Facile Synthesis of Cadmium Doped Graphite Carbon Nitride for Photocatalytic Degradation of Tetracycline Under Visible Light Irradiation

doi:10.21203/rs.3.rs-1118826/v1

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Facile Synthesis of Cadmium Doped Graphite Carbon Nitride for Photocatalytic Degradation of Tetracycline Under Visible Light Irradiation

https://doi.org/10.21203/rs.3.rs-1118826/v1

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Using semiconductor photocatalysts for antibiotic contaminants degradation under visible light has become a hot topic in recent years. Herein, a novel and ingenious cadmium doped graphite phase carbon nitride (Cd-g-C₃N₄) photocatalyst was successfully constructed via 60°C oil bath method to degrade tetracycline. Experimental and characterization results revealed that cadmium was well doped at g-C₃N₄ surface and exhibited high intercontact with g-C₃N₄. Additionally, the introduction of cadmium significantly improved the photocatalytic activity, and the optimum degradation efficiency of 10-Cd-g-C₃N₄ reached to 98.1%, which was exceeded 2.0 times than pure g-C₃N₄ (43.9%). Meanwhile, the 10-Cd-g-C₃N₄ sample presented higher electrical conductivity, light absorption property and photogenerated electron-hole pairs migration compared with bare g-C₃N₄. Additionally, the quenching experiments and electron spin-resonance tests were exhibited holes (h⁺), hydroxyl radicals (•OH) and superoxide radicals (•O₂⁻) were main active species involved in TC degradation. Equally, the effects of various conditions on photocatalytic efficiency, such as pH, initial TC concentrations and catalyst dosage, were also researched. This work gives a reasonable point to synthesize high-efficiency and economic photocatalysts.

Carbon nitride

Cadmium-doping

Tetracycline

Photodegradation

For the past few years, researchers have been seeking suitable ways to solve the overwhelming environmental problems, caused by the continuous increase of population and energy-intensive industry (Abbott 2010; McCauley & Stephens 2012; Wang & Yang 2016). Especially, the recklessly discharged of untreated wastewater and sewage have posed a huge threat to water environment. Photocatalysis, as an environmental-friendly process, has become a significant method applied in energy and environmental remediation field (Ge et al. 2011; Liu et al. 2016; Wang et al. 2008; Zhu et al. 2017). Undoubtedly, the most important tasks in photocatalysis research are designing high-efficient, non-toxic and low-cost photocatalysts, which should include sustainable light absorption capacity, narrow band gap, controllable morphology and good redox ability (Xiang et al. 2012). Recently, metallic oxides, metallic nitrides and metallic sulfides have developed as excellent photocatalytic material to remove organic pollutants (Chan et al. 2017; Shi et al. 2018; Song et al. 2019; Wang et al. 2018b; Xiao & Jiang 2017; Xu et al. 2019). Whereas, some obvious defects have hindered their further application, such as high cost, low degradation efficiency and time-consuming (Yan et al. 2010; Yu & Wang 2015; Yuan et al. 2014; Zhang et al. 2016c; Zhao et al. 2014).

In comparison, more recently discovered carbon nitride with several covalent CN allotropes is considered as one of the most promising photocatalysts than metallic photocatalysts (Dong et al. 2012; Samanta et al. 2014; Wang et al. 2008). Among the five phases (alpha, beta, cubic, quasi-cubic and graphite phase) of carbon nitride, graphite phase carbon nitride (g-C₃N₄), as a metal-free and environmentally benign conjugated catalyst (Lu et al. 2015; Zhou et al. 2013), was received much attention and widely used in organic pollutants degradation, particularly owing to its high physicochemical stability, adjustable electronic structure, molecular tunability, suitable specific surface area and appropriate band gap for absorbing blue light (Butchosa et al. 2014; Chai et al. 2012; Chen et al. 2020; Fan et al. 2016; Liu et al. 2015; Samanta et al. 2014; Wang et al. 2020a; Wang et al. 2019; Wang et al. 2020b; Yan et al. 2009a). Nevertheless, the disadvantages of g-C₃N₄ cannot be ignored, such as insufficient light absorption (Li et al. 2009a), low surface area and rapid recombination rate of photo-generated carriers (Chang et al. 2018; Zhang & Xia 2011), which limited the photodegradation properties.

Several strategies, including chemical doping, morphological control, electronic structural design and defects engineering have been explored to overcome the shortcomings of g-C₃N₄ and enhance the photocatalytic performance (Gao et al. 2012; Katsumata et al. 2014; Ou et al. 2015; Qi et al. 2018). Among these strategies, metal elements doping of g-C₃N₄ have been commonly exhibited much advantages and was regarded as a feasible method to improve photodegradation properties (Schwinghammer et al. 2014; Tahir et al. 2013; Thaweesak et al. 2017; Wang et al. 2018b). First of all, metal dopants can generate lone electron pairs in pure g-C₃N₄ as it interacts with the cations, accordingly, the ability to capture electrons was enhanced (Gao et al. 2013; Ong et al. 2016; Wang et al. 2009a). Simultaneously, metal doping can extend visible light absorption range, improve charge transfer efficiency, and enhance redox capacity (Jun et al. 2013; Liu et al. 2012; Samanta et al. 2014). Meanwhile, metal doping can effectively adjust the morphology and crystal phase of g-C₃N₄, consequently, the photocatalytic activity of the catalyst was enhanced by introducing oxygen vacancies or constructing heterogeneous structures (Fan et al. 2019; Gao et al. 2012; Huang et al. 2019; Zhu et al. 2017).

