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Photocatalytic degradation of tetracycline in aqueous systems under visible light irridiation using needle-like SnO2 nanoparticles anchored on exfoliated g-C3N4

Abstract

Background

Pharmaceuticals is one of the groups of contaminants of emerging concern that are resistant to decomposition or removal by most of the existing water and wastewater treatment procedures, hence the need to develop techniques to facilitate the removals of this group of organic contaminants from water systems. In this study, needle-like SnO2 nanoparticles was synthesised and loaded on exfoliated g-C3N4 nanosheet through a hydrothermal method, for use as sensitive visible light induce-photocatalyst for the decomposition of tetracycline in aqueous systems. The synthesised composites was characterized and analysed for the nature of the heterojunction between the SnO2 nanoparticle and g-C3N4 nanosheet using microscopic and spectroscopic techniques.

Results

The composites were of improved surface properties and enhanced visible-light absorption. The synthesised SnO2/g-C3N4 nanocomposites with various amounts of SnO2 (10–50 mg), employed in the degradation of tetracycline under visible light irradiation, were of good degradation efficiency. The degradation efficiencies of tetracycline by 1 wt.%, 2 wt.%, 3 wt.% and 5 wt.% SnO2/g-C3N4 photocatalyst were 81.54%, 90.57%, 95.90% and 92.15% as compared to g-C3N4 and SnO2 with 40.92% and 51.32% degradation efficiencies. The synergistic interaction between the needle-like SnO2 and exfoliated g-C3N4 nanosheet promoted the separation of photogenerated electron holes pairs, which enhanced their migration rate between SnO2 and g-C3N4 heterojunction, thereby facilitating the degradation of tetracycline. The ·O2 was noted to be the major reactive species in the photocatalytic of the 3 wt.% SnO2/g-C3N4 nanocomposite.

Conclusion

The fabricated SnO2 nanoparticles anchored on exfoliated g-C3N4 showed good performance for the decomposition of tetracycline in water, with possible application on other pharmaceuticals having same moiety (similar chemical structures).

Introduction

Low concentrations, ranging from ng/L to µg/L of over 3000 chemicals substances, including pharmaceutical compounds (PCs), have been detected in wastewater, groundwater, surface water, soil and even in drinking water world over, for over a decade now [1,2,3]. The occurrence of these PCs in water bodies is due to outfalls from their global usage as therapeutic and prophylactic medicines and nutrient supplements in humans, livestocks, and agricultural health management [4,5,6]. The growth in population, urbanization and lifestyle places pressure on PCs demand/supply shift dynamics, resulting in increase, in the production and consumption of tonnes of different pharmaceuticals in recent times.

Small to significant proprtion of pharmaceuticals administered/consumed by humans and animals are excreted from their bodies in metabolized/un-metabolized forms, and discharged via domestic wastewater into sewers and other drainage systems, from where they are transferred/translocated into freshwater surface systems and other environmental compartments. Industrial wastewater release, as well as indiscriminate disposal of unused and expired medicinal products, contributes to the total load of chemical contaminants in different environmental media [7, 8]. The environmental stability of these pharmaceutical substances (materials) coupled with the continuous unabated and uncontrollable environmental reception, has resulted in growing concerns about the potential adverse effects these compounds will have on the ecosystem and human health [9, 10].

The removal of pharmaceuticals from wastewater and other aqueous matrices, especially drinking water, is of intense focus. Unfortunately, most of the existing conventional potable water (PW) and wastewater treatment plant (WWTP) technologies encompassing biological, chemical, and adsorption processes are not designed to resolve/degrade residues of pharmaceutical compounds such as tetracycline and other antibiotics during the treatment process, prior to the release of treated water use into the environment [11]. The inability of WWTPs to eliminate many pharmaceuticals and chemical contaminants calls for the need for the development of new technologies and process designs for the degradation of these substances during water purification/treatment.

One of the possible technologies gaining traction for the resolution of pharmaceuticals and other organic contaminants in aqueous systems is the advanced oxidation procedures (AOP). AOP processes, which relies on the production of highly reactive free radicals, have proven to be a potential efficient means through which pharmaceuticals can be degraded [12, 13]. Advance oxidation procedures have been adapted in different forms ranging from ozonolysis [14], Fenton methods [15, 16], UV–Vis photo-catalysed [17], and many others. Most of these AOP adapted techniques are simple in operation; possess high catalytic activity with no selectivity for organic and inorganic compounds. They are however, limited by energy needs, non-recyclable, waste generation, incomplete decomposition and generation of solid sludge, which hinders their practical application in the degradation of pollutants within the environment [18, 19].

