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Article

Heterostructured S-TiO2/g-C3N4 Photocatalysts with High Visible Light Photocatalytic Activity

by
Yassine Alaya
1,
Bilel Chouchene
2,
Ghouti Medjahdi
3,
Lavinia Balan
4,
Noureddine Bouguila
1 and
Raphaël Schneider
2,*
1
Laboratoire de Physique des Matériaux et des Nanomatériaux Appliquée à l’Environnement, Faculté des Sciences de Gabès, Gabès University, Gabès 6072, Tunisia
2
Université de Lorraine, CNRS, LRGP, F-54000 Nancy, France
3
Université de Lorraine, CNRS, IJL, F-54000 Nancy, France
4
CEMHTI-UPR 3079 CNRS, Site Haute Température, 1D Avenue de la Recherche Scientifique, F-45071 Orléans, France
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(4), 226; https://doi.org/10.3390/catal14040226
Submission received: 4 March 2024 / Revised: 20 March 2024 / Accepted: 26 March 2024 / Published: 28 March 2024
(This article belongs to the Section Photocatalysis)
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Abstract

:
Novel heterojunctions associating graphitic carbon nitride g-C3N4 and S-doped TiO2 nanoparticles were successfully designed and prepared via a hydrothermal method and used for photocatalytic degradations. The loading in S-TiO2 nanoparticles on g-C3N4 was varied (5, 10 and 20 wt%), and the photocatalysts were characterized by XRD, FT-IR, solid-state UV-visible diffuse reflectance, photoluminescence, XPS, TEM and SEM. The S-TiO2 (5%)/g-C3N4 catalyst exhibits the highest activity for the photocatalytic degradation of the methylene blue (MB) dye under visible light irradiation. The high photocatalytic performance originates from the enhanced separation and transfer of photogenerated charge carriers. The S-TiO2 (5%)/g-C3N4 photocatalyst is stable and can be reused five times without a sharp drop in activity, indicating its high potential for wastewater remediation.

Graphical Abstract

1. Introduction

In recent years, various chemical, physical and biochemical processes including chlorination, Fenton reactions, coagulation, membrane separation or adsorption have been developed to treat wastewater and remove emerging pollutants like dyes and pigments, drugs, pesticides or heavy metals [1,2]. All these processes are not fully satisfactory because they require either powerful oxidants or only move pollutants from a liquid phase to a solid phase. Heterogeneous photocatalysis is an advanced oxidation process (AOP) and is considered as a valuable alternative to the treatments previously mentioned for wastewater treatment as it is environmentally friendly and requires only a light source and a catalyst for the degradation of harmful compounds present in water [3,4].
Upon light activation of a semiconductor photocatalyst, reactive oxygen species (ROS) like hydroxyl or superoxide radicals, singlet oxygen or hydrogen peroxide may be produced after oxidation of water molecules or reduction of molecular oxygen molecules adsorbed at the surface of the photocatalyst [4,5,6]. ROS exhibit strong oxidizing power, can fragment toxic organic molecules and, in the optimal conditions, mineralize them into CO2 and H2O. The choice of the photocatalyst and its ability to be photoactivated and produce ROS under illumination will therefore play a crucial role for the photodegradation of harmful compounds. One of the key points will be the inhibition of the charge carrier’s recombination to improve their diffusion to the surface of the photocatalyst. Graphitic carbon nitride (g-C3N4) is one of the most studied photocatalysts in recent years due to its easy synthesis, low cost and good photocatalytic performance [6,7,8,9,10].
g-C3N4 is an organic semiconductor easily prepared by thermal polymerization of N-rich precursors such as melamine, urea or dicyandiamide and exhibits high thermal and chemical stability [6,7,8,9,10]. Moreover, the energy bandgap of g-C3N4 is of ca. 2.7 eV, which allows an improved utilization of solar light compared to wide bandgap metal oxides such as ZnO or TiO2. However, the quantum efficiency of g-C3N4 is relatively modest due to the high recombination rate of photo-generated charge carriers. To overcome this limitation, many studies have focused on the construction of g-C3N4-based heterostructured photocatalysts [6,10,11,12,13]. The heterojunction can effectively decrease and even inhibit electron–hole pairs recombination thus enhancing their participation in reactions occurring at the surface of the photocatalyst (water oxidation and oxygen reduction to produce hydroxyl and superoxide radicals, respectively).
TiO2 is one of the most used photocatalysts due to its high photoreactivity, high stability, low toxicity and low cost, and has very often been coupled with g-C3N4 to construct a heterojunction due to the matched energy band between these semiconductors [14]. The engineered g-C3N4/TiO2 heterostructured photocatalysts were successfully used in various photocatalytic applications, such as pollutant degradation [15,16,17,18,19,20,21,22,23,24], organic synthesis [25], water splitting [15,26,27,28], CO2 reduction [28,29] and NOx oxidation [30].
The doping of TiO2 with S is an effective method to narrow the bandgap and enhance the visible light photoactivity because the S atom has a lower electronegativity and higher 3p orbital energy than that of the O atom. The S dopants can substitute either O as an anion or Ti as a cation which allows for the introduction of additional extrinsic electronic levels located in the energy band gap of TiO2 [31,32,33]. However, due to the high stability of TiO2 and to the larger ionic radius of S than that of O, the S-doping of TiO2 at an adequate concentration is usually difficult. To prepare S-doped TiO2, the layered TiS2 can be used as a precursor and oxidized in air at temperatures of 500–600 °C [34,35]. However, such high temperatures do not allow to maintain a high level of S doping in the TiO2 crystal lattice [34]. S-doped TiO2 can also be prepared via the hydrothermal oxidation of TiS2 under acidic conditions [36,37].
Only three recent reports describe the synthesis of S-doped TiO2 associated to g-C3N4. Biswal, Pany et al. prepared g-C3N4/S-doped TiO2 particles by co-pyrolysis at 500 °C of TiOSO4 and urea and further associated the composite to Ti3C2 to yield photocatalysts for water splitting [38,39]. Hossain et al. described the synthesis of S-doped TiO2 by treatment of TiCl4 with dye wastewater sludge followed by calcination at 600 °C. The produced S-doped TiO2 was further associated to g-C3N4 by annealing with melamine at 550 °C and the obtained photocatalysts used for NOx removal [40].
Herein, a mild hydrothermal process was developed for the preparation of anatase S-doped TiO2 (S-TiO2) containing ca. 11% S by hydrolysis of TiS2 at neutral pH. When this hydrolysis was conducted in the presence of g-C3N4, S-TiO2 nanoparticles with an average diameter of ca. 9.2 nm were found to be deposited on g-C3N4 sheets. The S-TiO2/g-C3N4 heterojunction exhibits high photocatalytic activity and stability for the degradation of the methylene blue (MB) dye under visible light irradiation. Based on optical and electronic data, the enhanced photocatalytic activity of the S-TiO2/g-C3N4 composite was demonstrated to originate from the enhanced visible light absorption and from the increased lifetime of photogenerated charge carriers.

