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

Using semiconductor photocatalysts for antibiotic contaminants degradation under visible light has become a hot topic in recent years. Herein, a novel cadmium doped graphite phase carbon nitride (Cd/g-C 3 N 4 ) 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-C 3 N 4 surface and exhibited high intercontact with g-C 3 N 4 . Meanwhile, Cd/g-C 3 N 4 presented excellent electrical conductivity and inhibited the recombination of electron-hole pairs. The introduction of cadmium signicantly improved the photocatalytic activity and the degradation eciency of 10 Cd/g-C 3 N 4 reached to 89.09%, which was exceeded 2.0 times than pure g-C 3 N 4 (43.99%). Additionally, the quenching experiments and electron spin-resonance tests exhibited holes (h + ), hydroxyl radicals (•OH) and superoxide radicals (•O 2− ) were dominated active species in TC degradation. Furthermore, the effects of various conditions on the reaction process, such as different pH, initial TC concentrations and catalyst dosage, were also researched. This work gives a reasonable point to synthesize high-eciency and economic photocatalysts.


Introduction
For the past few years, researchers have been seeking suitable ways to solve the overwhelming environmental problems, especially caused by the continuous increase of population and energyintensive industry (Abbott 2010, McCauley &Stephens 2012. Photocatalysis, as an environmental-friendly process, has become the signi cant method applied in energy and environmental remediation eld (Ge et al. 2011, Wang et al. 2008, Zhu et al. 2017. Undoubtedly, the most important tasks in photocatalysis research are designing high-e cient, stable and low-cost photocatalysts, which should include excellent light absorption capacity, non-toxic, narrow band gap, controllable morphology and good redox ability (Xiang et al. 2012). Recently, metallic oxides, metallic nitrides, metallic sul des were 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. Whereas, some obvious defects have hindered their further application. In comparison, more recently discovered carbon nitride in 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 ve phases (alpha, beta, cubic, quasi-cubic and graphite phase) of carbon nitride, graphite phase carbon nitride (g-C 3 N 4 ) was widely used in organic pollutants degradation, owing to its high physicochemical stability, adjustable electronic structure and molecular tunability (Butchosa et al. 2014, Fan et al. 2016, Samanta et al. 2014). Particularly, g-C 3 N 4 , as a metal-free and environmentally benign conjugated catalyst (Lu et al. 2015, Zhou et al. 2013, possesses narrow diffusion length (Yan et al. 2009a), suitable speci c surface area and appropriate band gap for absorbing blue light (Chai et  However, the disadvantages of g-C 3 N 4 cannot be ignored, such as insu cient light absorption , and rapid recombination rate of photo-generated carriers (Chang et al. 2018, Zhang &Xia 2011. Several strategies, including chemical doping, morphological control, electronic structural design and defects engineering have been explored to overcome inherent shortcomings of g-C 3 N 4 and subsequently enhance the photocatalytic performance (Gao et al., Katsumata et al. 2014, Ou et al. 2015, Qi et al. 2018).
Among these strategies, metal elements doping of g-C 3 N 4 is regarded as a fungible and feasible method to improve the photocatalytic performance. And the reported researches have been commonly exhibited the advantages of metal elements doping g-C 3 N 4 , rst of all, metal dopants can generate lone electron pairs in g-C 3 N 4 as it interacts with the cations, thus enhancing its ability to capture electrons (Gao et al. 2013, Ong et al. 2016, Wang et al. 2009a). In addition, metal doping can effectively adjust the morphology and crystal phase of g-C 3 N 4 , thus introducing oxygen vacancies or constructing heterogeneous structures to enhance the photocatalytic activity of the catalyst. Meanwhile, metal doping can extend visible light absorption range, improve charge transfer e ciency, and enhance redox capacity (Fan et al. 2019, Jun et al. 2013, Samanta et al. 2014, Schwinghammer et al. 2014, Wang et al. 2018b).