Oxidative degradation of pharmaceutical compounds using semiconducting photo-catalysts appears a viable option due to their simplicity, energy-saving, cheap and environmental friendliness, as well as their mild reaction condition requirements [20]. However, a major challenge arising from the use of photo-catalyst in advance oxidative photo-catalysed degradation is the narrow light response range and low quantum efficiency. It is therefore important to develop and fabricate an effective, stable and cheap visible-light-driven, efficient photo-catalyst for the degradation of organic pollutants [21]. Heterostructured semiconducting nanocomposites have a promising photocatalytic activity that is amenable to tuning for improved oxidative decomposition of hazardous pollutants within an aqueous environment [22]. This is because heterostructured nanocomposites possess heterojunction properties that improve the charge transfer mechanism and slow down the fast recombination of the charge carrier. They also possess excellent physical and chemical properties that enhance the activity of active radicals during the degradation process [23]. Despite these advantages, their photocatalytic performance is hindered by the quick recombination of photoinduced charge carriers and the low surface area that limits their quantum efficiency and photocatalytic activity. The development and use of hetero-structured semiconducting nanocomposites doped on g-C3N4 is suggested as a route to address the limited visible light adsorption, and to enhance charge separation associated with many photocatalysts [24]. However, information on the use of hetero-structured nanocomposites with improved photocatalytic potentials for the oxidative degradation of pharmaceutical compounds in aqueous systems is scanty.

In this study, visible light driven needle-like SnO2/g-C3N4 photocatalyst was synthesised using a facile and economical hydrothermal method by loading various amounts of SnO2 nanosheets on exfoliated g-C3N4, characterized for phase structures, morphologies, optical properties, chemical compositions and photocatalytic performance, and tested for the oxidative photodegradation of tetracycline (TCN), an organic pollutant, under visible light irradiation. Tetracycline (TCN) is one of the commonly used broad-spectrum, cheap and readily available antibiotic pharmaceuticals. As far as we know, the degradation of tetracycline using SnO2/g-C3N4 composites has not been fully explored, and very little knowledge is available concerning its use for the treatment of other pharmaceutical compounds in aqueous media.

Material and methods

Melamine (99.9%), hydrazine monohydrate (80%), ethanol, SnCl2.5H2O, benzoquinone, isopropyl alcohol, thiourea, potassium iodide, tetracycline, acetonitrile, methanol were purchased from Sigma- Aldrich, South Africa. All the reagents were of analytical grade and were used without further purification.

Preparation of thermally exfoliated g-C3N4 Nanosheet

The g-C3N4 nanosheet was synthesised by the calcination process as described by Yousaf, et al. [25] with slight modification. Briefly, 5 g of melamine was weighed into a crucible with a cover and transferred into a laboratory model muffle furnace. This was calcined at 550 °C for 4 h at a heating rate set at 10 °C/min. The resulting bulky yellow powder was allowed to cool to room temperature and thereafter crushed to a fine powder using a porcelain pestle and mortar. This resulting g-C3N4 fine was exfoliated by thermal treatment at 550 °C for another 2 h, at a heating rate of 10 °C/min to yield a thermal exfoliated g-C3N4 nanosheet.

Preparation of SnO2/g-C3N4 composite photocatalysts

About 1 g of the exfoliated g-C3N4 nanosheet was dispersed in 120 ml of deionized water in a 250 ml beaker and ultra-sonicated for 30 min at room temperature. The sonicated g-C3N4 suspension was stirred continuously at room temperature for 30 min, followed by the addition of different masses of tin chloride pentahydrate (10, 20, 30, 50 mg) to different aliquots to achieve, equivalent to 1, 2, 3, 5 wt.% equivalent loadings, respectively. The resulting homogenous mixtures were stirred, thereafter, 3 ml of hydrazine hydrate was added to each of them. The resulting suspensions were stirred again for another 30 min, and then transferred into 100 ml Teflon-lined stainless-steel autoclaves and kept at 180 °C in an oven for 24 h. The autoclave was removed from the oven and allowed to cool to room temperature. The resulting precipitates of the different coupled amounts of SnO2 nanoparticles were obtained via centrifugation, and afterwards, washed severally with deionized water and ethanol, and then dried overnight in a hot air oven at 80 °C to obtain needle-like SnO2/g-C3N4. The SnO2 nanoparticles were also synthesised using the same method but without the addition of g-C3N4.

Characterization

The X-ray diffraction pattern of the prepared nanocomposites were obtained using a Shimadzu 6100 X-ray diffractometer with a CU Kα radiation source (Bruker D6).

The UV–Vis diffuse reflectance spectra (DRS) of the synthesised SnO2/g-C3N4 composites were measured with Pelkin-Elmer UV–Vis spectrophotometer, the photoluminescence (PL) emissions were obtained via Perkin Elmer LS 55 spectrofluorometer.

The BET analysis (Micrometrics Tristar 3000) of the synthesised composites were carried out to determine their specific surface areas and their textural characteristics.

Their surface morphologies were also determined using the Scanning Electron Microscopy instrument (Zeiss 10 kV field). Elemental analysis of the composites were also carried out by energy dispersive spectrometry (EDS, Shimadzu), while the interfacial interaction within the synthesised composites were obtained via High-resolution transmission electron microscopy (JEOL TEM), along with the selected area electron diffraction (SAED) pattern.

The FTIR of the composites were measured on Perkin Elmer Series 100 Spectrum to determine the functional characteristics of the composites.

Electrochemical impedance spectroscopy (EIS) plots were performed by an electrochemical workstation (CHI660C, Shanghai Chenhua Instrument Corporation, China) with a three-electrode system consisting of glassy carbon electrode (GCE) as reference electrode, Pt wire as the counter electrode and ITO glass coated with SnO2/g-C3N4, in an electrolyte solution made up of a mixture of 0.5 mol/L Na2SO4, 2.5 mmol/L potassium hexacyanoferrate (III) (K3[Fe(CN)6]), and 2.5 mmol/L potassium ferrocyanide (K4Fe(CN)6•3H2O).

