Introduction

The extensive use of pharmaceuticals in the recent years is a growing concern among environmentalists and researchers worldwide (Luo et al. 2015). The pharmaceutical residues and hospital effluents discharged into the aquatic environment are perceived to be major environmental threats due to the uncontrollable release of harmful organic contaminants (Xue et al. 2015). Among pharmaceuticals, antibiotics used for their strong inhibitory effects on a variety of pathogenic microorganisms are major contributors in such environmental contamination (Wu et al. 2015). The tetracycline (TC) discovered in the 1940s is a family of antibiotics, exhibiting activity against a wide range of gram-positive and gram-negative bacteria and against disease-causing organisms as chlamydia, mycoplasmas, rickettsiae, and protozoan parasites (Jeong et al. 2010). Trace amounts of pharmaceutical agents like TC can cause varied issues among which the most threatening is presumed to be the emergence of drug-resistant bacteria and other organisms (Yan et al. 2015). Conventional wastewater treatment plants are not designed to address the removal of antibiotics present in such trace amounts (Ai et al. 2015). Photocatalysis characterized by efficient degradation rates, high mineralization efficiency, and low toxigenicity appears to be the greener approach as CO2 and H2O are produced as the only by-products (Reyes et al. 2006). In recent years, UV light irradiation for pollutant degradation has been extensively studied for metal oxides like TiO2, ZnO, etc. (Banerjee et al. 2014; Luévano-Hipólito et al. 2016; Smitha et al. 2013). However, as the ultraviolet region accounts for only 4% of the incoming solar energy, only a few practical applications are realized thus far (Bu and Zhuang 2013; Safari et al. 2014). Therefore, photocatalysis in sunlight irradiation emerges to be a solution for the effective removal and alleviation of environmental contaminants (Joy et al. 2016; Panneri et al. 2016; Yan et al. 2016).

Graphitic carbon nitride, generally known as C3N4 with a medium bandgap of about 2.7 eV, has received good attention in recent times as a new metal-free, visible light photocatalyst for organic pollutant degradation (Fagan et al. 2016; Liu et al. 2016; Suyana et al. 2016; Wang et al. 2009; Zhao et al. 2015). It is obtained by the thermal condensation of nitrogen-rich organic molecules melamine, cyanamide, thiourea, and urea (Ye et al. 2015). The photocatalytic degradation property of C3N4 has been amply demonstrated in the past years by employing powder obtained through various synthetic approaches and from varying precursors (Dong et al. 2013; Fan et al. 2015). In a notable commentary by Kroke, it is pointed out that the labeling of graphitic “C/N/H polymers” obtained by condensation reactions during the pyrolysis of the common precursors like cyanamide, dicyandiamide, and melamine as graphitic C3N4 (C3N4) is misleading (Kroke 2014). According to him, the formation of s-triazine and s-heptazine units is often misrepresented as C3N4 and pure C3N4 has not been obtained yet. The C3N4 claimed widely in the literature is believed to be non-stoichiometric. The first structural characterization of such materials is provided by Lotsch et al. employing the techniques of electron diffraction and solid-state NMR spectroscopy (Lotsch et al. 2007). The 2D structure of melon with heptazine building blocks and a polymeric nature makes it a defective C3N4 material with a graphite-like topology resulting from a network terminating NH and NH2 groups. In a recent review, it is reiterated that the polycondensation of nitrogen-rich precursors on thermolysis yield heptazine-based carbon nitride than the triazine-based ones, as has been originally postulated by Domen and Antonietti et al. (Ong et al. 2016; Wang et al. 2009). In view of the common terminology employed for the designation of such materials and to be consistent with literature, carbon nitride derived from the four precursors here shall be referred to as C3N4. A schematic illustration of the formation of C3N4 from the four different precursors is shown in Fig. 1.