2. Results

2.1. Synthesis and Characterization of S-TiO2/g-C3N4 Catalysts

TiS2 was prepared by reacting elemental S with TiCl4 in 1-octadecene (ODE) at 300 °C [41]. The synthesis of S-TiO2/g-C3N4 catalysts was performed by associating g-C3N4 with TiS2 in water under sonication followed by a hydrothermal treatment at 160 °C for 4 h (Figure 1). This hydrothermal treatment allows a partial hydrolysis of TiS2 into S-TiO2 nanoparticles which are deposited on the surface of g-C3N4 sheets.
The crystallographic structure of the prepared catalysts was determined by X-ray diffraction (XRD) (Figure 2a). The XRD pattern of S-TiO2 shows that all peaks correspond to the tetragonal crystalline structure of TiO2 (anatase form, JCPDS No 01-071-1167). For g-C3N4, the signal at 13.03° is associated with the (100) the interlayer structural packing, while the signal at 27.56° corresponds to the (002) interplanar staking of aromatic systems [6,10]. The intensity of the g-C3N4 (100) peak is markedly reduced after association with S-TiO2, indicating that the long-range order of the in-plane structural packing of g-C3N4 is altered likely due to the intralayer hydrogen bond breaking. A shift of the g-C3N4 (002) signal from 27.28° to 27.56° is also observed when increasing the loading in S-TiO2 from 0 to 20 wt%, suggesting a distortion of the g-C3N4 layered structure after association with S-TiO2 (Figure 2b). The interplanar distance decreases (0.3268, 0.3262, 0.3235 and 0.3230 nm for g-C3N4, S-TiO2 (5%)/g-C3N4, S-TiO2 (10%)/g-C3N4 and S-TiO2 (20%)/g-C3N4, respectively), indicating a more compact packing of g-C3N4 sheets which should improve the charge carrier separation. These results confirm the successful association of S-TiO2 with g-C3N4.
The chemical structure of S-TiO2 and S-TiO2/g-C3N4 samples was further investigated by Fourier transform infrared spectroscopy (FT-IR) (Figure 2c). For S-TiO2, the broad band located at 3500–3300 cm−1 originates from the O-H stretching vibration of chemisorbed water molecules. The O-H bending vibration can also be observed at 1630 cm−1. The signals observed at 2924 and 2856 cm−1 correspond to asymmetrical and symmetrical stretching vibrations of the C–H bonds, which suggests that ODE molecules used as solvent for the synthesis are is still adsorbed on the surface of S-TiO2 particles. The strong and broad band located in the 700–500 cm−1 range can be assigned to the Ti-O-Ti and Ti-S-Ti stretching vibrations [42]. For g-C3N4, the signals at 3346 and 3171 cm−1 correspond to the O-H stretching of adsorbed water molecules and to the N-H groups located at the periphery of g-C3N4 sheets, respectively. The stretching modes of C=N and C-N-C bonds in triazine heterocycles appear at 1621, 1545, 1456, 1398, 1314, and 1229 cm−1 while the sharp peak located at 803 cm−1 can be assigned to the out-of-plane breathing mode of triazine units in g-C3N4 [6,10]. All characteristic FT-IR signals of S-TiO2 and g-C3N4 can be observed in g-C3N4/S-TiO2 catalysts.
X-ray photoelectron spectroscopy (XPS) was further used to investigate the composition and the valence state of elements composing the S-TiO2/g-C3N4 photocatalysts using the catalyst loaded with 5 wt% S-TiO2 as representative. The XPS overview spectrum indicates the presence of C, N, O, S and Ti elements without impurity elements (Figure 3a). The N 1s signal can be deconvoluted into three peaks located at 398.62, 400.17 and 401.17 eV corresponding to bicoordinated N atoms (C=N=C bonds), bridging N atoms (N-(C)3 bonds) and peripheral N atoms (NH2 or NH bonds), respectively (Figure 3b) [6,10]. The signals observed at 404.50 eV is associated to the charging effects in heterocycles. The main signal observed at 288.81 eV in the HR-XPS spectrum of C 1s can be assigned to sp2-bonded carbon (N-C=N) bonds in aromatic units (Figure 3c) [6,10]. The signal observed at 284.81 eV originates from adventitious carbon. The weak signals observed at 286.31 eV and 288.61 eV correspond to carbon linked to one or two oxygen atoms, respectively, which suggests the presence of carbonyl or carboxyl groups on the edges of g-C3N4 sheets. The Ti 2p3/2 signal appears at 458.47 eV, value slightly lower than that of Ti(+4) in TiO2 (458.70 eV), which confirms that S atoms are incorporated into the TiO2 lattice which