Photocatalytic activity evaluation of the synthesised catalyst

The photocatalytic activity of the synthesised SnO2/g-C3N4 composite was evaluated for the photodegradation of tetracyclines under visible light irradiation (250 W xenon lamp with a cut-off filter of 420 nm) in a photocatalytic reactor (Lelesil Innovative System, India). About 50 mg of the SnO2/g-C3N4 composite materials were dispersed in 100 ml, 30 mg/L tetracycline solution. The resulting suspension of mixtures were stirred (using a magnetic stirrer) in the dark (to minimize the decomposition of tetracycline) for 30 min to attain the tetracycline—photocatalyst adsorption–desorption equilibrium. The resultant suspension solutions were exposed to continuous illumination under visible light irradiation (250 W, wavelength λ = 420 nm) for 120 min, with constant magnetic stirring to maintain cooling at temperatures between 25 and 30 °C. About 4 ml aliquots of the visible light irradiated suspensions were taken at 20 min intervals, centrifuged to remove the SnO2/g-C3N4 photocatalysts, and then analysed with UV–visible spectroscopy (UV-3600-Shimadzu, UV–Vis-NIR) at an absorbance of 356 nm to determine the extent of the degradation of tetracycline.

The removal efficiency of tetracyclines by the photocatalyst was calculated as follows;

$$\mathrm{Removal \,efficiency }= \frac{Co-C }{Co}\times 100\%$$

where Co is the initial concentration of tetracycline solution at time = 0 (t0) (initial time), degradation before treatment, while C is the concentration value at a final time after treatment.

Result and discussion

Morphology studies

The morphologies of the synthesised composites were characterised using SEM, EDX mapping, TEM, SAED, and HRTEM to confirm their crystallinity, composition, and the elemental distribution of the synthesized heterostructure composites. Figure 1(i); shows the SEM images of the exfoliated g-C3N4 nanosheet, SnO2 nanoparticle and SnO2/g-C3N4 heterostructure composites.

Fig. 1
figure 1figure 1

(iii): TEM images of a g-C3N4 nanosheet, b SnO2 nanoparticles, c SnO2/g-C3N4 and, d HR-TEM image SnO2/g-C3N4 composites

The prepared exfoliated g-C3N4 nanosheet are made up of lamellar structure with rough pore surfaces; the SnO2 nanoparticles appear as tiny grain-like particles with near uniform sizes, while the morphology of the exfoliated g-C3N4 sheet remains unchanged. The SEM image of synthesised SnO2/g-C3N4 heterostructure composites showed that the tiny grain-like SnO2 were evenly decorated on the surface of the exfoliated g-C3N4 nanosheet, which probably partly accounts for the suppression of the recombination of photogenerated electron–hole pairs. In addition, the composites were further evaluated to confirm the presence of SnO2 in the SnO2/g-C3N4 by the EDS analysis.

The EDS of 3 wt.% SnO2/g-C3N4 composites (Fig. 1(ii)), where the elements Sn, N, C and O were examined, further proved that the composites are composed of SnO2 and g-C3N4. Combined with the content distribution of each element in the surface analysis and comparison with the selected area, Sn is the most abundant, covering the surface of the composites, which is indicative that SnO2 was successfully loaded on g-C3N4.

The TEM micrographs of g-C3N4 (Fig. 1(iii)) showed a lamellar shape with several rough sheets while the synthesised needle-like SnO2 nanoparticles were compactly and uniformly distributed on the surface of the synthesised g-C3N4 in the SnO2/g-C3N4 composites. The HRTEM images further confirms the deposition of SnO2 on the surface of exfoliated g-C3N4 nanosheets, and the fabrication of heterostructured SnO2/g-C3N4 nanocomposites (Fig. 1(iii)). The crystallographic nanoparticles lattice fringe pattern of the heterostructures SnO2/g-C3N4 nanocomposites were measured to be 0.24 nm, 0.27 nm and 0.34 nm. The measured lattice fringe patterns with spacings of 0.24 nm, 0.27 nm and 0.34 nm could be respectively ascribed to the interplanar distance of (200), (101) and (110) planes of SnO2 nanocrystals, which when combined with the XRD patterns indicates the formation of heterostructured SnO2/g-C3N4 composites. Moreover, the selected area electron diffraction (SAED) pattern (Fig. 1(iii)d) displays several discrete concentric rings with superimposed bright spots, which confirms that SnO2 are deposited on the surface of the heterostructured SnO2/g-C3N4 nanocomposites. This confirms the successful formation of hybrid heterostructure between SnO2 nanoparticles and exfoliated g-C3N4 nanosheet.