In this paper, different proportions of cadmium were introduced into porous g-C 3 N 4 photocatalyst to degraded TC via oil bath and calcination method. To the best of our knowledge, there is rarely reported of cadmium doped g-C 3 N 4 for photocatalytic degradation. The experimental results of photodegrading TC under visible light irradiation demonstrated that Cd/g-C 3 N 4 possessed the higher photocatalytic performance in comparison with that of pure g-C 3 N 4 . This study offers an innovative viewpoint for g-C 3 N 4 modi cation for further chemistry doped and elaborates the mechanism of cadmium doping for photoactivity improvement in detail.
2.2. Synthesis of g-C 3 N 4 g-C 3 N 4 was prepared via traditional calcination method and the speci c steps were as follows (Wang et al. 2021): Weighing 10 g melamine into a porcelain alumina crucible, covered with tin foil, then the crucible was placed into muff furnace with a heating rate of 10°C /min and kept at 550°C for 3 h. After the porcelain alumina crucible was cooled down to room temperature, the obtained g-C 3 N 4 powder was grounded and collected for further use.
2.3. Synthesis of Cd/g-C 3 N 4 Cd/g-C 3 N 4 was prepared through the thermal polymerization of melamine in the presence of CdCl 2 . 1.0 g g-C 3 N 4 sample was taken and dissolved in 20 mL of ethanol/H 2 O solution (15/5, vol/vol), then put different amounts of CdCl 2 into the solution. After ultrasound for half an hour, the solvent evaporated in an oil bath under magnetic agitation at 60°C. The remaining solids were placed in a petri dish, covered with plastic wrap and dried 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 5°C/ min, kept at 550°C for 3 h. When the product was static cooled, Cd/g-C 3 N 4 composite photocatalysts with different cadmium contents were prepared. The product was ground into powder and marked as X Cd/g-C 3 N 4 , (X = 3, 5, 7, 10, 15, 20, 30) according to different mass ratios.

Characterization
X-ray diffraction (XRD) of Cd/g-C 3 N 4 was obtained 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 used 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 spectro uorometer 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 re ectance 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 also analyzed by the energy dispersive spectroscopy (EDS) mapping images captured on a Zeiss Sigma 300 atomic resolution analytical microscope.

Photocatalytic process
A 300 W Xenon lamp with 420 nm pass lter in the laboratory was employed to simulate sunlight for catalytic process. Typically, 0.04 g catalyst was weighed and put in a 100 mL beaker. Then 50 mL TC solution at 20 mg/L was poured in the beaker and put it on a blender and set the rotate speed 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 syringe and ltered with 0.22 µm membrane. The samples were taken 6 times in an hour. Then the concentration of suspension was tested by UV-Vis spectrophotometer with wavelength at 357 nm.

Photoelectrochemical process
The photoelectrochemical test of catalyst was obtained from the instrument of CHI0-660D workstation. The PEC performance of the g-C 3 N 4 and Cd/g-C 3 N 4 electrodes was evaluated using a normal threeelectrode system with 0.2 M Na 2 SO 4 solution under illumination or in the dark. In the three electrodes system, Ag/AgCl electrode was reference electrode, Pt electrode was counter electrode and working electrode was 150 µL diluted catalyst adhered to uorine-doped tin oxide (FTO). The load operation process was depicted as follows: FTO was rstly ultrasonic cleaned in ethanol, acetone and deionized water for three times, respectively. Secondly, 10 mg catalyst was uniformly imported into 150 uL naphthol for ultrasonic oscillation. Finally, 100 µL suspension was evenly dropped onto FTO and dried at 120°C. In the three-electrode system, a 300 W Xenon lamp with 420 nm pass lter in the laboratory was used as light source, then 0.2 M Na 2 SO 4 aqueous solution was poured into the system until liquid passed through the catalyst as electrolyte.