conducts to a shift to a lower binding energy (Figure 3d) [43]. The O 1s spectrum shows three signals at 529.91, 533.0 and 531.57 eV corresponding to O atoms in the TiO2 lattice [42] and to O atoms engaged in C-OH and C=O bonds, respectively, indicating that g-C3N4 sheets are functionalized by oxygenated groups (Figure 3e) [44]. The major signal observed for S 2p3/2 is located at 163.14 eV, indicating that S in the −2 oxidation state is present in S-TiO2 nanoparticles, which corresponds to Ti-S bonds in the TiO2 lattice (Figure 3f) [42]. The weaker signal observed at 166.49 eV corresponds to S in the +4 oxidation state and could be assigned to oxidized S atoms in the form of O=S=O bonds in the TiO2 lattice [45]. The XPS analysis also shows that TiO2 particles are doped by approximately 11% S, which allows to propose the TiO1.78S0.22 formula for these nanoparticles.
The microstructure and the morphology of g-C3N4, S-TiO2 and S-TiO2/g-C3N4 catalysts were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images of S-TiO2 (5%)/g-C3N4 catalyst show that g-C3N4 is composed of auto-assembled thin sheets with an average size of ca. 150 nm (Figure 4a,b). g-C3N4 assemblies are porous, which is conducive to catalytic activity. The SEM image in chemical contrast allows to distinguish g-C3N4 from S-TiO2 nanoparticles which appear as small aggregates deposited on the surface of g-C3N4 (Figure 4c). The SEM-associated energy dispersive X-ray (EDX) analysis confirms the presence of C, N, O, S and Ti elements (Figure 4d).
TEM images confirm the small size and the lamellar structure of g-C3N4 and the deposition of S-TiO2 nanoparticles with an average size of ca. 9.2 ± 1.8 nm on the surface of g-C3N4 sheets (Figure 5a–c). The interplanar distances of 0.18 nm (200) and 0.35 nm (101) correlate well with the anatase structure of S-TiO2 (Figure 5c). The elemental mapping confirms the presence of C, N, Ti, O and S elements and the presence of S-TiO2 clusters of nanoparticles dispersed on g-C3N4, which is beneficial for the charge transfer between S-TiO2 and g-C3N4 (Figure 5d–i). The chemical composition determined by the EDX analysis gives an atomic ratio of Ti/S of 8.44, indicating that the doping percentage of S in TiO2 is of 11.8 (Figure 5j). This value is close to the doping percentage of 11% determined by XPS.
Figure 6a shows the UV-visible diffuse reflectance spectra (DRS) of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 catalysts recorded in the range 250–1000 nm. After association of S-TiO2 with g-C3N4, the visible light absorption is increased compared to pure g-C3N4. The plots of (αhν)1/2 vs. photon energy (eV), where α, h and ν are the absorption coefficient, the Plank constant and the light frequency, respectively, were used to determine the bandgap energies S-TiO2, g-C3N4 and S-TiO2/g-C3N4 materials (Figure 6b). Pure S-TiO2 and g-C3N4 exhibit bandgap energies of 3.09 and 2.80 eV, respectively. The decreased bandgap of S-TiO2 compared to pure TiO2 (Eg of 3.2 eV for pure anatase TiO2) confirms the presence of S atoms in the crystal lattice, which agrees well with literature [46]. With the increase of the S-TiO2 loading in the photocatalysts, the bandgap gradually decreases from 2.80 to 2.75, 2.71, and 2.67 eV for g-C3N4 associated 5, 10 and 20% S-TiO2, respectively. The increased light absorption in the visible range of S-TiO2/g-C3N4 composites should be beneficial for the formation of charge carriers under visible light irradiation and thus increase the photocatalytic activity.
The separation efficiency of photogenerated charge carriers was examined through PL measurements that provide valuable information on the separation and the lifetime of photogenerated electron–hole pairs (Figure 7) [6,10]. After excitation at 350 nm, g-C3N4 exhibits a strong PL emission centered at 433 nm while S-TiO2 is non-fluorescent. The PL of S-TiO2/g-C3N4 catalysts decreases gradually with the increase of the S-TiO2 loading from 5 to 20 wt%. As the PL emission originates from the recombination of electron–hole pairs in g-C3N4, the decrease of the PL intensity observed after association with S-TiO2 nanoparticles is indicative of an enhancement of the charge carriers lifetime originating from the charge transfer between g-C3N4 and S-TiO2 nanoparticles.

