fig. 10a and b). The control experiment performed over 150 mg GZ-3 in the dark shows a negligible MB concentration decrease. Since the GZ-3 contains 100 mg of ZSM-5, the control experiment using 100 mg of ZSM-5 under light illumination was also performed and the result shows significantly slower MB concentration changes compared to the GZ-3. All of these results confirm that the MB degradation is mainly attributed to the activity of GZ-3, due to the synergistic effect between ZSM-5 and g-C3N4.
Apart from the synergistic effect between the g-C3N4 and ZSM-5 in GZ-3, the enhanced photocatalytic performance can also be attributed to the higher specific surface area of GZ-3 due to its higher ZSM-5 content. The higher specific surface area and the abundant mesopores in GZ-3 both originated from ZSM-5 and g-C3N4 enhance the multilight scattering. In this case, the large mesopores, especially in g-C3N4, induce a multiple-reflection effect on incident light, which facilitates the light transfer onto inner surface of GZ-3. As a result, the light absorption is improved.18,37,38 In addition to the enhanced multilight scattering, the large mesopores also facilitates fast mass transport.43–47
Based on the aforementioned results, the reaction mechanism can be predicted. When the GZ-3 composite was irradiated by UV light, the electrons (e−) are excited from valence band (VB) to conduction band (CB) with immediately generated h+ in the VB (eqn (4)). The h+ reacts with water to produce ˙OH (eqn (5)).25 The ˙OH and e− species finally involves in MB degradation as presented in eqn (6) and (7).
GZ-3 + UV irradiation → eCB− and hVB+ | (4) |
hVB+ + H2O → ˙OH + H+ | (5) |
˙OH + MB → mineralized products | (6) |
eCB− + MB → mineralized products | (7) |
To further investigate the charge transfer and conductivity of the composites, the EIS analysis was performed. Fig. 11c shows the Nyquist plots of the composites samples with their fitted circuit. In general, the smaller semicircle in the high-frequency region indicates the smaller charge transfer resistance (RCT). In this regard, the Rs represents the series resistance, mainly from the solution impedance, while the RCT represents the charge transfer resistance within the material.28 As shown in Table 3, the Rs values are relatively comparable among the measured samples. Therefore, the resistance is dominantly affected by the RCT. It can be seen from Fig. 11c that all of the GZ composites generally have lower semicircles compared to the bare g-C3N4, suggesting the lower charge transfer resistance.52–54 It is worth noting that the ZSM-5 exhibits the smallest charge transfer resistance, which indicates the highest conductivity among the samples. Based on this observation, the presence of ZSM-5 in the GZ-composites can potentially facilitate the charge carrier transfer from g-C3N4. Possibly, the more conductive ZSM-5 facilitates the charge transfer from the bulk phase of g-C3N4 to the composite's surface.
Samples | Rs (ohm) | RCT (ohm) |
---|---|---|
g-C3N4 | 43.26 | 274![]() |
GZ-1 | 46.82 | 73![]() |
GZ-2 | 49.05 | 116![]() |
GZ-3 | 43.27 | 169![]() |
ZSM-5 | 52.63 | 40![]() |
The PL spectra were then performed to investigate the separation and recombination of the photoinduced excitons. In general, a lower PL peak intensity reflects lower electron–hole pair recombination.55 As shown in Fig. 11d, the PL emission peaks of bare g-C3N4 was observed at 473 nm and exhibits the highest emission intensity, suggesting the rapid recombination of the electron–hole pairs. Upon combination with ZSM-5, the intensity of the PL emission peaks decreases and exhibits a blue shift, which agrees with the UV-vis DRS spectra.56 The decrease of the intensity is the following order: g-C3N4 > GZ-1 > GZ-2 > GZ-3. The more ZSM-5 portion in the composites, the lower the intensity. The results suggest that the presence of ZSM-5 in the g-C3N4-based composites reduces the recombination of electron–hole pairs, which agrees with the previous report.25 Considering a more inhibition of electron–hole pair recombination along with a more portion of ZSM-5 in the composites, it seems that more ZSM-5 in the GZ composites facilitates the electrons and hole transfer to the surface. Specifically, when the electrons and holes are generated in g-C3N4, the electrons and holes can be directly migrated to the ZSM-5, hence promoting the contact between the charge carriers (electrons and holes) with the reactant molecules. Note that ZSM-5 also has a porous structure, which provides more surface area. Because of this, the recombination can be suppressed by the presence of more ZSM-5 in the composites.
The results from PL analysis seem to have a different trend from the EIS results. However, it can be rationalized based on the charge transfer migration within the g-C3N4 and ZSM-5 particles. Based on EIS analysis, the more ZSM-5 portion in the composites, the higher charge transfer resistance. In this regard, the resistance in the composites may be mainly contributed by the interface resistance between the g-C3N4 and ZSM-5 particles. That is why the GZ-3 shows the highest charge transfer resistance since it has more contact area or interface between the ZSM-5 and g-C3N4 particles. When the charge carriers migrate from the bulk g-C3N4, they can be highly resisted in the interface between g-C3N4 and ZSM-5 particles. However, once the electrons or holes reach the ZSM-5 particles, the charges carrier can be migrated efficiently to the surface and makes contact with reactants (Fig. 12). Note that the ZSM-5 shows the lowest charge transfer resistance (Fig. 11c). In other words, in the GZ composites, the g-C3N4 serves as the main photoactive material, which generates the electrons and holes, while the ZSM-5 facilitates the transfer of photogenerated charge carriers to the surface and optimizes the contact with the reactant by providing a larger surface area. Moreover, because of the facilitated charge transfer migration in the ZSM-5 particles, either volume recombination or surface recombination of electron and hole pairs can be significantly suppressed. Therefore, although GZ-3 has the highest charge transfer resistance than GZ-1 and GZ-2, it shows the lowest charge carriers recombination due to the significantly suppressed charge carrier pairs recombination.
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