In this paper, an ingenious cadmium doped graphite phase carbon nitride photocatalyst was successfully constructed via 60°C oil bath method, different proportions of cadmium were introduced into porous g-C₃N₄ photocatalyst. To the best of our knowledge, there is rarely reported of cadmium doped g-C₃N₄ for photocatalytic degradation. The experimental and characterization results of photodegrading TC under visible light irradiation demonstrated that cadmium doped graphite phase carbon nitride exhibited a higher light absorption ability and surface area, fast transfer of photo-generated carriers and correspondingly improved photocatalytic performance. This study elaborated the mechanism of cadmium doping for photoactivity improvement in detail and offers an innovative viewpoint for further chemistry doped of g-C₃N₄.

2.1. Chemicals

All the chemicals were of analytical grade. Cadmium chloride (CdCl₂), polyvinyl alcohol (PVA), melamine, tetracycline (TC), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), anhydrous ethanol, isopropanol (IPA), 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPO), deionized water (H₂O), were employed without further purification.

2.2. Synthesis of g-C₃N₄

The g-C₃N₄ powder was prepared via traditional calcination method with little revision, the specific steps were as follows (Wang et al. 2021): Weighing 10 g melamine into a porcelain alumina crucible (50 ml), covered with tin foil. Then the crucible was placed into muff furnace with a heating rate of 10°C /min, kept at 550°C for 3 h. After the porcelain alumina crucible was cooled down to room temperature, the obtained g-C₃N₄ powder was grounded and collected for further use.

2.3. Synthesis of Cd-g-C₃N₄

All Cd doped g-C₃N₄ samples was prepared through the thermal polymerization of melamine in the presence of CdCl₂. 1.0 g g-C₃N₄ sample was taken and dissolved in 20 mL of ethanol/H₂O solution (15/5, vol/vol), then put different amounts of CdCl₂ into the solution. After ultrasound for half an hour, the solvent evaporated in an oil bath under magnetic agitation at 60°C. Whereafter, the remaining solids were placed in a petri dish, covered with plastic wrap and desiccated in vacuum drying oven at 60°C for 24 h, when cooled to room temperature, the obtained samples were poured into a mortar to thoroughly ground. Then the samples were placed into muff furnace with a heating rate of 10°C/ min, kept at 550°C for 3 h. When the product was static cooled, the product was marked as X-Cd-g-C₃N₄ photocatalysts (X = 5, 10, 15) with different cadmium chloride contents. The cadmium chloride contents in carbon nitride material samples were listed in Table S1 and Scheme S1 illustrated the brief diagram of 10-Cd-g-C₃N₄ prepared process.

2.4. Characterization

X-ray diffraction (XRD) was recorded by D8 advance via Cu-Kα radiation, the scan ranges of 2θ extended from 10 º to 80 º with a scan speed of 8°/min. The morphological structure was obtained on JSM-7800F scanning electron microscopy (SEM). Escalab 250Xi spectrometer via Al Kα X-ray source was employed to measure X-ray photoelectron spectroscopy (XPS). The morphologies of the catalyst were observed through Jeol 2100F transmission electron microscopy (TEM). The photoluminescence spectra (PL) spectra were investigated by Fluoromax-4 spectrofluorometer at 380 nm excitation wavelength. Fourier transform infrared spectroscopy (FT-IR) was measured by Bruker spectrometer, with the wavenumber ranging from 500 to 4000 cm⁻¹. Ultraviolet–visible diffuse reflectance absorption spectroscopy (UV-Vis DRS) was performed on Cary 300 UV-Vis spectrophotometer, scanning from 200 ~ 800 nm. The Brunauer–Emmett–Teller (BET) surface area was carried out by Micromeritics ASAP2460. Electrochemical impedance spectroscopy (ESR) was analyzed by Bruker a300. The total organic carbon (TOC) was tested via Analytikjena multi N/C 2100. The chemical element compositions were analyzed by the energy dispersive spectroscopy (EDS) mapping images, captured on a Zeiss Sigma 300 atomic resolution analytical microscope.