2.2. Photocatalytic Activity

The photocatalytic performance of S-TiO2/g-C3N4 catalysts was evaluated in the degradation of the methylene blue (MB) dye under visible light irradiation (intensity of 30 mW/cm2) and at room temperature. MB was chosen as model dye as it is present in colored aqueous effluents from textile industries. MB is also a carcinogenic pollutant and its negative impact on both humans and aquatic life is well known. A control experiment showed that less than 4% of the dye was bleached in these experimental conditions without the photocatalyst. The adsorption–desorption equilibrium between the photocatalyst and MB molecules was reached by stirring in the dark for 60 min prior to irradiation. Figure 8a shows that the S-TiO2 (5%)/g-C3N4 composite exhibits the highest photodegradation rate of MB under visible light (quantitative photodegradation after 240 min irradiation). Further increase of S-TiO2 content in the photocatalyst (10 and 20 wt%) decreases the photocatalytic activity of the composite catalysts likely due to the blocking of g-C3N4 active sites by S-TiO2 nanoparticles deposited on their surface and/or to the enhanced photogenerated charges recombination. These photocatalytic results are in good accordance with PL measurements.
The obtained kinetic data could be fitted by a first-order kinetic model, ln(C0/C) = kt, where C0 is the initial concentration of MB, C is the MB concentration at time t and k is the apparent pseudo-first-order rate constant (Figure 8b). The apparent first-order rate constants k were determined from the slope of the ln(C0/C) vs. irradiation time curves and were found to be 0.0057, 0.0207, 0.0194 and 0.0107 min−1 for g-C3N4 and S-TiO2/g-C3N4 catalysts containing 5, 10 and 20 wt% S-TiO2, respectively, further confirming that the g-C3N4/S-TiO2 (5%) catalyst exhibits the highest activity.
The monitoring of the UV-visible absorption of the MB solution during its photocatalytic degradation using the S-TiO2 (5%)/g-C3N4 catalyst shows the decrease of the characteristic UV-visible absorption signal of MB at 664 nm (Figure 8c). The blue shift observed for the absorption peak of MB (from 664 to 625 nm) originates from the N-demethylation of the dye [47].
The stability and the reusability of the photocatalyst are crucial, especially for practical applications. The S-TiO2 (5%)/g-C3N4 catalyst was used for the photodegradation of MB in five cycles under visible light irradiation. After each run, the catalyst was recovered by centrifugation and reused without any treatment. As shown in Figure 8d, only a small decrease in photocatalytic performance is observed after each cycle (from 100 to 85% after the fifth cycle). The weak decrease in photocatalytic activity could be related to the adsorption of by-products from MB photodegradation as the catalyst was not washed or dried between two cycles. These results show not only that the catalyst is stable but also of interest for real environmental applications.
Figure 9a shows the influence of the catalyst loading (15, 30 or 45 mg of S-TiO2 (5%)/g-C3N4 in 30 mL of BM) was also investigated. With low catalyst load (15 mg in 30 mL of MB solution), MB photodegradation is slow (k = 0.0063 min−1) and incomplete after 240 min irradiation. The highest photodegradation rate was reached using 30 mg of catalyst in 30 mL of the MB solution (k = 0.0207 min−1). A decrease of the photodegradation rate is observed when the catalyst loading was increased to 45 mg in 30 mL of the MB solution likely due to light scattering and screening effects (k = 0.0129 min−1) [48].
The influence of the MB concentration (5, 10 or 20 mg/L) on the photodegradation rate was also studied (Figure 9b). Similar results were obtained for MB concentrations of 5 and 10 mg/L (k = 0.0267 and 0.0207 min−1 for concentrations of MB of 5 and 10 mg/L, respectively), indicating that the surface of the catalyst is not saturated by MB molecules adsorbed on its surface and that the production of reactive species responsible for MB photodegradation is not hindered. However, at a concentration of 20 mg/L, the degradation rate decreased markedly (k = 0.0019 min−1), suggesting that the catalyst surface is saturated by MB or by intermediates generated from MB photocatalytic degradation.
The surface charge of the photocatalyst has a very important role on the photocatalytic performance because it does not only influence the adsorption of pollutants as well as that of photodegradation intermediates on the catalyst surface but also the nature of the reactive species produced for the photodegradation [49]. Figure 10a shows the surface Zeta potential vs. pH of g-C3N4 and S-TiO2 (5%)/g-C3N4 catalysts. The pH of the point of charge (pzc) of g-C3N4 is 4.7, which agrees with previous reports [6,10]. A slight increase of the pzc to 5.6 is observed upon association with S-TiO2 particles as the pzc of TiO2 is of ca. 7.5 [50]. In this context, the influence of the pH of the MB solution on the photocatalytic degradation was also investigated (Figure 10b). The pH of the MB solution was adjusted from 2 to 11 using 0.1 M HCl or NaOH before the photocatalytic experiment. The adsorption of MB at the surface of the catalyst is weak (ca. 10–15%) at pH 2 and 4 due to the electrostatic repulsion between the catalyst and the dye, both positively charged. As the pH further increases, the adsorption is more marked (up to 85% at pH 11) because the charge of the catalyst becomes more and more negative. For pH values between 5 and 9, the photodegradation of MB requires approximately 180 min of irradiation to be complete. At pH 11, approximately 85% of the dye is adsorbed to the catalyst surface after 60 min in the dark and the remaining MB in solution is decomposed in 60 min. The FT-IR analysis of the photocatalyst after reaction shows that a high proportion of MB is not degraded but simply adsorbed. The k values are the highest for pH values ranging from 5 to 9 (k = 0.0207, 0.0239 and 0.0265 min−1 at pH 5, 7 and 9, respectively).
The photocatalytic performances of the S-TiO2 (5%)/g-C3N4 catalyst were compared to those of recently developed heterostructured TiO2/g-C3N4 photocatalysts for MB degradation (Table 1). Even if it is difficult to compare the performances because the quantities of catalyst, the concentration in MB or the irradiation conditions are not identical, the S-TiO2 (5%)/g-C3N4 catalyst is one of the most effective for the degradation of MB under visible irradiation.