Structure and morphology
Structure and crystalline phase of pure g-C 3 N 4 and Cd/g-C 3 N 4 were analyzed by XRD. Figure 1a showed two diffraction peaks appear at vicinity of 12.9 ° (100) and 27.5 ° (002), which were labeled as the typical graphitic carbon nitride in pure g-C 3 N 4 and in good agreement with the standard XRD pattern of graphitic carbon nitride (JCPDS 87-1526). The in-planar tri-s-triazine structural ordering of the conjugated aromatic system was con rmed by the peak at 12.9 ° and the distance between the in-plane layers was about 0.69 nm. The peak at 27.5 ° represented the inter-planar periodic lamellar ordering of typical graphite-like carbon nitride, and the distance between the inter-planar layers was about 0.33 nm, calculated according to Bragg's Law (Lu et al. 2015, Zou et al. 2019). However, the intensities of both two peaks of Cd/g-C 3 N 4 were signi cantly decreased with the increasing cadmium doping, and the (100) peak almost disappeared, indicating that the introduction of the Cd affected the chemical structure and reduced hydrogen bond effect in the intralayer of g-C 3 N 4 , thus decreasing the distance between the inter-planar layers and loose packing of layers in g-C 3 N 4 (Thaweesak et al. 2017, Yu et al. 2016. It is worth noting that the (002) peak displayed a slight shift to higher angles in Cd/g-C 3 N 4 samples, suggesting the decrease of interlayer distance in Cd/g-C 3 N 4 . Apparently, there were no peak of cadmium in any form in Cd/g-C 3 N 4 composites compared with pure g-C 3 N 4 , which was due to the small content and high dispersion in solution (Jia et al. 2019, Wang et al. 2020b).

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The possible functional groups in the g-C 3 N 4 and Cd/g-C 3 N 4 were determined by FT-IR spectroscopy. As illustrated in Fig. 1b, the Cd/g-C 3 N 4 and g-C 3 N 4 exhibited almost similar FT-IR features, indicating the primary framework of g-C 3 N 4 well preserved after Cd doping. 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 veri ed the presence of triazine units (Yan et al. 2009b) and was consistent with previous ndings (Wang et al. 2018a). Four characteristic peaks were found in the region of 1200-1600 cm − 1 , which appeared at 1245, 1327, 1412 and 1573 cm − 1 , respectively, indicating that the aromatic C-N heterocycle (C 6 N 7 ) had a distinctive stretching mode (Park et al. 2011). In addition, the O-H bending vibration was observed in all samples at 1630 cm − 1 (Sobhana S S et al. 2011). Speci cally, the peak at about 1645 cm − 1 were the stretching vibration of typical C = N units. The characteristic band at 2173 cm − 1 corresponded to terminal cyano groups (C ≡ N) (Yang et al. 2013). The wide region from 3000 to 3500 cm − 1 were ascribed to the stretching mode of the N-H in aromatic groups and O-H stretching vibrations, which may be due to partial hydrogenation of exposed nitrogen and the adsorption of water molecules on the surface of the catalyst (Tonda et al. 2014, Wang et al. 2018a. The spectra of Cd/g-C 3 N 4 were basically as same as the pure g-C 3 N 4 , which con rmed that Cd doping did not change the functional group of g-C 3 N 4 . The morphology and microstructure of g-C 3 N 4 and Cd/g-C 3 N 4 were examined by SEM, and the image of g-C 3 N 4 without pre-treatment with CdCl 2 were presented in Fig. 2a and Fig. 2b, which displayed an aggregated, thick irregular platelets and scattered crystals particle structure stacking with bulky nanosheets. The image revealed the pure g-C 3 N 4 was consists of randomly folded and stacked tri-striazine layers, which were piled up in an irregular manner to formed a porous structure (Narkbuakaew &Sujaridworakun 2020). After the Cd doping, great difference could be seen in Fig. 2c and Fig. 2d, Cd/g-C 3 N 4 was composed of layer structure with some uniformly smaller particles on the surface. The thickness of the layer was thinner after employment of CdCl 2 , revealing a certain degree of exfoliation of the aggregated thick nanosheets. Additionally, the porosity of the 10 Cd/g-C 3 N 4 was obviously increased, which might be related to the produce of light layer cracks during crystal growth process and evaporation of chlorine gas during heating process. Consequently, the increased pores could provide more reaction active sites, thus enhancing the photocatalytic activity.