X-ray diffraction (XRD) analysis

The phase analysis and the crystal orientations of the synthesised SnO2 nanoparticles, g-C3N4 and the corresponding needle-like SnO2/g-C3N4 heterostructure composites, comprising different mass loading of SnO2 were examined by powder X-ray diffraction (XRD). The diffraction patterns of the needle-like SnO2 nanoparticles, g-C3N4 nanosheet and each of the composites were slightly variable, with those for the composites having similar 2θ diffraction (Additional file 1: Fig. S1). The major diffraction peaks in the SnO2 nanosheet diffractogram are 26.54, 34.02, 38.03, 51.72, 54.50, 65.88 and 78.54 2θ degrees. The 2θ diffraction angles correspond to the (110), (101), (200), (211), (002), (301) and (202) crystal planes, which is consistent with the formation of pure cassiterite SnO2 crystallite, devoid of impurities (JCPDS No 041-1445) [26]. Two significant peaks were observed on the diffractogram of the synthesised g-C3N4 nanosheet at 13.19 and 27.39 2θ degrees; indexed to the (002) and (100) diffraction planes assigned to the tris-s-triazine units, which is a characteristic of interlayer stacking of conjugated aromatics systems of graphitic materials (JCPDS No 87-1526) [27].

The diffractogram of the synthesized heterostructure needle-like SnO2/g-C3N4 composites had two characteristic 2θ diffraction peaks at 13.02 and 27.22 2θ, attributed to the crystalline nature of g-C3N4 with a reduced intensity devoid of distinct diffraction peaks corresponding to the diffraction peaks of SnO2 nanoparticles could be observed (Additional file 1: Fig. S2). This might be due to the small amount of SnO2 contents, in the surface of exfoliated g-C3N4 nanosheet and their high dispersion in the interior of polymeric g-C3N4 nanosheet during the preparation of the nanocomposites. This is indicative that a good synergistic interaction occurs between the exfoliated g-C3N4 nanosheet and SnO2 nanoparticles [28].

However, on increasing the amount of SnO2 loaded on g-C3N4 from 1 wt.% and 2 wt.% to 3 wt.% and 5 wt.%, two weak peaks at 33.60° and 51.55° 2θ were observed on the XRD pattern, which were assigned to the (101) and (211) planes of tetragonal SnO2. This is indicative that the as-synthesised photocatalyst demonstrates high crystallinity without any impurity [27, 29]. Additionally, the values of the crystallite sizes for the synthesized heterostructure composites were calculated using the Scherrer equation (D = \(\frac{\mathrm{K}\lambda }{\beta \mathrm{Cos}\theta }\)), as given [30]; where D is the crystallite size in nm, K is Scherrer’s constant ≈ 0.9, λ is the wavelength of the X-ray radiation (CuKα = 0.15406 nm), β is the corrected band broadening (full width at half-maximum (FWHM)) of the diffraction peak, and θ is the diffraction angle. The particle particle size distribution of the synthesized materials generally falls within 8–23 nm (Table 1), which is indicative that they are polycrystalline in nature.

Table 1 Specific surface area, pore volume, pore diameter and crystallite sizes of the synthesized SnO2, g-C3N4 and SnO2/g-C3N4 composites

Optical characteristics: absorption and photoluminescence

The optical properties of the synthesised composites were studied to estimate their ability to absorb visible light. The bandgap of all the synthesised composites were calculated according to the Kubelka–Munk equation, αhѴ = A(hѴ-Eg)1/2 [31]; where α represents the adsorption coefficient; h, Planck’s constant; ν, for light frequency; while Eg and A are the bandgap energy, and constantly called band tailing parameter respectively.

The synthesised needle-like SnO2 nanoparticles absorb in the visible light region at about 478 nm (Fig. 2). Therefore, the bandgap energy of the as-synthesised needle-like SnO2 nanoparticles were calculated to be approximately 2.64 eV. The observed absorption and calculated bandgap may be attributed to the higher oxygen concentration due to the formation of extra ionized oxygen vacancies within the synthesised SnO2 nanoparticles when compared to other synthesised SnO2 with bandgap energy of about 3.60 eV [32, 33]. The g-C3N4 nanosheet absorbs at approximately around 458 nm, with a calculated energy bandgap of about 2.70 eV; while the energy bandgap of the synthesised composites with different weights of SnO2 loaded on exfoliated g-C3N4 are close to 2.52 eV for 10% SnO2/g-C3N4, and 2.45 eV for each of 2, 3, 5 wt.% SnO2/g-C3N4 heterostructure composites, respectively. The calculated energy band gap values for various composites of SnO2/g-C3N4 composites imply that they are photosensitive, and have the ability to respond to visible light. This also signifies that the fabrication of heterostructure composites can hinder the recombination of charge carriers, thereby causing a shift in their light response-ability towards the visible region of the spectrum. Thus, the materials synthesised SnO2/g-C3N4 possess narrow bandgap, which consequently, lead to improved light harvesting capacity of the material, compared to 2.50 eV and 2.68 eV reported for the degradation of some organic pollutants [34, 35].