2.3. Reaction Mechanism

To investigate the photocatalytic degradation mechanism, trapping experiments were conducted to identify the active species produced by the S-TiO2 (5%)/g-C3N4 catalyst at neutral pH. Tert-butanol (t-BuOH), DMSO, ammonium oxalate (AO), TEMPOL and NaN3 were used as hydroxyl ●OH radicals, electrons (e−), holes (h+), superoxide O2●− radicals and singlet oxygen 1O2 scavengers, respectively. The percentage of MB photodegradation was estimated by UV-visible spectroscopy after 240 min visible light irradiation. As shown in Figure 11a, the addition of DMSO or TEMPOL causes a decrease in photodegradation, indicating that electrons (e−) and superoxide O2●− radicals are involved in the photodegradation mechanism. Holes play the major role on the photodegradation as the largest decrease in photocatalytic activity is observed after addition of AO. Other scavengers have only a minor influence on photodegradation.
The band edge positions of S-TiO2 and g-C3N4 were determined using Equations (1) and (2):
EVB = χ − Ee + 0.5 Eg
ECB = EVB − Eg
where ECB and EVB are the edge potentials of the conduction band (CB) and of the valence band (VB), respectively, χ is the Mulliken absolute electronegativity of the semiconductor (χ (g-C3N4) = 4.72 eV and χ(S-TiO2) = 5.72 eV using the TiO1.78S0.22 formula determined by XPS) [62], Eg is the bandgap energy determined from UV-visible DRS, and Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV). The determined ECB and EVB values for g-C3N4 are −1.16 and 1.59 eV vs. NHE, respectively, and ECB and EVB values for S-TiO2 are −0.33 and 2.76 eV vs. NHE, respectively, which allows us to propose the energy diagram described in Figure 11b. The following Type II heterojunction mechanism for the S-TiO2 (5%)/g-C3N4 catalyst can be proposed. Both semiconductors are photoactive under visible irradiation. Electrons photogenerated in the conduction band of g-C3N4 can either react with adsorbed O2 molecules to produce O2●− radicals that were demonstrated to play an important role in the photocatalytic degradation or be transferred to the conduction band of S-TiO2 due to a mixed phases junction where the reduction of O2 into O2●− can also take place. Simultaneously, the holes generated in the valence band of S-TiO2 under light illumination are transferred to the valence band of g-C3N4. As holes play a key role in the photocatalytic mechanism, they likely oxidize MB into [MB]+. These mutual transfers of electrons and holes allowed by the construction of the Type II heterojunction reduce the probability of electron–hole pair recombination and favor the production of the reactive species responsible for the photocatalytic degradation of MB.