Fig. 1
figure 1

Schematic diagram illustrating the formation of polymeric C3N4 synthesized from different precursors

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A comparative evaluation of the C3N4 powders obtained from different precursors, for a single application, is thus far not available. To the best of our knowledge, there are only two reports on tetracycline (TC) degradation using C3N4 as a photocatalyst (Li et al. 2015; Xue et al. 2015). Xue et al. synthesized a plasmonic photocatalyst, Au/Pt/ C3N4, and Li et al. prepared C3N4-ZnO/halloysite nanotubes (HNTs) as a nanocomposite photocatalyst. They evaluated the prepared samples for the photocatalytic degradation of TC. However, the use of expensive metal catalysts like Au and Pt makes the material unaffordable for an environmental remediation process. The second report involved a cumbersome preparative process less suitable for bulk applications. Herein, we propose a metal-free, regenerative, and visibly active photocatalyst C3N4 synthesized from four different precursors by an elementary thermal condensation process. The physicochemical properties of the as-prepared samples were studied, and the photocatalytic activity of the same was evaluated by TC degradation.

Materials

Melamine (C3H6N6, 99.0%) and cyanamide (CH2N2) were purchased from Sigma-Aldrich (India) while urea (CH4N2O) and thiourea (CH4N2S) were purchased from Merck (India). Tetracycline (TC) was purchased commercially. All reagents for synthesis and analysis were used without further purification. Deionized water was used throughout this work.

Preparation of photocatalyst

C3N4 was prepared by the thermal condensation of nitrogen-rich precursors; melamine, cyanamide, thiourea, and urea. Weighed amounts of different precursors were put into an alumina crucible. The alumina crucible, kept in a semi-closed system to decrease the volatilization of the precursors, was then heated at 550 °C in a muffle furnace for 2 h, in an air atmosphere, with a ramping rate of 3 °C/min. After the alumina crucible was cooled down to room temperature, the prepared sample was ground and the powder thus obtained was used as such for further studies.

Photocatalytic degradation of tetracycline

The photocatalytic activities of C3N4 synthesized from different precursors were evaluated by photodegradation of tetracycline (TC) under sunlight irradiation (October 7, 2015, Trivandrum, India, between 11 p.m. and 1 p.m.). In a typical photocatalytic experiment, 0.04 g photocatalyst was dispersed in 80 ml of aqueous TC (10−4 M) solution. The mixed suspension was magnetically stirred for 30 min in the dark to attain absorption-desorption equilibrium before irradiation. Aliquots were taken at a 15-min interval to measure the concentration of TC with a UV-Vis spectrophotometer in the spectral range of 200 to 800 nm. Tetracycline is characterized by strong absorption peaks at 278 and 357 nm. The reduction in intensity of the peak at 357 nm is monitored with time for the degradation profiles. After every degradation experiment, the photocatalyst was centrifuged washed, dried, and reused.

Results and discussions

The XRD patterns of carbon nitride (C3N4) derived from the four different precursors (melamine, cyanamide, thiourea, and urea) are shown in Fig. 2. The two distinct peaks at 27.4° and 13.1° can be assigned to the (002) and (100) diffraction planes of g-C3N4 (JCPDS 87-1526) (Cao et al. 2015). The strong peak at 27.4° with an interplanar distance d = 0.335 nm corresponds to the stacking of aromatic systems. Another peak at 13.1° (d = 0.675 nm) represents the interlayer structural packing (Yan et al. 2009). XRD patterns of C3N4 synthesized from different precursors illustrated identical peaks, implying formation of phase pure C3N4 from all the materials. A closer observation suggests that the intensity of the peak at 27.4° varies between samples, following the order urea < thiourea < cyanamide < melamine, and can be attributed to the varying degrees of condensation. A sharp intense peak indicates that there are more regular repetitions between graphitic layers (Cao et al. 2015).