Fig. 2
figure 2

a UV–Vis DRS and, b Kubelka–Munk model of SnO2, g-C3N4 and its corresponding heterostructured composites

The photoluminescence (PL) spectra of the synthesised composites were assessed to evaluate the migration, transfer, and recombination rate of photogenerated electron–hole pairs. This is important as it helps to understand the interaction between the catalyst involved in the formation of heterojunction structure within the composites photocatalyst and the separation of the electron-holes pair, which leads to their photocatalytic activity in photodegradation application [36]. The maximum absorption wavelength used as PL excitation wavelength, was 350 nm as obtained from the UV DSR analysis. The exfoliated g-C3N4 nanosheets showed a strong emission peak which appears at about 425 nm (Fig. 3a). This is characteristic, for fast recombination of photogenerated charge carriers in g-C3N4 [37]. The observed emission bands for the synthesized needle-like SnO2 nanoparticles were 425, 460, 484 and 532 nm (inserts in Fig. 3a). The above emission bands can be attributed to phenomenons such as crystal defects or surface defects, oxygen vacancies, tin vacancies and tin interstitials attributes in the materials [38]. Pure SnO2 nanoparticles showed a less intense peak when compared to the intensity of the peak of the synthesised exfoliated g-C3N4. This suggests that the synthesized needle-like SnO2 nanoparticles consist of a lower electron–hole recombination rate or slow recombination of photoinduced charge carriers.

Fig. 3
figure 3

a PL spectra of g-C3N4, SnO2 and SnO2/g-C3N4 composites with different compositions of SnO2 and, b their corresponding EIS analysis plot

Moreover, the introduction of different masses of SnO2 nanoparticles on the exfoliated g-C3N4 nanosheets significantly reduced the emission peak of the exfoliated g-C3N4 nanosheet. The decrease in the emission intensities of the different needle-like SnO2/g-C3N4 composites result from the formation of heterojunction between needle-like SnO2 nanoparticles and the g-C3N4 nanosheet, which lower electron–hole recombination or slows recombination of photoinduced charge carriers, weakened by the addition of SnO2 nanoparticles [39]. This suggests that the synergistic interaction between SnO2 nanoparticles and g-C3N4 nanosheet considerably leads to the decrease in the recombination of the photoinduced charge carrier, which is evident in its ability to degrade tetracycline under visible light.

Furthermore, electrochemical impedance spectroscopy (EIS) analysis was conducted to confirm the relevant charge transfer process and the recombination rate between the photogenerated electrons and holes. The EIS plot (Fig. 3b) reveals that the steady state perturbation in the different SnO2/g-C3N4 heterostructure composites (indicated by the magnitude of the electrical impedance frequency) are much smaller than that of g-C3N4. This indicates that a higher charge transfer rate occurred in SnO2/g-C3N4 composites, resulting in less obstruction in the transfer of electrons-holes, and thus, a more efficient separation of the charge [40]. Therefore, the % mass content of SnO2 nanoparticles in the SnO2/g-C3N4 composites clearly influences the magnitude of EI, which is consistent with the PL analysis. The 3 wt.% SnO2/g-C3N4 showed lower resistance and higher separation efficiency for the photogenerated electrons and holes, thereby enhancing their potential photocatalytic efficiency for organic pollutants' degradation compared to other composites.

The FTIR spectra of the synthesised needle-like SnO2 nanoparticles, g-C3N4 nanosheet and all the SnO2/g-C3N4 composites were measured (Additional file 1: Fig. S3). The spectra patterns provide information and insight into the changes in the structure of the exfoliated g-C3N4 nanosheet. The FTIR spectrum of SnO2 showed a broad peak around 527 cm−1, characteristic of Sn–O stretching vibration of Sn–O-Sn, while the peaks at 1648 and 3362 cm−1 are attributed to the molecular water bending vibration and hydroxyl groups stretching vibration, respectively [39]. FTIR spectrum of g-C3N4 shows a peak at 804 cm−1 which is considered as an out-of-plane bending vibration characteristic of a tris-s-triazine ring of the g-C3N4 main building block. Peaks from 1204 to 1626 cm−1 are related to the stretching vibration modes of CN heterocycles, while the broad peak at around 3086 cm−1 and 3158 cm−1 are attributed to the stretching mode of N–H bond from the uncondensed amino groups and O–H band from absorbed water molecules [41]. The FTIR spectrum of SnO2/g-C3N4 composites was made up of the main characteristic peaks of g-C3N4, with a slight shift in their wavenumbers, which indicates a strong interaction between exfoliated g-C3N4 nanosheet and SnO2 particles.

Sorption–desorption studies

The N2 adsorption–desorption isotherms of the synthesised composites are shown in Fig. 4. The four composites of SnO2/g-C3N4 composites with the different masses of SnO2 particles with SnO2 and g-C3N4 showed type IV isotherms, while its hysteresis loops are categorized as type H3. According to the IUPAC classification, heterostructured composite materials of this type are listed as mesoporous in nature [42]. Mesoporous materials are of high surface area, and when used as photocatalyst; they have been reported to enhanced visible-light harvesting capacity vis-a-viz mesostructure light trapping in their molecules, which in turn enhances their photocatalytic efficiency by the generation of more reactive species that improves reactant adsorption for better photon-reactant molecules interactions [41].