2.5. Photocatalytic process

A 300 W Xenon lamp with 420 nm pass filter was employed to simulate sunlight for catalytic process. Typically, 0.04 g catalyst was weighed in a 100 mL beaker, 50 mL TC solution at 10 mg/L was poured in the beaker and stirred at 450 rpm/s for photocatalysis studies. In order to eliminated the effect of adsorption, we placed the mixture solutions in the dark and stirred for 0.5 hour until the adsorption-desorption equilibrium. During the light source was added, TC suspension was collected every 10 min with a 0.22 µm filter membrane. Then the concentration of suspension was tested by UV-Vis spectrophotometer with wavelength at 357 nm.

2.6. Photoelectrochemical process

The photoelectrochemical (PEC) test of catalyst was conducted on CHI0-660D workstation. A 300 W Xenon lamp with 420 nm pass filter was used as light source. The PEC performance of g-C₃N₄ and 10-Cd-g-C₃N₄ were evaluated using a normal three-electrode system with 0.2 M Na₂SO₄ solution. In the three electrodes system, Ag/AgCl electrode was reference electrode, Pt electrode was counter electrode and working electrode was fluorine-doped tin oxide (FTO) with diluted catalyst. The adhered process was depicted as follows: FTO was firstly ultrasonic cleaned in ethanol, acetone and deionized water for three times, respectively. Secondly, 10 mg catalyst was uniformly immersed into 150 uL naphthol for ultrasonic oscillation. Finally, 100 µL suspension was evenly dropped onto FTO and dried at 120°C.

3.1. Structure and morphology

The structure and crystalline phase were analyzed by XRD. Fig. 1a confirmed g-C₃N₄ has two diffraction peaks at vicinity of 12.9° (100) and 27.5° (002), which were in good agreement with the standard XRD pattern of graphitic carbon nitride (JCPDS 87-1526). The peak at 12.9° was confirmed by the in-planar tri-s-triazine structural ordering of the conjugated aromatic system, and the peak at 27.5° was represented the inter-planar periodic lamellar ordering of typical graphite-like carbon nitride (Li et al. 2016). According to Bragg’s Law, the distance between the in-plane layers was about 0.69 nm, and the distance between the inter-planar layers was about 0.33 nm (Lu et al. 2015; Zou et al. 2019). On the contrary, the intensities of both two peaks were significantly abated, and the peaks were minor shifted in 10-Cd-g-C₃N₄ samples, indicating that the Cd elements incorporated into the g-C₃N₄ lattice and resulting to the reduced hydrogen bond effect (Sher et al. 2021), as a consequence, the distance between the inter-planar layers of g-C₃N₄ was increased (Thaweesak et al. 2017; Yu et al. 2016). These findings confirmed the successful doping of Cd, and the introduction of Cd decreased the crystallinity of g-C₃N₄ sample (Abu Hanif et al. 2021). Apparently, there were no peak of cadmium in 10-Cd-g-C₃N₄ sample, which was due to the small content of cadmium and high dispersion in solution (Jia et al. 2019; Wang et al. 2020b).

The functional groups in the g-C₃N₄ and 10-Cd-g-C₃N₄ were determined by FT-IR. As illustrated in Fig. 1b, the pure g-C₃N₄ presented numerous absorption bands. The peak at 809 cm⁻¹ was assigned to the typical out-of-plane bending vibration of tri-s-triazine-based structure (Liu et al. 2020), which verified the presence of triazine units (Wang et al. 2018a; Yan et al. 2009b). Analogously, five characteristic peaks were found in the region of 1200–1700 cm⁻¹, which appeared at 1245, 1327, 1412, 1573 and 1645 cm⁻¹, respectively, indicating the distinctive stretching mode of aromatic C–N heterocycle (C₆N₇) (Park et al. 2011; Sobhana S S et al. 2011). Similarly, the wide region from 3000 to 3500 cm⁻¹ were ascribed to the stretching mode of the partial hydrogenation of exposed nitrogen (N-H) and the adsorbed water molecules (O-H) (Tonda et al. 2014; Wang et al. 2018a; Yan et al. 2016). Obviously, the 10-Cd-g-C₃N₄ sample exhibited almost similar FT-IR features with g-C₃N₄, indicating the primary framework of g-C₃N₄ was well preserved. Specifically, a weak peak was found in the region of 2150 cm⁻¹, which may assign to the additional stretching mode caused by the interaction of cadmium and g-C₃N₄ (Sher et al. 2021). Additionally, the stretching mode of the partial hydrogenation of exposed nitrogen (N-H) was shifted and weakened, which corresponded to the Cd dopant was incorporated into g-C₃N₄ (Yang et al. 2013; Li et al. 2016).