The detailed microstructure features and subtle morphologies of materials were further surveyed by TEM.
As depicted in Fig. 3a and Fig. 3b, it could be clearly seen that the structure of pure g-C 3 N 4 was transparent or non-transparent nanosheets because of ultrathin thickness or the overlap of a few nanosheets. Moreover, we could clearly observe a piece of a typical silk nanosheet with smooth external surface and at layer structures of the obtained g-C 3 N 4 , which manifested that the graphene-like layers were uniformly distributed and like randomly oriented. Obviously, randomly folded and stacked layer were easily piled up in an irregular manner to an agglomerate. As shown in Fig. 3c and Fig. 3d, the 10 Cd/g-C 3 N 4 exhibited a multilayered nanosheets structure and laminated crystallizes with abundant microporous. Further, the cadmium nanoparticles were well doped at g-C 3 N 4 surface and exhibited high intercontact with g-C 3 N 4 , there were many small holes in the nanosheet, which was bene cial for increasing the speci c surface area signi cantly. Fig. S1 and Table S1 illustrated the weight ratio of Cd/g-C 3 N 4 composites, elements C was 25.73%, N was 61.98% and Cd was 12.28%, which illustrated that the cadmium was well doped in the g-C 3 N 4 with a slight atomic ratio. All these results proved that Cd/g-C 3 N 4 composites were successfully constructed.
The speci c BET surface area and the pore size distribution were investigated and depicted in Table S2 and Fig. S2. Obviously, the Cd/g-C 3 N 4 sample obtained a larger speci c surface area than the monomer g-C 3 N 4 . Table S2 illustrated the speci c surface areas and pore volume of Cd/g-C 3 N 4 was 16.46 m 2 /g and 0.123 cm 3 /g, whereas monomer g-C 3 N 4 was 13.467 m 2 /g and 0.077 cm 3 /g. Undoubtedly, larger surface area and increased pore volume of Cd/g-C 3 N 4 could increase the contact area and attachment active sites for pollutant, and accelerate the transfer of photogenerated carriers, thus achieving a more e cient degradation rate (Chang et al. 2015). The results of N 2 adsorption-desorption isotherm of g-C 3 N 4 and Cd/g-C 3 N 4 were exhibited that g-C 3 N 4 and 10 Cd/g-C 3 N 4 possessed a type IV curve with H3 hysteresis hoop, which represented the presence of slit shaped mesoporous structure accumulation of particles (Iqbal et al. 2017). These results were consistent with the previous TEM images, further con rmed that Cd was successfully doped into g-C 3 N 4 .
The element chemical bonding states and electronic structure details of g-C 3 N 4 and 10 Cd/g-C 3 N 4 were investigated by XPS analysis. The full spectrum (Fig. 4a) indicated that the composite sample 10 Cd/g-C 3 N 4 was consisted of Cd ions and pure g-C 3 N 4 , which could be seen from the strong peaks of C 1s, N 1s, Cd 3d and a small number of O element. C and N elements were mainly derived from g-C 3 N 4 , and the presence of Cd was caused by Cd doping. Carbon nitride thermal polymerization process and surface absorption of water and oxygen were the main sources of oxygen element (Chou et al. 2016, Fang et al. 2016. Additionally, there was no signal of Cl atom (around 200 eV), since Cl atoms were evaporated during heating process (Amirthaganesan et al. 2010).  Figure 4c showed the high-resolution C1s spectra, and the two sharp peaks were situated at 284.8 and 288.3 eV, respectively. The former peak at 284.8 eV was speci cally 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 sp 2 C atoms bonded with adjacent N atoms inside the aromatic structure (N-C = N) . Three main peaks could be perceived from the high-resolution N 1s spectra (Fig. 4d), and the peak at 398.8 eV originated from sp 2 -bonded N atoms in triazine units (C = N-C) , Takanabe et al. 2010, and peak at 401.1 eV corresponded to amino groups (C-N-H) from the surface uncondensed bridging N atom (Liu et al. 2010, 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 charging or positive charge localization in the heterocycles (Kong et al. 2018), or the π-π* excitations between the stacking interlayers . Both the high-resolution C 1s and N 1s spectra con rmed the presence of g-C 3 N 4 .