Fig. 4
figure 4

N2 adsorption of a g-C3N4, b SnO2, c 1% SnO2/g-C3N4, d 2% SnO2/g-C3N4, e 3% SnO2/g-C3N4, f 5% SnO2/g-C3N4 g and with their respective pore size distribution plot inserted

The textural properties of the synthesised SnO2, g-C3N4, and the different masses of SnO2/g-C3N4 heterostructure composites such as; BET surface area, pore volume and average pore diameter are listed in Table 1. The synthesised needle-like SnO2 nanoparticles display a high surface area of 38.06 m3/g when compared to the synthesised exfoliated g-C3N4 nanosheet with surface area of 25.22 m3/g. However, the introduction of SnO2 into the g-C3N4 nanosheet results in a slight improvement in the surface area with an increase in their pore volume and pore diameter of the different composites. The 3 wt.% SnO2/g-C3N4 composites showed the best textural properties of 34.37 m3/g, 0.137 cm3/g, 28.62 nm for it’s surface area, pore volume and pore diameter, respectively. The improvement in the surface area of the composites indicates that some changes occurred during the incorporation of SnO2 on g-C3N4, which could facilitate the transfer of photoinduced electron–hole pairs, hence, improving their photocatalytic efficiency for the degradation of organic pollutants.

Thermogravimetric (TGA) analysis

The thermal stability of the synthesised materials were analysed by thermogravimetric analysis in an atmosphere of air at a heating rate of 10.00 °C/min. An initial weight loss at the temperature of 50–200 °C was observed for all materials (Additional file 1: Fig. S4a). This can be attributed to the loss of absorbed water and other substances on the surface of the materials [43]. The gradual weight loss for g-C3N4 after 550 °C may be associated with the combustion of the carbon skeleton and the decomposition of defects and edge functional groups like uncondensed amine functional groups and the edge cyano-group that exist within g-C3N4 nanosheet [44]. Meanwhile, the weight loss of the different masses of SnO2 nanoparticle loaded on the g-C3N4 sheet between the temperature range of 400 and 550 °C may be ascribed to the combustible carbon and the decomposition of the functional groups in the g-C3N4 structure, while the SnO2 nanoparticle maintains its stability with a nearly straight line [45]. The specific decomposition temperatures of the synthesised materials are 703 °C, 658 °C, 635 °C with 50% SnO2/g-C3N4 showing the lowest stability at 595 °C, while the g-C3N4 nanosheet shows the highest stability of 720 °C (Additional file 1: Fig. S4b). The TGA revealed that the synthesised materials are thermally stable, while its composition are consistent with the original materials.

Photocatalytic evaluation

The photocatalytic activity of the composites with different needle-like SnO2 nanoparticles mass loads on exfoliated g-C3N4 nanosheets for the photocatalytic degradation of tetracycline under visible light irradiation the effects of SnO2 mass loads was investigated. The visible light-assisted degradation of tetracycline in reaction systems in which the different synthesised composites were dispersed and dark stabilised for 30 min to attain adsorption–desorption equilibrium prior to visible light exposure were fairly. The exposure of the tetracycline solution to visible light irradiation caused minimum degradation of about 26.90%, while a 40.92% and 51.32% degradation was observed when exfoliated g-C3N4 nanosheet and needle-like SnO2 were added to the tetracycline solution as a catalyst, respectively, after 120 min (Fig. 5a).

Fig. 5
figure 5

a photocatalytic activities of the synthesized composite, b kinetics studies

The concentration of tetracycline decreased significantly when the different composites SnO2/g-C3N4 were used as catalysts. The degradation efficiencies of tetracycline by the 1 wt.%, 2 wt.%, 3 wt.% and 5 wt.% SnO2/g-C3N4 photocatalyst were 81.54%, 90.57%, 95.90% and 92.15%, respectively. It is important to note that the photocatalytic ability of the different SnO2/g-C3N4 composites increased with respect to the amount of SnO2 nanoparticles added to the g-C3N4 nanosheet until 3 wt.% SnO2/g-C3N4 composite, which displayed the highest degradation of 95.90% tetracycline degradation within 120 min irradiation under visible light, after which a decrease was observed with increase in % wt. SnO2 load. This implies that an increase in the content of SnO2 in the SnO2/g-C3N4 composites plays an important role in catalysing the degradation of tetracycline molecules vis-a-viz the formation of heterojunction within the structural configuration of the SnO2/gC3N4 composites. The heterostructured characteristics of the SnO2/gC3N4 play a crucial role in improving the photocatalytic activity of the composites. Hence, the photocatalytic efficiency can be optimized by adjusting the content of SnO2 nanoparticles on exfoliated g-C3N4 nanosheet.

More also, the efficiently high tetracycline degradation of 95.90% by the 30% SnO2/g-C3N4 composite could be because of their promoted charge separation, which results in increased quantum confinement leading and low recombination of photogenerated charge, as well as its larger surface area when compared to other composites (Table 1). On the other hand, increasing the SnO2 composite content to 5 wt.% in the SnO2/g-C3N4 led to a decrease in the photocatalytic activity. This may be due to the shielding of the active sites as a result of the excess addition of SnO2 on the exfoliated g-C3N4 nanosheet, which in turn hinders the light-capturing capacity of g-C3N4 under visible light irradiation during the photodegradation process [46].

The photocatalytic kinetics for the degradation of tetracycline was studied. The degradation process obeyed the pseudo-first order kinetics model (\(-In(\frac{C}{Co})=kt\)); where k represents the reaction rate constant while Co and C stand for the initial concentration and final concentration of tetracycline at time t, respectively.