Fig. 2
figure 2

XRD pattern of C3N4 synthesized from different precursors

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The morphological features as seen by TEM, however, revealed significant differences. A TEM image of thiourea-derived C3N4 (T-C3N4) in Fig. 3a showed large thick paper-folded sheets while the urea-derived C3N4 (U-C3N4) in Fig. 3b showed crinkly paper-folded thinner sheets (Dong et al. 2013). A TEM image of melamine-derived C3N4(M-C3N4) in Fig. 3c can be characterized as an aggregate of planar sheets, but the formed aggregate has a smooth surface due to the layered structure of C3N4 (He et al. 2014). Cyanamide-derived C3N4 (C-C3N4) in Fig. 3d revealed layered platelet-like morphology (Thomas et al. 2008). The TEM images show distinct morphological differences between all the C3N4 derived from different precursors. The differences in the morphology can be attributed to the varying sources of C3N4 and its thermal decomposition pathways. The thin and thick folded sheets of U- and T-derived C3N4, respectively, can be attributed to the presence of heteroatoms oxygen and sulfur. The condensation pathway of these precursors is longer than that of M- and C-derived C3N4 which causes multiple exfoliations of the planar sheets and forms folded thin and thick sheets. The curling of these sheets is anticipated to reduce the surface tension. On the other hand, M- and C-derived C3N4 restricts the formation of the planar aggregates (Dong et al. 2013). The EDS spectrum of C3N4 shown in Fig. S1 indicates the presence of only elements carbon (C) and nitrogen (N).

Fig. 3
figure 3

TEM images of C3N4 synthesized from a thiourea, b urea, c melamine, d cyanamide as precursors

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The optical properties and band structure of C3N4 synthesized from different precursors, as determined by UV-Vis diffuse reflectance spectra (DRS), are presented in Fig. 4. The optical absorption patterns were characteristic of an intrinsic semiconductor like absorption in the blue region of the visible spectra (Liu et al. 2010). Melamine-derived C3N4 showed the extension of absorption in the visible region while the other precursors yielded the least extension among them. The absorption edge positions were at 440, 452, 464, and 468 nm for U-C3N4, T-C3N4, C-C3N4, and M-C3N4, respectively. All the samples however had a weaker absorption tail.

Fig. 4
figure 4

UV-Vis absorption spectra of C3N4 synthesized from different precursors

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The differences in the absorption edges of all the C3N4 samples can be attributed to different factors such as the difference in the degree of condensation, condensation temperature, different local structures, and packing defects formed during the preparation process (Wang et al. 2012). The higher order of condensation creates defects in local structures and packing, leading to a stronger conjugative effect (Zhang et al. 2012b).

The FTIR spectra of C3N4 derived from different precursors are shown in Fig. 5. All the spectra showed matching patterns confirming the identical nature of C3N4 formed. In the spectrum below, several sharp bands were observed between 1200 and 1650 cm−1. The characteristic absorption bands at 1648.9, 1571.8, 1458.0, and 1398.2 cm−1 are ascribed to the stretching vibration modes of the carbon nitride heterocycle. Another significant absorption is observed around 801 cm−1 and corresponded to the bending vibration mode of triazine units (Zhao et al. 2005).

Fig. 5
figure 5

FTIR spectra of C3N4 synthesized from different precursors

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The nitrogen adsorption-desorption isotherms of C3N4 from different precursors as shown in Fig. 6 indicate that the specific surface area of C3N4 had a strong dependence on the precursor type and their synthesis conditions used. The BET surface area values obtained are 153, 67, 11, and 8 m2 g−1 for U-C3N4, T-C3N4, C-C3N4, and M-C3N4, respectively. The difference in the surface area values can be attributed to the various decomposition pathways followed by the respective precursors. M-C3N4 and C-C3N4 polycondense into big and smooth layered sheets resulting in a smaller surface area (Zhang et al. 2012b). U-C3N4 and T-C3N4 have significantly higher surface areas, as they form small thin and thick crinkly paper-folded sheets, respectively. Moreover, the evolution of volatile decomposition products (SO2 and CO2) during the pyrolysis of thiourea and urea induces the formation of porous aggregates (Dong et al. 2013; Zhang et al. 2012a). The enhanced surface area facilitates active sites for surface-dependent reactions, such as photodegradation (Cao et al. 2015). As shown in Fig. 6, the adsorption-desorption isotherms of all the samples corresponded to type II B isotherms (BDDT classification), indicating the presence of macropores, which allows unrestricted monolayer-multilayer adsorption.