Obviously, both C 1s and N 1s spectra of 10 Cd/g-C 3 N 4 sample exhibited an upwards shifted to higher binding energies compared with pure g-C 3 N 4 . Remarkably, the peak at 404.4 eV of N 1s shifted to 405.6 eV and possessed higher intensity. All these positive shifts may be ascribed to the electrons transfer from g-C 3 N 4 to doped Cd 2+ , further proving the strong interaction between Cd and g-C 3 N 4 (Ji et al. 2019).

Optical properties
The optical absorption characteristics was an evaluation criterion to optical properties and electronic band structures of catalyst. Additionally, the absorption range and capacity of catalysts to light affected the photocatalytic performance. The UV-vis diffuse adsorption spectrum of g-C 3 N 4 and Cd/g-C 3 N 4 were revealed in Fig. 5. It can be found that there was a threshold value of 465 nm in the intrinsic absorption of bare g-C 3 N 4 , which was assigned to the bandgap of 2.57 eV (Kong et al. 2018, Wang et al. 2009b, based on Tauc equation listed (Luo et al. 2017): (αhν) 1/2 = A(hν -E g ) Where α is the absorptivity, h is the Planck's constant, ν is the light frequency, A is the constant. Obviously, the absorption edge of 10 Cd/g-C 3 N 4 had a signi cantly redshift compared with bare g-C 3 N 4 , which illustrated that 10 Cd/g-C 3 N 4 had a narrower band gap (2.29 eV), thus reducing the excitation energy for photogenerated carriers. Moreover, Cd/g-C 3 N 4 extended the adsorption edge to near 700 nm and the intensity of light adsorption was signi cantly enhanced, leading to a signi cant improvement in the visible light response. The spectral range covered extension is conducive to increasing the generation of electron-hole pairs, which showed effective visible light absorption activity and meant that cadmium doping into g-C 3 N 4 lattice greatly enhanced the optical response of g-C 3 N 4 .
The band gap (E g ) of 10 Cd/g-C 3 N 4 was smaller than g-C 3 N 4 , which further con rmed the improvement of light absorption performance. The above analysis indicated that, with the doping of Cd, the light absorption performance was improved and the band gaps tended to be smaller. Based on this result, it can be inferred that doping g-C 3 N 4 with Cd can signi cantly affect the electronic band structure of g-C 3 N 4 and further improves the photocatalytic performance (Ge et al. 2012).
The charge separation, migration and recombination were analyzed by photoluminescence (PL) spectra.
Commonly, the weakened PL intensity meant enhanced photoinduced charge carries separation and transfer e ciency. PL spectra of 10 Cd/g-C 3 N 4 and g-C 3 N 4 excited at 380 nm were indicated in Fig. 6a.
Apparently, the strong absorption peak exhibited by pristine g-C 3 N 4 at 460 nm was consistent with the absorption edge of the UV diffuse re ection spectrum, which indicated that the recombination of the photogenerated electron − hole pair was severe (Yu et al. 2013). Once cadmium was introduced, the highest intensity position showed a gradual red-shifted from 460 nm of g-C 3 N 4 to 490 nm of Cd/g-C 3 N 4 , which was associated with the band gap narrowing effect (Zou et al. 2019) (Gu et al. 2018). Obviously, the PL signals of Cd/g-C 3 N 4 samples signi cantly fell with the doping of cadmium and 10 Cd/g-C 3 N 4 was the lowest, which illustrated that Cd doping restrained the recombination of photoinduced charge carriers and further proved that 10 Cd/g-C 3 N 4 composite possessed the best removing e ciency . Figure 6b was a Mott-Schottky plots with at band potentials at a frequency of 1000 Hz (conduction band potential is equivalent to at band potential for an n-type semiconductor).