The calculated rate constant (k) and the mean regression coefficients (R2) values for the kinetics of the degradation of tetracycline with different amounts of SnO2 nanoparticles loaded on the g-C3N4 sheets are presented in Fig. 5b. The calculated k values are 0.0062, 0.00671, 0.0107, 0.0215, 0.0235 and 0.0222 min−1 for 1 wt.%, 2 wt.%, 3 wt.% and 5 wt.% SnO2QDs/g-C3N4, respectively (Table 2). The as-synthesised 3 wt.% SnO2/g-C3N4 nanocomposites exhibited a superior k value when compared to the other nanocomposites, generating a rate constant that is 6.32 and 4.33 times better than those of pure g-C3N4 nanosheet and SnO2 nanoparticle, respectively, for the degradation of tetracycline. This showed that loading different amounts of SnO2 on g-C3N4 can improve the degradation of tetracycline under visible light. This can be attributed to a decrease in the recombination of the electron–hole pairs through the formation of localized states between the conduction and valence bands.

Table 2. Summary of photocatalytic degradation of tetracycline visible light irradiation using SnO2/g-C3N4 heterostructured composites

Photocatalyst reusability

Recyclability and stability are among the important parameters considered in the practicality of a photocatalyst. This was evaluated for the practical application of the synthesised SnO2/g-C3N4 heterostructured composites in five cycling experiments, for the degradation of tetracycline under the same reaction condition (50 mg of the catalyst dispersed in 100 ml, 30 mg/L tetracycline solution). At the end of each reaction cycle, the spent solutions were centrifuged to recover the SnO2/g-C3N4 composites used as catalysts and washed severally with water and ethanol, followed by overnight drying in the oven at 80 °C. About 86.78% of the tetracycline was degraded on the fifth run after 120 min, showing a slight reduction in the photocatalytic efficiency of 3 wt.% SnO2/g-C3N4 compared to the first run which had 95.90% degradation, under the same photocatalytic condition (Additional file 1: Fig. S5). This might be due to the loss of the photocatalyst charge during the recycling process.

Furthermore, the crystal structure of the 3 wt.% SnO2/g-C3N4 composite photocatalyst was investigated prior to- and after use in five-cycle experiments of the photocatalytic process, to evaluate and confirm whether there was any distortion or changed in the crystal structure, using XRD analysis. The results showed that there was no significant change in their crystalline structure, thus suggesting that the chemical structures of the photocatalysts were not affected after use, hence their stability. It can therefore be inferred that the 3 wt.% SnO2/g-C3N4 displayed high stability under visible light for the degradation of tetracycline, probably because of the π-π stacking interaction that exists between the photocatalyst and the tested pollutants [47].

To understand the photocatalytic mechanisms and elucidate the reactive species involved during the photocatalytic degradation of tetracycline using 3 wt.% SnO2/g-C3N4 heterostructured composite under visible light irradiation, various scavengers such as potassium iodide KI for H+ scavenging, p-benzoquinone (PBQ) for ·O2, Isopropyl alcohol (IPA) for ·OH scavenging and lastly thiourea for scavenging OH·, H+ and e were used as suggested by Bui, et al. [48]. The same experimental procedure for the photocatalytic activity were used. Results revealed that the introduction of thiourea into the reaction medium led to a considerable decrease in tetracycline degradation (65.54%); KI addition led to 87.62% removal efficiency, PBQ caused 56.41 removals. However, the addition of IPA significantly suppresses tetracycline degradation by 41.38% when compared to degradation without scavenger (Additional file 1: Fig. S6). These results indicate that the presence of •OH and •O2 radical proved to be the major reactive species for the degradation of tetracycline using 3 wt.% SnO2/g-C3N4 as photo-catalyst.

The calculated conduction band (CB) and valence band (VB) of SnO2 are 3.04 eV and 0.40 eV while that of g-C3N4 nanosheet are − 1.12 eV and 1.58 eV, respectively; showing that the compositing semiconductors have suitable band potentials that led to the formation of heterojunction structures to restrain the recombination of electron-holes pairs. Based on the characterisation and experimental data obtained in this study, a tentative mechanism for the photodegradation of tetracycline using SnO2/g-C3N4 composites under visible light irradiation is proposed (Additional file 1: Fig. S7). Upon reception of visible light irradiation on the composites, both g-C3N4 and SnO2 generated electrons and holes, leading to te migration of the photoexcited electrons from the CB of g-C3N4 nanosheet to the CB of SnO2 nanoparticles. In contrast, the photoexcited holes migrates from the VB of g-C3N4 to the VB of SnO2 resulting in the effective separation of the photoexcited charge carriers, which enhances their photocatalytic performance for the degradation of tetracycline under visible light irradiation.