The morphologies of as-prepared samples were examined by SEM. Fig. 2a and Fig. 2b presented the image of pure g-C₃N₄, which were layers and porous structure piled up in an irregular manner (Narkbuakaew & Sujaridworakun 2020). Also, there were large cross-sectional area between layers, which were excellent doping sites (Sher et al. 2021). After Cd doping, 10-Cd-g-C₃N₄ sample was composed of thin layer structure stacking with scattered crystals particles Fig. 2(c-d), revealing the aggregated nanosheets of g-C₃N₄ was exfoliated, the crystallinity of g-C₃N₄ was decreased (Abu Hanif et al. 2021). Elemental mapping illustrated that Cd element was homo-dispersed at the surface of g-C₃N₄, suggested the success doping of Cd (Figure S1, supporting information). The elements weight ratio of 10-Cd-g-C₃N₄ sample were illustrated in Table S2, where the elements C was 25.73%, N was 61.98% and Cd was 12.28%, indicating the low concentration of Cd compared with others elements, which confirmed the cadmium doped on the g-C₃N₄ with a slight atomic ratio. The energy dispersive X-ray spectra (EDS) were illustrated in Fig. S1a, and only three peaks (C, N, Cd) were presented in the EDS mapping spectrum, confirming that the C, N, Cd elements were homogeneous distribution and the prepared 10-Cd-g-C₃N₄ sample was highly pure.

The detailed morphologies and microstructure were further surveyed by TEM. All the prepared samples were nanometer range particle size. As depicted in Fig. 3a and Fig. 3b, it could be apparently seen that the pure g-C₃N₄ was transparent or non-transparent, owing to the overlap of multilayered nanosheets. Simultaneously, as confirmed in Fig. 3c and Fig. 3d, the 10-Cd-g-C₃N₄ exhibited an ultrathin nanosheets structure, which was beneficial for increasing the specific surface area. Analogously, a typical silk nanosheet with smooth external surface of g-C₃N₄ was observed, which manifested that the graphene-like layers were uniformly distributed and randomly oriented. Obviously, the elemental mapping (Fig. S1 and Fig. 3(e-i)) confirmed the cadmium elements were uniform interspersing at the surface of g-C₃N₄, and exhibited high intercontact with g-C₃N₄ due to the electrostatic attraction. The results were consistent with previous study that the electron-rich Cd or CdS clusters were firmly anchored on the electron-unsaturated g-C₃N₄ surface, which illustrated that the cadmium was doped on the g-C₃N₄ surface (Zhang et al. 2016). Fig. 3i illustrated the amorphous structure of 10-Cd-g-C₃N₄ sample. The selected diffraction pattern (Fig. 3j) shows two distinct amorphous diffraction rings, which correspond to two peaks of carbon nitride in the XRD pattern and correspond to the XRD data. All these results clarified that 10-Cd-g-C₃N₄ samples were successfully constructed.

The BET specific surface area, pore volume and pore size distribution were tested by N₂ adsorption-desorption and the results were depicted in Table S3 and Fig. S2. The specific surface area of 10-Cd-g-C₃N₄ was 16.46 m²/g, and the pore volume was 0.123 cm³/g, whereas the surface area of pure g-C₃N₄ was 13.467 m²/g, and the pore volume was 0.077 cm³/g. Obviously, the 10-Cd-g-C₃N₄ sample obtained a larger surface area and pore volume than pure g-C₃N₄. Analogously, the porosity of 10-Cd-g-C₃N₄ was obviously increased, which related to the cracks of light layer during crystal growth process or the evaporation of chlorine gas during heating process. Undoubtedly, larger surface area and pore volume could provide more reaction active sites, increase abundant light absorption area and accelerate the transfer of photogenerated carriers, accordingly, achieving a higher photocatalytic degradation rate (Chang et al. 2015). The N₂ adsorption-desorption isotherm (Fig. S2a) of 10-Cd-g-C₃N₄ was exhibited a type IV curve with H3 hysteresis hoop, which represented the presence of slit shaped mesoporous structure (Iqbal et al. 2017).

The element chemical bonding states and electronic structure details were investigated by XPS analysis. Fig. 4a characterized the full survey spectrum of g-C₃N₄ and 10-Cd-g-C₃N₄. Obviously, the pure g-C₃N₄ sample was consisted of carbon (C), nitrogen (N), oxygen (O) element and the 10-Cd-g-C₃N₄ sample was consisted of C, N, O and cadmium (Cd) element. The presence of O element was caused by carbon nitride thermal polymerization process or catalyst surface absorption of water (Chou et al. 2016; Fang et al. 2016; Xue et al. 2019). Equally, no signal of chlorine (Cl) element (around 200 eV) could be observed, since Cl atoms were evaporated during heating process (Amirthaganesan et al. 2010).