Both g-C 3 N 4 and 10 Cd/g-C 3 N 4 displayed a positive slope, and the slope of 10 Cd/g-C 3 N 4 was decreased compare with g-C 3 N 4 , indicating g-C 3 N 4 and 10 Cd/g-C 3 N 4 were both n-type semiconductors and the electron donor density in 10 Cd/g-C 3 N 4 increased (Jun et al. 2013, Yang et al. 2013, Yuan et al. 2018, Zhou et al. 2014. Higher donor density is very helpful for improving photocatalytic performance because of the increased electrical conductivity and the mobility of charge carriers (Zhou et al. 2014). The conduction band potential of g-C 3 N 4 and 10 Cd/g-C 3 N 4 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 in uenced 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 ), which were formed by N2p and C2p orbital of g-C 3 N 4 , respectively (Xiong et al. 2016). The E CB and E VB could obtained from following (Zhang et al. 2010): Where X is electronegativity and E e is 4.5 eV. Fig. S3 revealed the E VB and E CB positions of as-prepared materials. For pure g-C 3 N 4 , the E VB was situated at 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-C 3 N 4 decreased to 1.47 eV and the E CB climbed to -0.82 eV.
Other photoelectrochemical techniques such as EIS and photocurrent response measurement have been employed to investigate the movement process of photoinduced electrons in prepared materials. Figure 6c revealed the photocurrent responses of g-C 3 N 4 and Cd/g-C 3 N 4 were repeatable and outstanding photostability under successive on/off illumination cycles. In addition, the photocurrent of g-C 3 N 4 could be negligible, and signi cantly increased photocurrent response density of 10 Cd/g-C 3 N 4 was observed, which indicated Cd doping increased the conductivity and separation of electron and hole of g-C 3 N 4 (Ren et al. 2017). Electrochemical impedance spectroscopy (EIS) was another method to evaluated the separation and transfer e ciency of photoinduced carriers. The smaller arc radius suggested smaller transfer resistance, namely the better separation e ciency, which meant lower recombination of photoinduced carriers (Lu et al. 2017). The equivalent electrical circuit is illustrated in Fig. 6d, which contained electrolyte solution resistance (Rs), double-layer capacitance (CPE), and charge-transfer resistance (Rp) (Yang et al. 2002). As depicted in Fig. 6d, the arc radius of 10 Cd/g-C 3 N 4 was smaller than pristine g-C 3 N 4 , which re ected that the charge-transfer resistance of Cd/g-C 3 N 4 was much lower than that of pristine g-C 3 N 4 , and Cd/g-C 3 N 4 possessed high-e ciency photoinduced carriers separation ability and faster interfacial charge transport level (Zhu et al. 2015).