Fig. 6
figure 6

Nitrogen adsorption-desorption isotherms of C3N4 synthesized from different precursors

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The chemical state of the elements in the various C3N4 samples is investigated using X-ray photoelectron spectroscopy (XPS). Figure 7 represents the XPS spectra of C3N4 derived from the four different precursors. Peaks of carbon, nitrogen, and oxygen are displayed in the survey spectrum (Fig. 7a). The peaks of O1s appeared due to adsorbed H2O on the surface of C3N4. [30] The high-resolution spectra of C1s are presented in Fig. 7b–e. The C1s spectrum shows two dominant peaks at 284.4 and 288.1 eV (Angleraud et al. 2001). The peak at 284.4 eV is ascribed to C=C coordination and the peaks at 288.1 eV for N–C=N bonds, which is the major carbon species (Fang et al. 2015). Similarly, the high-resolution spectra of N1s (Fig. 7f–i) showed two dominant peaks at 398.3 and 400.8 eV. The peak at 398.3 eV is ascribed to the C–N=C bond in triazine rings, and the peak at 400.8 eV is ascribed to tertiary nitrogen N–(C)3 bonds. A weak peak at 403.9 eV (Fig. 7f) is attributed to charging effects or positive charge localization in the heterocycle (Ye et al. 2013). No predominant peaks of sulfur (S) and oxygen (O) can be observed in T-C3N4 and U-C3N4 XPS spectra, respectively. This indicates the complete release of sulfur in thiourea and oxygen in urea during the heating treatment at 550 °C/2 h in air (Dong et al. 2013).

Fig. 7
figure 7

a Survey spectrum. b–e C1s. f–i N1s of C3N4 synthesized from different precursors

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Photocatalytic application

The photocatalytic activity of C3N4 derived from the different precursors was comparatively evaluated by the degradation of tetracycline (TC) under sunlight irradiation. TC represents a major section of the antibiotics currently in use and can thus be considered as a model pollutant for evaluating the effectiveness of C3N4 photocatalysts (Addamo et al. 2005).

The degradation profile of TC (10−4 M) using C3N4 samples under sunlight irradiation is illustrated in Fig. 8. The blank experiment showed no photolysis of TC. It can be observed that after 120 min of irradiation, the degradation ratio (Fig. 8) of TC followed an order of U-C3N4 (93%), T-C3N4 (70%), C-C3N4 (65%), and M-C3N4 (62%).

Fig. 8
figure 8

Degradation pattern of TC using C3N4 synthesized from different precursors (the error bars in the figure represent the standard deviations of three independent measurements for each data point)

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It is thus easy to conclude that the degradation efficiency of TC improved with the increased surface area of the C3N4 used. The enhanced surface area provided a necessary increase in activity for a surface-associated phenomenon. The highest degradation efficiency was shown by the C3N4 sample derived from urea and can be attributed to the surface area value in excess of 150 m2/g. The surface area values of other samples are much below the value of urea-derived C3N4, and consequently, their photocatalytic activity is also less. It is to be noted that despite having higher visible light absorption ranges (Fig. 4) for the C3N4 samples derived from melamine and cyanamide, their photocatalytic activity was much lower than the urea-derived one. The dominating role of the surface area in determining the photocatalytic efficiency of C3N4 is thus evident from this result. The photocatalytic degradation of the organic pollutant generally is regarded as pseudo-first-order kinetics when C0 is in millimolar concentration.

$$ \ln \left(\mathrm{C}0/\mathrm{C}\right)=\mathrm{Kt} $$
(2.1)

Where C0 is the concentration of TC after initial adsorption-desorption equilibrium, C is the concentration after visible light irradiation and K is the first-order kinetics rate constant. The linear relationship between ln (C0/C) and t is depicted in Fig. 9. The rate constants of C3N4 from U, T, C, and M are 0.01759, 0.01008, 0.00902, and 0.00846 min−1, respectively. U-C3N4 exhibits the best degradation efficiency and has the highest rate constant among the above four samples.

Fig. 9
figure 9

Photodegradation rate of TC using C3N4 synthesized from different precursors

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The recyclability of photocatalysts is evaluated for its utility in practical applications. The recyclability of C3N4 derived from all the precursors was checked by TC degradation for 4 cycles and is shown in Fig. 10. The urea-based C3N4 gave the highest degradation rate and only showed a slight decline in photocatalytic efficiency compared to the notable reduction in efficiencies observed for the other samples. This shows the stability of the photocatalysts for 4 cycles. The XRD pattern and IR of U-C3N4, before and after four cyclic tests, are given in Fig. 11a, b. The results indicated no change in the phase and chemical composition of the catalyst used.