3. Materials and Methods

3.1. Reagents

Titanium tetrachloride TiCl4 (99.9%, Merck, Darmstadt, Germany), sulfur S (99.98%, Merck), 1-octadecene ODE (90%, Fischer, Rider Increis, Austria), melamine (99%, Fischer), cyanuric chloride (98%, Fischer), methylene blue MB (Merck), tert-butanol t-BuOH (>99%, Merck), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl TEMPOL (95%, Merck), ammonium oxalate OA (99.5%, Fischer), DMSO (99.9%, Fischer), sodium azide NaN3 (99%, Fischer), acetone (Carlo Erba, Cornaredo, Italy), methanol (VWR, Atlanta, GA, USA), ethanol (VWR) and toluene (Carlo Erba) were used as received.

3.2. Preparation of TiS2

TiS2 was prepared according to ref. [41] with slight modifications. Briefly, elemental sulfur (30 mmol) was added to 50 mL of ODE and the reaction flask was purged with argon and heated to 300 °C. Next, titanium tetrachloride (5 mmol) was injected and the reaction mixture was maintained at 300 °C for 30 min. After cooling to room temperature, the black precipitate produced was filtered and washed using acetone (30 mL) and a combination of methanol, toluene and acetone (2 × 30 mL) to remove any residual organic solvent and any by-products that may have formed during the reaction.

3.3. Synthesis of g-C3N4

Melamine (20 mmol) and cyanuric chloride (13.5 mmol) were solubilized in 20 mL of a THF/water (1/1) solution and the mixture was stirred at room temperature for 5 h until a white colored gel is obtained. The gel was dried in an oven at 60 °C for 12 h before being placed in a crucible with a lid and calcined for 1 h at 250 °C, 1 h at 350 °C, and 1 h at 450 °C to obtain bulk g-C3N4. After natural cooling, the powder was recovered and used without further treatment.

3.4. Preparation of S-TiO2/g-C3N4 Photocatalysts

The S-TiO2/g-C3N4 catalysts were prepared by impregnating g-C3N4 (250 mg) dispersed by sonication in 30 mL of water with TiS2 (5, 10, and 20 wt%). The mixture was first ultrasonicated for 30 min, transferred into a stainless-steel autoclave and heated at 160 °C for 4 h. Finally, after natural cooling to room temperature, the S-TiO2/g-C3N4 catalyst was washed with deionized water (4 × 20 mL) and dried at 70 °C for 15 h.

3.5. Photocatalytic Tests

The photodegradation of methylene blue (MB) used at a 10 mg/L concentration was investigated by using S-TiO2/g-C3N4 photocatalysts under visible light irradiation (irradiance of 30 mW/cm2). Photodegradations were conducted in a batch reactor at 20 °C. Typically, the S-TiO2/g-C3N4 catalyst (30 mg) was dispersed under sonication in 30 mL of the MB solution and the mixture was stirred in the dark in order to achieve adsorption-desorption equilibrium. After 60 min, the suspension was illuminated. Aliquots (1 mL) were taken from the solution at regular time intervals, centrifuged (4000 rpm for 5 min) to separate the catalyst and the concentration of MB in the supernatant was monitored by UV-visible absorption spectroscopy at 664 nm.
To identify the species involved in the photocatalytic degradation of MB, DMSO, t-BuOH, NaN3, TEMPOL and AO were used to scavenge electrons, hydroxyl ●OH radicals, singlet oxygen 1O2, superoxide O2●− radicals and holes, respectively. The experimental procedure is similar to that described above, except that scavengers were added at a 10 mM concentration.

3.6. Characterization

XRD analyses were performed on a Panalytical (Tokyo, Japan) X′Pert pro-MPD diffractometer with Cu Kα radiation in the 2θ range from 20° to 90°. The structure and the size of the photocatalysts were characterized by SEM (JEOL (Tokyo, Japan) JSM-6490 LV and JEOL JSM IT800 instruments coupled with EDS) and TEM (Philips (Amsterdam, The Netherlands) CM200 equipment). SAED images were obtained using the Philips CM200 equipment. The functional groups and the valence states of elements composing the S-TiO2/g-C3N4 catalysts were investigated by XPS (Gammadata (Uppsala, Sweden) Scienta SES 200-2 spectrometer) and by FT-IR (ALPHA Brucker (Billerica, MA, USA) Optik spectrometer). The zeta potential of the catalysts was determined using a Malvern (Malvern, UK) Zetasizer Nano ZS instrument. UV-visible absorption spectra in liquid phase were recorded on a Thermo Scientific (Waltham, MA, USA) Evolution 220 spectrometer. UV-visible DRS of the catalysts were recorded using a Shimadzu (Kyoto City, Japan) 2600–2700 spectrometer with BaSO4 as reference. The separation of charge carriers was investigated by PL using a Horiba (Kyoto City, Japan) Fluoromax-4 Jobin Yvon spectrofluorimeter.