Photocatalytic activities
To test the effect of different content of Cd doping on the degradation performance of the catalyst, we carried out a group of comparative degradation experiments with only changing the content of cadmium under visible-light irradiation (λ > 420 nm) at room temperature. In this paper, TC was selected as the main contaminant due to the high stability under visible light irradiation. Undoubtedly, the more TC degraded, the better degradation performance of the catalyst. As illustrated in Fig. 7a, the degradation rate of TC without catalysts can be neglected. Furthermore, the adsorption capacity of the doped catalyst is slightly larger than that of pristine g-C 3 N 4 during the dark stage, which corresponds to an increase in the speci c surface area of the catalyst after doping with Cd. However, the photocatalytic activity of Cd/g-C 3 N 4 composites rst increases and then decreases with the increase of Cd content, which probably indicates that excessive Cd could serve as photo-carrier's recombination center and inhibited the photocatalytic performance. Moreover, the introduction of excessive Cd may reduce the photo-adsorption ability and cover the surface active site of catalyst, thus hindering the effective migration of charge carriers , Zhang et al. 2016b, Zhang et al. 2010). Obviously, Cd/g-C 3 N 4 samples exhibits great increased photo-decomposition e ciency compared with pure g-C 3 N 4 (45.1%) and the 10 Cd/g-C 3 N 4 sample showed the greatest photocatalytic performance (89.09%) after an hour light irradiation, which can be attributed to the effective separation and transfer of photogenerated charges originating from the composite interface. All these results concluded that Cd load had signi cantly improved the photodegradation e ciency.
The pseudo-rst-order kinetic model was an alternative analytical method, which could further investigate the performance of the catalyst, the formula is express as the following: ln (C t /C 0 ) = -K app t C 0 is the initial concentrations and C t is the concentrations at time t of TC, K app is reaction rate constant (min − 1 ), respective. Fig. S4 presented the pseudo-rst-order kinetic plots of Cd/g-C 3 N 4 and pristine g-C 3 N 4 . Under the same condition, the degradation rate constant K app of 10 Cd/C 3 N 4 (3.605 × 10 − 2 min − 1 ) is about 3.6 times that of pristine g-C 3 N 4 (0.99 × 10 − 2 min − 1 ), demonstrating that 10 Cd/g-C 3 N 4 had the best photocatalytic performance, which consisted with the above experimental results. In conclusion, the introduction of cadmium could enhance photodegradation e ciency under light condition.
The effects of different reaction conditions, such as initial concentration of TC, different catalyst dosage and pH value, were studied to meet practical application. Figure 7b displayed the in uence of different catalyst dosages on TC photodegradation. It was apparent that the removal rate of TC was enhanced slightly as the dosages increased (0.4, 0.6, 0.8 and 1.0 g/L) and the catalyst dosages at 1.0 g/L showed the best photocatalytic performance.
The complete removal of TC in the photocatalytic process are crucial to prevent secondary pollution. Therefore, the ability of mineralization is also an important indicator of photocatalysts. Fig. S5 clearly indicated that the mineralization e ciency of TC reached to 34% within 60 min visible light irradiation, which con rmed that Cd/g-C 3 N 4 could actually degrade TC into small molecule intermediate compounds or CO 2 and H 2 O. The results indicated that Cd/g-C 3 N 4 is an excellent photocatalyst for TC degradation.
In this experiment, diluted hydrochloric acid and sodium hydroxide were used to adjust the initial pH. Figure 7c indicated that 10 Cd/g-C 3 N 4 performed well in wide pH range. Obviously, the degradation e ciency of TC was 77.2% at pH = 11. While the pH decreased to 7, the degradation e ciency got up to 79.9%, and while the pH decreased to 5, the degradation e ciency increased to 88.5%. It was concluded that photodegradation e ciency was higher in acidic environment than neutral environment and alkaline environment. Initial concentration of TC also had a strong effect on photocatalyst activities in practical application. It could be clearly detected that the removal rates of TC were 88.5%, 78.6%, 56.9% and 50.3% when the initial concentration of TC was 10, 20, 30 and 40 mg/L (Fig. 7d). This result indicated that the degradation rate was higher at low pollutant concentrations and then decreased with pollutant concentration increased. Two possible reasons could be proposed to elucidate this tendency. Firstly, high concentrations of contaminants might accumulate at the surface of catalyst, thus inhibiting the light absorption capacity. Secondly, intermediate products produced in the process of pollutant degradation might occupy the active site, so that pollutant had no contact with the catalyst.