From the Mott-Schottky plot, as shown in Additional file 1: Fig. S8, the flat band potential for the g-C3N4, SnO2 and the 3 wt.% SnO2/g-C3N4 composites were estimated to be − 1.33, − 0.12 and − 1.15 eV. Generally, the conduction band potential of a semiconductor is − 0.1 or − 0.2 eV, more negative than the flat band potential. Therefore the estimated conductor band from the recorded flat band potentials are − 1.43, − 0.22, − 1.25 eV. Moreover, the valence band potential of g-C3N4, SnO2 and SnO2/g-C3N4 were calculated (1.27, 2.42 and 1.20 eV) using the Butler and Ginley equations [49] as given in Eqs. 1 and 2:

$$ {{E}}_{{{\text{VB}}}} = \chi - {\text{E}}^{{\text{e}}} + 0.{\text{5E}}_{{\text{g}}} $$
(1)
$$ {{E}}_{{{\text{CB}}}} = {\text{E}}_{{{\text{VB}}}} - {\text{E}}_{{\text{g}}} $$
(2)

where the value of Ec is 4.5 eV, X value of SnO2 is 6.22 eV, while that of g-C3N4 is 4.73 eV which was similar to the values reported by Sun et al. [50]. The bandgap energy (Eg) of g-C3N4 and SnO2 as obtained from UV–Vis DRS are 2.70 eV and 2.64 eV.

The electronic structure of the composites was influenced by the large percentage composition of g-C3N4 being the base material, while the small quantity of SnO2 in the composites led to the suppression of its electronic structure. From the above discussion, it is evident that the conduction band potential of the composites is higher than the required electron reduction potential (− 0.33) for the conversion of an oxygen molecule to superoxide anions radicals. Hence the reason superoxide anion radical is the determining oxygen species for the photocatalytic degradation of tetracycline. On the other hand, the valence band (1.20 eV) is less positive than the required oxidative potential for the conversion of H2O molecule and OH ions to hydroxyl radicals (HO) which explains the insufficient amount of hydroxyl radicals in the system. Moreover, the conduction band position of g-C3N4 nanosheet is −1.12 eV vs. NHE, which is more negative than that of SnO2 nanoparticle (0.04 eV vs. NHE) with a redox potential of O2/•O2 (+ 0.28 eV vs NHE). This shows that the photogenerated electrons could react with the adsorbed O2 to produce active oxygen species •O2 radicals.

Conclusions

Series of visible light-driven SnO2/g-C3N4 heterostructure composites were synthesised via a facile hydrothermal method. The loading of different masses of SnO2 nanoparticles on the synthesised exfoliated g-C3N4 sheets produces a strong heterojunction and improved specific surface area coupled with good optical properties. As revealed by the SEM, TEM and HR-TEM the synthesised composites generated a needle-like crystal structure with the uniform distribution of the SnO2 nanoparticles on the surface of the g-C3N4 nanosheet while the PL result confirm the decrease in the recombination rate due to the introduction of SnO2 nanoparticle on to the surface of the exfoliated g-C3N4 sheet. After comparison, 3 wt.% SnO2/g-C3N4 composites exhibited a higher photocatalytic performance than other composites as a result of improved specific surface area and charge separation efficiency. From the photocatalytic analysis, it was observed that the •O2 and •OH, radicals are the main active species generated during the photocatalysis process due to the action of electron reduction.

Moreover, the kinetic rate constant, recyclability and stability of the synthesised composites for the degradation of tetracycline were also analysed with obtained results showing excellent capability and robustness of the composites towards the degradation of tetracycline under visible light irradiation. The heterostructure composite mechanism pathway for the degradation of tetracycline under visible light irradiation were also proposed. Attributable to the improved performances of the synthesised composites for visible-light responsive applications, materials with such efficiency have potential environmental applications.

Availability of data and materials

The data sets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Supplementary Information

Additional file 1: Fig. S1.

XRD patterns of (a) SnO2 nanoparticles, (b) g-C3N4 nanosheet and, (c) different mass combinations of SnO2/g-C3N4 heterostructure composites. Fig. S2. Diffractograms of synthesized heterostructure needle-like SnO2/g-C3N4 composites devoid of distinct diffraction peaks of SnO2 nanoparticles. Fig. S3. FTIR Spectra of pure SnO2, g-C3N4, and SnO2/g-C3N4 composites. Fig. S4. (a) TGA thermograms and (b) the corresponding derivative thermograms of the synthesized heterostructured composites. Fig. S5. (a) recyclability test of 30% SnO2/g-C3N4 composite and (b) XRD pattern of 30%SnO2/g-C3N4 before photoreaction and used after five repeated cycles of photodegradation experiments. Fig. S6. showing the effects of various scavengers on the tetracycline degradation using 3% SnO2/g-C3N4 under visible light irradiation. Fig. S7. Reaction mechanism pathway for the degradation of tetracycline through 30% SnO2/g-C3N4. Fig. S8. Mott-Schottky plots measured at 1 kHz in 5 mM [Fe(CN)6]4−/3− solution.

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Oluwole, A.O., Olatunji, O.S. Photocatalytic degradation of tetracycline in aqueous systems under visible light irridiation using needle-like SnO2 nanoparticles anchored on exfoliated g-C3N4. Environ Sci Eur 34, 5 (2022). https://doi.org/10.1186/s12302-021-00588-7

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Keywords

  • Needle-like SnO2
  • SnO2/g-C3N4
  • Nanocomposite
  • Tetracycline degradation
  • Reactive species