The peaks of Cd 3d were situated at 405.6 eV and 412.4 eV in Fig. 4b, which were ascribed to the Cd 3d_5/2 and Cd 3d_3/2 (Reddy et al. 2021), indicating the existence of Cd²⁺ in 10-Cd-g-C₃N₄ (Zhang et al. 2016a), and further confirming the presence of Cd dopant (Abu Hanif et al. 2021). Similarly, Fig. 4c elucidated the high-resolution C1s spectra, two sharp peaks were situated at 284.8 and 288.3 eV, respectively. The former peak at 284.8 eV was specifically on behalf of carbon atoms (C–C bonds), namely graphite or amorphous carbon (Liu et al. 2010; Takanabe et al. 2010), and the latter sharper peak at 288.4 eV resulted from sp² C atoms bonded with adjacent N atoms inside the aromatic structure (N–C=N) (Thomas et al. 2008). Obviously, three peaks could be perceived from the high-resolution N 1s spectra (Fig. 4d), the peak at 398.8 eV originated from sp²-bonded N atoms in triazine units (C=N–C) (Li et al. 2009b; Takanabe et al. 2010), the peak at 401.1 eV corresponded to amino groups (C–N–H) from the surface uncondensed bridging N atom (Liu et al. 2010; Thomas et al. 2008), which were similar to previously published literature (Chao et al. 2014; Matanović et al. 2015). The peak at 404 eV was assigned to the effects of surface charge localization in the heterocycles or the π–π* excitations between the stacking interlayers (Kong et al. 2018; Li et al. 2018; Zhang et al. 2014; Zhang et al. 2012). Both the high-resolution C 1s and N 1s spectra confirmed the presence of g-C₃N₄. Simultaneously, both C 1s and N 1s spectra of 10-Cd-g-C₃N₄ sample exhibited an upwards shifted compared with pure g-C₃N₄, all these positive shifts could be ascribed to the electrons transfer of 10-Cd-g-C₃N₄ sample, further proving the strong interaction between Cd elements and g-C₃N₄ (Ji et al. 2019; Yan et al. 2019).

3.2. Optical properties

The optical absorption characteristics was an evaluation criterion to optical properties and electronic band structures of catalyst. Simultaneously, the optical absorption range and capacity of catalysts affected the photocatalytic performance. Fig. 5a revealed the UV–vis diffuse adsorption spectrum of g-C₃N₄ and 10-Cd-g-C₃N₄. An absorption threshold at 465 nm was found in pure g-C₃N₄. Obviously, the absorption threshold of 10-Cd-g-C₃N₄ had a significantly redshift toward longer wavelengths compared with pure g-C₃N₄, which illustrated that Cd doping may form a new level in g-C₃N₄ and cause a narrower band gap (Abu Hanif et al. 2021). Moreover, the intensity of light adsorption was significantly enhanced in 10-Cd-g-C₃N₄ sample, which further confirmed the improvement of light absorption performance. Additionally, 10-Cd-g-C₃N₄ extended the adsorption edges to near 600 nm, this extension is conducive to increasing the light absorption of g-C₃N₄ and increasing the generation of electron–hole pairs (Chen et al. 2021). Simultaneously, the UV–vis diffuse adsorption spectrum of other samples with different cadmium contents was clarified in Fig. S3a. Among these samples, the 10-Cd-g-C₃N₄ sample displayed the highest visible light absorption in a long range.

Both g-C₃N₄ and 10-Cd-g-C₃N₄ sample are direct-gap materials. The band gap (E_g) of g-C₃N₄ and 10-Cd-g-C₃N₄ were calculated based on the related Tauc plots in Fig. 5b (Luo et al. 2017). Noticeably, the band gap of pure g-C₃N₄ was assigned to 2.57 eV (Kong et al. 2018; Wang et al. 2009b; Zhang et al. 2012), and the band gap of 10-Cd-g-C₃N₄ was 2.29 eV. Undoubtedly, the narrower band gap reduced the excitation energy for photogenerated carriers, leading to a significant improvement in the visible light response. As a consequence, Cd doping can significantly affect the light absorption performance of g-C₃N₄ (Ge et al. 2012).

The charge separation, migration and recombination were analyzed. Fig. 6a indicated the PL spectra of 10-Cd-g-C₃N₄ and g-C₃N₄ samples. Commonly, the weakened PL intensity meant enhanced photoinduced charge separation and transfer efficiency. Apparently, the strong peak at 460 nm in pure g-C₃N₄ was consistent with the absorption edge of the UV-vis DRS, and indicating the severe recombination of the photogenerated electron−hole pair (Yu et al. 2013). Equally, the highest intensity peak demonstrated a red-shifted from 460 nm to 490 nm in 10-Cd-g-C₃N₄, which was associated with the band gap narrowing effect (Gu et al. 2018; Zou et al. 2019). Analogously, the PL spectra of other samples with different cadmium contents was manifested in Fig. S3b. Obviously, the PL spectra of Cd-g-C₃N₄ sample significantly fell with the doping of cadmium, which could ascribe to the fast interfacial charge migration. Additionally, the PL spectra of 10-Cd-g-C₃N₄ was lowest, which illustrated that Cd doping restrained the recombination of photoinduced charge due to the strong contact between cadmium and g-C₃N₄ (Thomas et al. 2008).