4. Conclusions

In summary, a cheap and efficient hydrothermal method was developed for the preparation of new heterostructured photocatalysts associating S-TiO2 and g-C3N4. TEM and XPS analyses show that S-TiO2 particles with an average diameter of ca. 9.2 nm and doped with 11% S were deposited on g-C3N4 sheets. The photocatalytic performance was evaluated in the degradation of the MB dye under visible light irradiation. With the optimum loading of 5 wt% S-TiO2, the S-TiO2/g-C3N4 heterostructured photocatalyst exhibits high and stable photocatalytic activity. Scavenging experiments show that holes and superoxide radicals play a major in the photodegradation of dye and that the heterojunction between g-C3N4 and S-TiO2 is of Type II. The enhanced spatial separation of photo-generated electron–hole pairs account for the high photocatalytic activity of the S-TiO2 (5%)/g-C3N4 catalyst.

Author Contributions

Conceptualization, N.B. and R.S.; methodology, Y.A. and B.C.; validation, R.S.; formal analysis, Y.A. and B.C.; investigation, Y.A., B.C., G.M. and L.B.; data curation, Y.A., B.C., G.M. and L.B; writing—original draft preparation, Y.A. and B.C.; writing—review and editing, R.S.; supervision, N.B. and R.S.; project administration, R.S.; funding acquisition, R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors are grateful to the expertise and the facilities of the Platform MACLE-CVL, co-funded by the European Union and Centre-Val de Loire Region (FEDER). The authors also acknowledge thank Khalid Ferji (LCPM, Université de Lorraine) for Zeta potential analyses.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic representation of the synthesis of S-TiO2/g-C3N4 photocatalysts.
Figure 1. Schematic representation of the synthesis of S-TiO2/g-C3N4 photocatalysts.
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Catalysts 14 00226 g002
Figure 2. (a) XRD patterns, (b) magnification of the 22.5–32.5° zone and (c) FT-IR spectra of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 photocatalysts.
Figure 2. (a) XRD patterns, (b) magnification of the 22.5–32.5° zone and (c) FT-IR spectra of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 photocatalysts.
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Figure 3. (a) Overview XPS spectrum of the S-TiO2 (5%)/g-C3N4 sample and HR-XPS spectra of (b) N 1s, (c) C 1s, (d) Ti 2p, (e) O 1s and (f) S 2p.
Figure 3. (a) Overview XPS spectrum of the S-TiO2 (5%)/g-C3N4 sample and HR-XPS spectra of (b) N 1s, (c) C 1s, (d) Ti 2p, (e) O 1s and (f) S 2p.
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Figure 4. (a,b) SEM images of the S-TiO2 (5%)/g-C3N4 catalyst, (c) SEM image in chemical contrast and (d) the SEM-associated EDX analysis.
Figure 4. (a,b) SEM images of the S-TiO2 (5%)/g-C3N4 catalyst, (c) SEM image in chemical contrast and (d) the SEM-associated EDX analysis.
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Figure 5. (a,b) TEM and (c) HR-TEM images, (d–i) elemental mapping and (j) EDX analysis of the S-TiO2 (5%)/g-C3N4 material. The yellow circle corresponds to a cluster of S-TiO2 particles and each white circle corresponds to a S-TiO2 particle.
Figure 5. (a,b) TEM and (c) HR-TEM images, (d–i) elemental mapping and (j) EDX analysis of the S-TiO2 (5%)/g-C3N4 material. The yellow circle corresponds to a cluster of S-TiO2 particles and each white circle corresponds to a S-TiO2 particle.
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Figure 6. (a) UV-visible DRS of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 catalysts and (b) plots of [αhν]1/2 vs. photon energy to determine the energy bandgap. The tangent of the linear portion of the curve was used to determine the bandgap energy.
Figure 6. (a) UV-visible DRS of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 catalysts and (b) plots of [αhν]1/2 vs. photon energy to determine the energy bandgap. The tangent of the linear portion of the curve was used to determine the bandgap energy.
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Figure 7. Room temperature PL emission spectra of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 photocatalysts (λex = 350 nm).
Figure 7. Room temperature PL emission spectra of S-TiO2, g-C3N4 and S-TiO2/g-C3N4 photocatalysts (λex = 350 nm).
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Catalysts 14 00226 g008