Fig. 10
figure 10

Recyclability of the C3N4 from a melamine, b cyanamide, c thiourea, and d urea precursors

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Fig. 11
figure 11

The XRD pattern and IR of U-C3N4, before and after 4 cycles

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ROS scavenging

The trapping experiments for the determination of active species formed during the photocatalytic reaction of U-C3N4 are shown in Fig. 12. It is found that photocatalytic degradation of TC was not affected by the addition of isopropanol (IP) while significant changes were observed on addition of AgNO3. The degradation was drastically affected by benzoquinone (BQ) and triethanolamine (TEA) (Hao et al. 2016; Ma et al. 2016). It can therefore be concluded that photogenerated holes, O2 −, and electrons are the main active species behind the TC degradation under visible light irradiation on the surface of C3N4.

Fig. 12
figure 12

Results of species trapping experiment performed using IPA, TEA, BQ, and AgNO3

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The mass normalized PL spectra of graphitic carbon nitride (C3N4) derived from the four different precursors are presented in Fig. 13. The PL spectrum is a measure of the electron-hole recombinations, and a lower PL intensity indicates lower recombination rate of electron-hole pairs (Ma et al. 2016; Niu et al. 2012). All the C3N4 samples were excited at 370 nm, and the main emission peak of C3N4 appeared at 454 nm with M-C3N4 showing the highest fluorescence intensity and U-C3N4 the least. Thus, it can be deduced that U-C3N4 has the lowest rate of electron-hole pair recombination compared to other precursor-derived C3N4. The defect-rich surface of urea-derived C3N4 provided a necessary spatial barrier for the excitons and prevented the easy recombination, which necessarily improved the photocatalytic activity.

Fig. 13
figure 13

PL spectra of C3N4 synthesized from different precursors

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Photocatalytic mechanism

Based on the results of active species trapping experiments, a possible mechanism, illustrating the degradation pathway of TC using C3N4, is shown in the schematic below (Fig. 14). C3N4 has a band gap of 2.65–2.8 eV, depending on the type of precursor used. Under visible light irradiation, C3N4 generates electrons in the conduction band and holes in the valence band. The reduction potential of O2/O2 − is −0.33 eV. The ECB of C3N4 is −1.2 eV. Hence, the photogenerated electrons in the conduction band react with O2 existing in the photodegradation system and are reduced into superoxide radical (O2 −) anion. This O2 − anion degrades the TC molecules. On the other hand, the EVB value of C3N4 is +1.57 V vs. SHE and is lower than the standard redox potential of OH*/H2O (+2.68 eV vs. SHE) and OH*/OH− (1.99 V vs. SHE). Thus, the photogenerated holes present in the valence band of C3N4 cannot react with H2O or OH− to generate active oxidative species OH*. Hence, holes (h+) in the valence band of C3N4 degrade TC molecules directly (Ren et al. 2014; Xu et al. 2013).

Fig. 14
figure 14

Mechanism, illustrating the degradation pathway of TC using C3N4

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Conclusions

In this study, a comprehensive evaluation of the role of precursors on the characteristics of C3N4 derived from four different precursors (melamine, cyanamide, thiourea, and urea) is made. XRD patterns illustrated identical peaks confirming the similarity in products formed. The BET measurements indicated highest surface area for urea-derived C3N4 and least for melamine-derived C3N4. However, the DRS data showed extended absorption for melamine-derived C3N4 due to the enhanced conjugative effect. The photocatalytic activity of the prepared samples was evaluated by photocatalytic degradation of tetracycline (TC). Urea-derived C3N4 showed the highest activity, and melamine-derived C3N4 showed the least efficiency. Thus, the degradation efficiency of C3N4 against TC improved with the increased measure of the surface area. A possible degradation mechanism indicated that the holes and superoxide radical anions as the main reactive oxidative species. In summary, a novel, efficient, metal-free, regenerative, and visibly active photocatalyst C3N4 is prepared which serves the purpose of TC degradation.