Figure 6b was a Mott-Schottky plots with flat band potentials at 1000 Hz. Commonly, conduction band potential is equivalent to flat band potential for an n-type semiconductor. Both g-C₃N₄ and 10-Cd-g-C₃N₄ displayed a positive slope, indicating g-C₃N₄ and 10-Cd-g-C₃N₄ are both n-type semiconductors. Meanwhile, the Mott-Schottky slope of 10-Cd-g-C₃N₄ was abated compare with pure g-C₃N₄, owing to the increased electrical conductivity and the quick mobility of charge carriers, which revealing the electron donor density was increased in 10-Cd-g-C₃N₄ (Jun et al. 2013; Pan et al. 2018; Yang et al. 2013; Yuan et al. 2018; Zhou et al. 2014). As a consequence, higher electron donor density is helpful for improving photocatalytic performance (Zhou et al. 2014). The conduction band potential of g-C₃N₄ and 10-Cd-g-C₃N₄ concluded from the Mott-Schottky plots was -0.84 eV and -1.02 eV, compared with Ag/AgCl electrodes, corresponding to -0.64 eV and -0.82 eV vs normal hydrogen electrode (NHE).

Metal elements doping influenced the band edge of catalyst to a great extent, and the redox capacity of semiconductors were evaluated via the band edge position of conduction (E_CB) and the valence (E_VB) (Xiong et al. 2016). The E_VB could obtained from following equation (Zhang et al. 2010):

E _VB = E_CB + E_g

Figure S4 revealed the E_VB and E_CB positions of as-prepared samples. For pure g-C₃N₄, the E_VB was 1.93 eV, on the basis of the experimental E_g (2.57 eV), the E_CB was -0.64 eV. Meanwhile, the E_VB of 10-Cd-g-C₃N₄ was 1.47 eV and the E_CB was -0.82 eV, according to the experimental E_g (2.29 eV).

Other photoelectrochemical techniques such as electrochemical impedance spectroscopy (EIS) and photocurrent response measurement were employed to investigate the movement of photoinduced electrons. Fig. 6c revealed the photocurrent responses of as-prepared samples, both g-C₃N₄ and 10-Cd-g-C₃N₄ testified an outstanding reproduceible photostability under successive on/off irradiation cycles. Homoplastically, the photocurrent response density of 10-Cd-g-C₃N₄ was significantly increased compare with pure g-C₃N₄, indicating the introduction of Cd element increased the conductivity of g-C₃N₄ and accelerated the separation of electron-hole pair (Ren et al. 2017). EIS was another method to evaluated the separation and transfer efficiency of photoinduced carriers. Commonly, the smaller arc radius suggested smaller transfer resistance, namely the better separation and transfer efficiency of photoinduced carriers (Lu et al. 2017; Yang et al. 2002). As depicted in Fig. 6d, the arc radius of 10-Cd-g-C₃N₄ was smaller than pure g-C₃N₄, which reflected that 10-Cd-g-C₃N₄ possessed high-efficiency photoinduced carriers separation ability and faster interfacial charge transfer level (Zhu et al. 2015).

3.3. Photocatalytic activities

To testing the effect of different content of Cd doping on degradation performance, a group of comparative degradation experiments were carried out under the simulated visible-light irradiation (λ > 420 nm). In this paper, TC was selected as contaminant because of the high stability under visible light. As illustrated in Fig. 7a, the degradation rate of TC without catalysts can be neglected. Apparently, the adsorption capacity of all Cd doped g-C₃N₄ samples were slightly larger than that of pure g-C₃N₄ during the dark stage, which corresponding to an increase in the specific surface area of the catalyst. Obviously, all Cd doped g-C₃N₄ samples exhibits great increased photocatalytic degradation performance after 60 min visible-light irradiation compared with pure g-C₃N₄ (43.9%), confirming the doping of Cd was truly enhanced the photocatalytic efficiency (Abu Hanif et al. 2021). Apparently, the 10-Cd-g-C₃N₄ sample demonstrated the significantly improved photocatalytic degradation performance (98.1%), which can be attributed to the effective separation and transfer of photogenerated charges originating from the photocatalyst interface. Additionally, the photocatalytic degradation performance within 10 min of 10-Cd-g-C₃N₄ sample was tested (Fig. S5a), the result illustrated that the TC was degraded almost 80% within 10 minutes. Table S4 illustrated the comparison with other similar types researches, we found the removal rate of this work was excellent, by compare this work with other similar types of work.

Furthermore, the degradation performance of Cd doped g-C₃N₄ samples first enhanced with the increase of Cd content. As the content of Cd increased, the utility efficiency of visible light was promoted, as a consequence, more photoinduced carriers were provided. Unfortunately, when the doping content of Cd continuously increased, the degradation performance was decreased, the photocatalytic degradation performance of 15-Cd-g-C₃N₄ sample was only 75.9%, indicating the excessive Cd element could serve as photo-carrier’s recombination center or blocked the surface active site to restrained the light absorption capacity and thus inhibiting photocatalytic degradation performance (Li et al. 2009a; Zhang et al. 2016b; Zhang et al. 2010).