Figure 8. (a) Photocatalytic performances of g-C3N4, and S-TiO2/g-C3N4 catalysts for the degradation of MB, (b) plots of ln(C0/C) vs. irradiation time, (c) UV-visible spectral changes of MB during its photodegradation by the S-TiO2 (5%)/g-C3N4 catalyst under visible light irradiation (intensity 30 mW/cm2). (d) Recycling runs of the S-TiO2 (5%)/g-C3N4 photocatalyst.
Figure 8. (a) Photocatalytic performances of g-C3N4, and S-TiO2/g-C3N4 catalysts for the degradation of MB, (b) plots of ln(C0/C) vs. irradiation time, (c) UV-visible spectral changes of MB during its photodegradation by the S-TiO2 (5%)/g-C3N4 catalyst under visible light irradiation (intensity 30 mW/cm2). (d) Recycling runs of the S-TiO2 (5%)/g-C3N4 photocatalyst.
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Figure 9. Influence of (a) the S-TiO2 (5%)/g-C3N4 catalyst loading on MB photodegradation, and (b) influence of MB concentration on the photodegradation rate.
Figure 9. Influence of (a) the S-TiO2 (5%)/g-C3N4 catalyst loading on MB photodegradation, and (b) influence of MB concentration on the photodegradation rate.
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Figure 10. (a) Zeta potential of g-C3N4 and S-TiO2 (5%)/g-C3N4 catalysts vs. pH. (b) Influence of pH on the photocatalytic degradation of MB using the S-TiO2 (5%)/g-C3N4 catalyst.
Figure 10. (a) Zeta potential of g-C3N4 and S-TiO2 (5%)/g-C3N4 catalysts vs. pH. (b) Influence of pH on the photocatalytic degradation of MB using the S-TiO2 (5%)/g-C3N4 catalyst.
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Figure 11. (a) Influence of scavengers of ●OH radicals, electrons, holes, O2●− radicals and 1O2 on the photocatalytic degradation of MB using the S-TiO2 (5%)/g-C3N4 catalyst under visible light irradiation. All scavengers were used at a 10 mM concentration. (b) Schematic representation of the band diagram of the g S-TiO2 (5%)/g-C3N4 photocatalyst, of the charge transfer and of the redox potentials of O2/O2●− and H2O/●OH couples.
Figure 11. (a) Influence of scavengers of ●OH radicals, electrons, holes, O2●− radicals and 1O2 on the photocatalytic degradation of MB using the S-TiO2 (5%)/g-C3N4 catalyst under visible light irradiation. All scavengers were used at a 10 mM concentration. (b) Schematic representation of the band diagram of the g S-TiO2 (5%)/g-C3N4 photocatalyst, of the charge transfer and of the redox potentials of O2/O2●− and H2O/●OH couples.
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Table 1. Photocatalytic performance of recently developed heterostructured catalysts used for the degradation of MB.
Table 1. Photocatalytic performance of recently developed heterostructured catalysts used for the degradation of MB.
PhotocatalystPhotocatalyst Amount
(g/L)		[MB]
(mg/L)		Light SourceRemoval
(%)		Reaction Time
(min)		Ref
g-C3N4/TiO2Immobilized catalyst10Visible light97.6240[51]
g-C3N4/TiO20.110Solar light illumination80180[52]
TiO2/g-C3N4Not provided5Simulated sunlight84.6120[53]
g-C3N4/TiO2-ZnO110Visible light62.4120[54]
Sm-doped TiO2/g-C3N411000Visible light91.8120[55]
TiO2/g-C3N40.210Visible light10070[56]
B-doped g-C3N4/TiO21Not providedVisible light81225[57]
g-C3N4/TiO20.230Simulated sunlight100120[58]
GO/g-C3N4/TiO20.110Visible light98.8240[59]
TiO2/g-C3N40.310Simulated sunlight10060[60]
g-C3N4/TiO20.420Visible light96.660[61]
S-doped TiO2/g-C3N4110Visible light100240This work
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MDPI and ACS Style

Alaya, Y.; Chouchene, B.; Medjahdi, G.; Balan, L.; Bouguila, N.; Schneider, R. Heterostructured S-TiO2/g-C3N4 Photocatalysts with High Visible Light Photocatalytic Activity. Catalysts 2024, 14, 226. https://doi.org/10.3390/catal14040226

AMA Style

Alaya Y, Chouchene B, Medjahdi G, Balan L, Bouguila N, Schneider R. Heterostructured S-TiO2/g-C3N4 Photocatalysts with High Visible Light Photocatalytic Activity. Catalysts. 2024; 14(4):226. https://doi.org/10.3390/catal14040226

Chicago/Turabian Style

Alaya, Yassine, Bilel Chouchene, Ghouti Medjahdi, Lavinia Balan, Noureddine Bouguila, and Raphaël Schneider. 2024. "Heterostructured S-TiO2/g-C3N4 Photocatalysts with High Visible Light Photocatalytic Activity" Catalysts 14, no. 4: 226. https://doi.org/10.3390/catal14040226

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