Stability is an effective criterion to evaluated the practicability for semiconductor catalyst, thus, it is of great signi cance to assess stability of catalysts. The stability tests were carried out by reusing the catalyst under equal condition. It should be noted that the sample was cleaned with ethanol and deionized water, and then, it was dried in an oven at 60°C after each experiment. Figure 8a depicted that the TC degradation rates were slightly descending with the reuse of Cd/g-C 3 N 4 , which could be attributed to the small loss of sample during the collection after each cycle. The nial removal e ciency still maintained at a relative high level, which was about 80.03%. Moreover, no obvious peak changes were found by XRD analysis of the used photocatalyst, indicating the outstanding stability of Cd/g-C 3 N 4 . Therefore, the Cd/g-C 3 N 4 photocatalyst could be deemed as stable materials during the photocatalytic degradation of TC.

Mechanism
The trapping experiments was used to determine the decisive active species participating in the degradation process . Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), anhydrous ethanol, isopropanol (IPA), 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) were added into reaction solution as scavengers to capture h + , •OH and •O 2 − , respectively. Figure 9a and Fig. 9b illustrated that the degradation rate of TC was obviously suppressed by EDTA-2Na and descended from Page 12/30 90 % to 75.38 %, indicating that h + was the main active species of Cd/g-C 3 N 4 material. On the other hand, the degradation rates decreased slightly when the TEMPO and IPA were added, revealing that •O 2 − and •OH played auxiliary roles in photocatalytic degradation (Yan et al. 2010).
Electrochemical impedance spectroscopy (ESR) was carried out to verify the above results. As we could see from Fig. 9c and Fig. 9d, it is hard to seen the ups and downs signals of free radicals under dark conditions in Cd/g-C 3 N 4 . However, a strong intensity signals of •OH and •O 2 − radicals could be found under the visible light illumination, which proved that light is prerequisite for catalyst to product active species.
According to all aforementioned results, the introduction of Cd speed up the transmission of electron-hole pairs and decrease the recombination rate of photogenerated charge carriers, providing more reactive sites and accelerating cross-plane diffusion in g-C 3 N 4 nanosheets (Iqbal et al. 2017), thus achieving the enhanced photocatalytic property. A brief mechanism of TC degradation was summarized in Scheme 1. The electrons and holes were rstly generated under light irradiation, and the electrons transferred from valence band to conduction band spontaneously, thus leaving holes at valence band and electrons accumulated at conduction band (Eq. (1)). Then, the absorbed O 2 reacted with electrons to formed superoxide radical (Eq. (2)). Further, the surplus electrons reacted with superoxide radical to formed hydrogen peroxide (H 2 O 2 ) (Eq. (3)). As listed in Fig. S3

Conclusion
In summary, Cd/g-C 3 N 4 composites were prepared by mixing dopant agent CdCl 2 with g-C 3 N 4 in ethanol/H 2 O solution. The degradation experiments demonstrated explicitly that the photocatalytic activity of g-C 3 N 4 could be enhanced signi cantly with the doping of Cd elements, and the best removal e ciency was 89.09%, which was 2.0-times enhanced in comparison with pure g-C 3 N 4 (45.1%). Based on the characterization analysis, the enhanced photocatalytic performance could be ascribed to the improved the migration and separation of the photogenerated carriers. Meanwhile, the increased surface area and mesoporous structure extended the photo-absorption region, accelerating the transfer e ciency of light-generated charges. Concurrently, the ultrathin 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. The present work is expected to provide a new way for metal doped photocatalyst. Funding All the authors would like to thank the Program for the National Natural Science Foundation of China which nancially supported this study. All authors are grateful to their representative universities/institutes for providing experimental facilities and nancial support.

Declarations
Availability of data and materials All data and materials generated or analyzed during this study are included in the manuscript Ethical approval Not applicable.
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Competing interests The authors declare that they have no competing interests. (a) XRD patterns of pure g-C3N4 and Cd/g-C3N4 composites; (b) FT-IR spectra of pure g-C3N4 and Cd/g-C3N4 composites.