The pseudo-first-order kinetic model could further investigate the degradation performance of the as-prepared catalyst, and the formula was expressed as follow:

ln (C_t/C₀) = – K_appt

C₀ is the initial concentrations, C_t is the concentrations at time t, and K_app is reaction rate constant (min⁻¹), respectively. Fig. S5b presented the pseudo-first-order kinetic plots of Cd-g-C₃N₄ samples and pure g-C₃N₄. Under the same condition, the K_app of Cd-g-C₃N₄ samples were higher than pure g-C₃N₄ and the K_app of 10-Cd-g-C₃N₄ sample was the highest, demonstrating that 10-Cd-g-C₃N₄ had the best photocatalytic performance, which was consisted with the above experimental results.

The effects of different reaction conditions, such as initial concentration of TC, catalyst dosage and pH, were studied to meet practical application. Fig. 7b displayed the influence of different catalyst dosages on TC photodegradation. Apparently, as the catalyst dosages increased between 0.6 and 0.8 g/L, the removal rate of TC was enhanced monotonically, which was ascribed to the increased contact active sites between contaminants and materials, and more contaminants were absorbed on the surface of materials. Apparently, the catalyst dosages at 0.8 g/L elucidated the best photocatalytic degradation performance. Nevertheless, when the catalyst dosages over 0.8 g/L, the removal rate of TC was abated slightly, the photocatalytic degradation efficiency was 82.3% when the catalyst dosages at 1.0 g/L, which owing to the excessive catalyst reduced the photo-adsorption ability, as a result, hindering the effective migration of charge carriers (Zhang et al. 2016a).

The effect of different pH on TC photodegradation was clarified in Fig. 7c, and diluted hydrochloric acid and sodium hydroxide were used to adjust the initial pH. Obviously, the best degradation efficiency of TC was 94.4% at pH=5. Consequently, it was concluded that degradation efficiency was decreased in low pH values and alkaline environment, which can be explained by the surface charge of contaminants and materials. Additionally, the effect of initial concentration of TC was illuminated in Fig. 7d. It could be notably detected that the removal rates were abated with the initial concentration increased, and the best removal rate was 96.7% at 10 mg/L of TC. Two possible reasons could be proposed to elucidate this tendency. Firstly, high concentrations of contaminants might accumulate at the surface of catalyst, inhibiting the light absorption capacity. Secondly, intermediate products produced in the process of contaminants degradation might occupy the surface active site, so that contaminants had no contact with the catalyst.

In order to evaluated the practicability for catalysts, the stability tests were carried out by reusing the catalyst. Fig. 8a depicted the degradation rates of TC in four consecutive cycles. Obviously, the degradation rates were slightly decreased with the reuse of 10-Cd-g-C₃N₄, which could be attributed to the catalyst loss during the collection process. Whereas, the degradation rates still maintained a high level (89.5%) at the fourth cycle. Simultaneously, Fig. 8b demonstrated the XRD analysis of catalyst, no obvious peak changes were found in used catalyst, indicating the outstanding stability of 10-Cd-g-C₃N₄.

The ability of mineralization of note is an important index in photocatalytic process, which is used to ascertain the degree of contaminants removal. As intriguingly indicated in Fig. S4b, the mineralization efficiency of TC reached to 34% within 60 min visible light irradiation, which confirmed that 10-Cd-g-C₃N₄ could actually degrade TC into small molecule intermediate compounds (CO₂, H₂O).

3.4. Mechanism

The trapping experiments were used to determine the active species participating in the degradation process (Li et al. 2015). EDTA-2Na, IPA and TEMPO were added as scavengers to capture h⁺, •OH and •O₂⁻, respectively. Fig. 9a and Fig. 9b illustrated that the degradation rate of TC was suppressed by EDTA-2Na, indicating that h⁺ was the main active species of 10-Cd-g-C₃N₄. Similarly, the degradation rate of TC abated slightly when the TEMPO and IPA were added, revealing that •O₂⁻ and •OH played auxiliary roles in photocatalytic degradation process (Yan et al. 2010; Yu et al. 2018).

The ESR was carried out to verify the above results. As illustrated in Fig. 9c and Fig. 9d, it is hard to see the ups and downs signals of active species in 10-Cd-g-C₃N₄ under dark conditions. Paradoxically, the strong intensity 1:2:2:1 signals of •OH and 1:1:1:1 signals of •O₂⁻ radicals could be found under the visible light irradiation, which proving the visible light is prerequisite for 10-Cd-g-C₃N₄ to product •OH and •O₂⁻ active species.

According to all aforementioned results, the introduction of Cd element speeding up the separation and transfer of electron-hole pairs, decreasing the recombination rate of photogenerated charge carriers, providing more reactive sites and accelerating cross-plane diffusion in g-C₃N₄ nanosheets (Iqbal et al. 2017), thus achieving the enhanced photocatalytic property. A brief mechanism of 10-Cd-g-C₃N₄ sample during TC degradation was summarized in Scheme 1. The electrons and holes were firstly generated and separated under light irradiation, and the electrons transferred from valence band to conduction band spontaneously, as a consequence, leaving holes at valence band and electrons accumulated at conduction band (Eq. (1)). Then, the absorbed O₂ reacted with electrons to formed superoxide radical (Eq. (2)). Similarly, the surplus electrons reacted with superoxide radical to formed hydrogen peroxide (H₂O₂) (Eq. (3)). As listed in Fig. S4a, the E_CB of 10-Cd-g-C₃N₄ was negative than O₂/•O₂⁻ (-0.33 eV vs NHE), as a result, the accumulated electrons could react with O₂ to formed •O₂⁻. But the E_VB of 10-Cd-g-C₃N₄ was negative than OH⁻/•OH (2.40 eV vs NHE), consequently, the accumulated holes could not react with OH⁻ to formed •OH, which was not consistent with ESR result. Consequently, the •OH probably derived from H₂O₂ decomposition (Eq. (4)). From what has been discussed above, •O₂⁻, •OH and h⁺ were worked together to degrade TC into small molecules (Eq. (5)). The reaction equations involved in the photocatalytic reaction process are manifested below (Jia et al. 2020).

10-Cd-g-C₃N₄ + hv → e⁻ + h⁺ (1)

O₂ + e⁻ → •O₂⁻ (2)

•O₂⁻ + e⁻ + 2H⁺ → H₂O₂ (3)

H₂O₂ + e⁻ → •OH + OH⁻ (4)

•O₂⁻ + h⁺ + •OH + TC → H₂O + CO₂ + intermediate products (5)

In summary, the commendable 10-Cd-g-C₃N₄ photocatalyst were successfully prepared via oil bath by mixing dopant agent CdCl₂ with g-C₃N₄ in ethanol/H₂O solution. The degradation experiments demonstrated explicitly that the photocatalytic activity of g-C₃N₄ could be enhanced significantly with the doping of Cd element, and the best removal efficiency was 98.1% under visible light irradiation, which was 2.0-times enhanced in comparison with pure g-C₃N₄ (43.9%). Based on the characterization analysis, the enhanced photocatalytic performance could be ascribed to the following reasons: firstly, the enhanced photo-absorption ability, accelerating the migration, separation and transfer efficiency of photogenerated carriers. Meanwhile, the ultrathin mesoporous structure and the increased surface area could provide more active sites along with a shortened electron-transfer pathway, which was conducive to inhibiting the recombination of the photogenerated electrons and holes. As a consequence, this paper has demonstrated the 10-Cd-g-C₃N₄ sample can the significantly photocatalytic degradation tetracycline.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52070077, U20A20323, 51521006, 51879101), the National Program for Support of Top–Notch Young Professionals of China (2014), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), the Fundamental Research Funds for the Central Universities (531118010226).

Availability of data and materials All data generated or analyzed during the current study are included in this article and its supplementary information files.

Authors contributions YZ: Responsible for writing papers, conducting experiments, drawing the chart and analyzing the characterization part. HQ, ZW, HW, YH, QT, QL, PX: Responsible for reviewing the article and providing suggestions for revisions. The first draft of the manuscript was written by YZ and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.”

Ethical approval Not applicable.

Consent to participate Not applicable.

Consent to publish Not applicable.

Competing interests The authors declare that they have no competing interests.

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Scheme 1 are available in the Supplemental Files section

Scheme1.jpg
The transfer and separation of charges and degradation mechanisms in the 10-Cd-g-C₃N₄ under visible-light irradiation.
SupportingInformation.docx

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Editorial decision: Major Revision
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25 Feb, 2022
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03 Dec, 2021
First submitted to journal
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You are reading this latest preprint version

Facile Synthesis of Cadmium Doped Graphite Carbon Nitride for Photocatalytic Degradation of Tetracycline Under Visible Light Irradiation

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2.1. Chemicals

2.2. Synthesis of g-C₃N₄

2.3. Synthesis of Cd-g-C₃N₄

2.4. Characterization

2.5. Photocatalytic process

2.6. Photoelectrochemical process

3. Results And Discussion

3.1. Structure and morphology

3.2. Optical properties

3.3. Photocatalytic activities

3.4. Mechanism

4. Conclusion

Declarations

References

Schemes

Supplementary Files

Status:

Version 1

Facile Synthesis of Cadmium Doped Graphite Carbon Nitride for Photocatalytic Degradation of Tetracycline Under Visible Light Irradiation

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2.1. Chemicals

2.2. Synthesis of g-C3N4

2.3. Synthesis of Cd-g-C3N4

2.4. Characterization

2.5. Photocatalytic process

2.6. Photoelectrochemical process

3. Results And Discussion

3.1. Structure and morphology

3.2. Optical properties

3.3. Photocatalytic activities

3.4. Mechanism

4. Conclusion

Declarations

References

Schemes

Supplementary Files

Status:

Version 1

2.2. Synthesis of g-C₃N₄

2.3. Synthesis of Cd-g-C₃N₄